Anything wrong with this linear PSU design? (now selecting parts)

[Powermax]

What if I use a single discrete op amp to power a small resistor, and I used the actual +Vcc as the current sink? My guess is that the op amp has not been parameterized to sink current itself, and would probably draw an additional few tens of uA's on it's own (unless it was one of those ultra low power ones) Probably not a good idea, but could that be useful for other applications?

As for the fast current limit, wouldn't the speed be limited by the output filter capacitors? Those could deliver a pretty large current transient when shorted, although long leads from a remote supply might have enough inductance and loop area to attenuate that spike. It would seem that using a low impedance pass output pass element is going to be doomed when it comes to current control, as the op amp will have to do all the regulating, vs the voltage control  where the transistor's high current gain (and unity voltage gain) make the voltage error amp's job much easier.

I don't know how I would go about modifying it if the transistors I added were not enough. What if I somehow bypassed the NPN on the pass element tapping directly into the base of the MJE2955 to set current? Surely that'd be faster (?)

[ZeTeX]

Maybe you want a down-programmer: (aka make the supply a limited quadrant 2 output and complicate the shit out of it)
http://electronicdesign.com/test-amp-measurement/if-your-power-supply-needs-fast-rise-and-fall-times-try-down-programmer

But just so you know,  Rigol DP832 has 1000uF output capacitance, and it counts as a really good use-able power supply, so your supply is already much better and probably fast enough for 99% of the things.

[Kleinstein]

Usually the output capacitor can deliver quite a lot of extra current, so there is a point on how fast current limiting has to be. No need to be much faster. The trouble is a little that with a fast voltage loop, that the current can go really up for the worst case of having a dead short from a not so low voltage. Here the peak current can go to the 10s or even 100 A in the simulation - though only for a few µs or so. This could be enough to blow a transistor. Some overshoot at the current is Ok, but it should not be much more than the normal current rating. At least limiting to such a fixed limit is relatively easy.

To bring the voltage back to normal after a steep load step, there needs to be phased when the current is higher than the new current level. This is to give back the charge to the input cap it used before the regulator worked. So a limited overshoot in the current is often even desired.

With slower voltage control, the current spike will also be smaller. So it could help, to not make the voltage control so super fast. Especially with the sziklai type output stage with low output impedance (essentially the shunt resistance) this is a very real option. A 2-quadrant output stage can help keep impedance of the power stage low, even for very low currents. With some slow ringing (which is hard to avoid) it is hard to ensure enough current through the power stage otherwise.

For the speed of the circuit, one also has to take into account parasitic inductance. At a low output impedance in the 0.1 Ohms range, inductance in the 100 nH range has quite an influence. So there are limits on how fast one can get at such a low impedance level - high speed is something for 50 Ohms impedance, not 1 Ohms and below.

[Powermax]

Maybe you want a down-programmer: (aka make the supply a limited quadrant 2 output and complicate the shit out of it)
http://electronicdesign.com/test-amp-measurement/if-your-power-supply-needs-fast-rise-and-fall-times-try-down-programmer

But just so you know,  Rigol DP832 has 1000uF output capacitance, and it counts as a really good use-able power supply, so your supply is already much better and probably fast enough for 99% of the things.

Good point. I honestly can't see why it would need to be hugely much faster, although if it can be done with a few extra BJTs, then why not!? If I have to entirely change my design to have a different type of current limiting, then I'm not going to bother. I think capping the maximum voltage output of the op amps so they do not saturate to Vcc - 2V will be good enough. A near-short from Vcc to ground will cause a (slow) LM741 will take about 30 µS to travel the full supply voltage. Interestingly, 0.5V/µS (the slew rate of the LM741) would cause the output capacitor do draw or source 5A of current, which happens to be my maximum current limit! Just by coincidence. So whether the thing is in CC or CV mode when initially turned on (as the output capacitor charges) when I is set to 5A may depend on the particular op amp and it's speed.

To make the supply faster I could use better op amps (although that comes at the cost of stability.) If I use the LT1007 (I might in in a later design) I should be able to achieve 11V/uS which means  I could travel the whole supply rail in just under 1.5µS! 

[Kleinstein]

The fast current limit is relatively easy: just an extra shunt at either side of the power stage and a BJT to limit the current.

If you have a negative supply anyway, a down programmer is relatively easy: the minimal version is just a diode to let the OPs also pull down the output if they get more negative. Current is limited by the OPs. The more powerful version somehow collides with anti-windup, as the OPs in this case would have to provide a more negative voltage.

The slew rate is not only limited by the OPs, but also from the compensating caps: the actual limit could be slower.

[Yansi]

Why is it called a "downprogrammer"?  It seems to me it is just like adding a two quadrant operation, with the current sink side less powerful.

[David Hess]

But just so you know,  Rigol DP832 has 1000uF output capacitance, and it counts as a really good use-able power supply, so your supply is already much better and probably fast enough for 99% of the things.

That is an insane amount of output capacitance for a 3 amp output.  My starting rule of thumb is from 50 to 100 microfarads per amp (1) and my usual goal is to minimize the amount of output capacitance used consistent with transient response including mode transitions.  A more careful and complex design might operate with even less capacitance.

(1) I never bothered to look until now but the same output transistors, the Tektronix PS501 is 200uF per amp with a Darlington pair and the PS503A is 50uF per amp with a Sziklai pair which is suggestive.  It would be interesting to see the schematic to the older PS503.

[Powermax]

The fast current limit is relatively easy: just an extra shunt at either side of the power stage and a BJT to limit the current.

Yay! But I am not a fan of having an additional shunt resistor pissing away power. Could I used the already-existing shunt? Are you referring to the classic example of this!?

If you have a negative supply anyway, a down programmer is relatively easy: the minimal version is just a diode to let the OPs also pull down the output if they get more negative. Current is limited by the OPs. The more powerful version somehow collides with anti-windup, as the OPs in this case would have to provide a more negative voltage.

What is a "down programmer"? The and what is "anti-windup"? A quick google seemed to lead me to PID control theory Which is appropriate given this is a feedback system. What I understood was that it is essentially the "inertia" of a fast changing signal in this case. A signal that has the tendency to move fast but reluctant to change in dv/dt. Seems to be related to bandwidth, given that an integral function in the S domain or frequency domain looks like a downward sloping like (a sort of lo-pass filter)

The slew rate is not only limited by the OPs, but also from the compensating caps: the actual limit could be slower.

Somehow I knew you were going to point out that oversimplification. But good, because I did almost neglect that. Luckily the 'slow' UA741's don't need any compensation components at all in practice and in simulation. I'd imagine the LT1007's I might use even with compensation will be much faster.

[David Hess]

The fast current limit is relatively easy: just an extra shunt at either side of the power stage and a BJT to limit the current.

Yay! But I am not a fan of having an additional shunt resistor pissing away power. Could I used the already-existing shunt? Are you referring to the classic example of this!?

This is not a bad idea but I agree about adding an additional current shunt; I would rather use the existing current shunt or the ballast resistors if they exist.  If the existing current shunt is used, then I might add another diode or two in series with the base or emitter of the fast current limit transistor to raise the threshold allowing greater sensitivity of the current shunt.  There are more complex fast current limit circuits which provide a more accurate threshold but they are only common in integrated parts.  If an integrated regulator is used in place of the drive transistor, then its internal current limiting can be used.

Bipolar output transistors have an advantage over power MOSFETs as far as the fast current limiting because their gain falls at high currents naturally helping to protect the transistor.  If the normal current limit is fast enough which is not difficult to achieve, then the decrease in current gain and thermal capacity of the bipolar output transistor will be enough to protect it during the momentary short which is reflected in the pulsed safe operating area curves.

What is a "down programmer"? The and what is "anti-windup"?

I think he means that an active load is used to pull the output down when the programmed output voltage is lower than the existing output voltage providing two quadrant operation.  This is less necessary if the output capacitance is minimized.  Some power supplies which include this can be used as active loads.

The active clamps you added implement anti-windup preventing the external or internal compensation capacitors from charging when one or the other error amplifier is not driving the output.  Although a lessor concern, this also prevents the operational amplifiers from saturating which in some cases can also increase response time.

The slew rate is not only limited by the OPs, but also from the compensating caps: the actual limit could be slower.

Somehow I knew you were going to point out that oversimplification. But good, because I did almost neglect that. Luckily the 'slow' UA741's don't need any compensation components at all in practice and in simulation. I'd imagine the LT1007's I might use even with compensation will be much faster.

This is why no more compensation than necessary should be used.

Operational amplifiers which support external compensation might allow even better performance but they are uncommon and will make the design less universal.  The same goes with operational amplifier which directly support clamping to prevent windup.

[Kleinstein]

If more than one output transistor in parallel is used, which is very likely at 5 A, the current sharing resistors can be used to make the fast current limit. Having the fast limit with the low side shunt could work too, not as fast, but still faster than the normal regulation loop and fast enough.

For the output capacitance, it makes a difference which type of circuit is used. The circuits with a low impedance output stage (like here), can work with very little output capacitance. They may not even need it for stability of the CV loop, but just to get the CC loop stable and to get better pulse response. So there could be output capacitance in the 1 µF range if you really want to, especially if a 2 quadrant output stage is used. So the 10 µF in the simulation can be realistic.
Depending on the speed of the CC loop, this can add considerably to the measured impedance in the CC mode. Up to the point of having a large apparent output capacitance, if the CC loop is very slow. The shown current loop is reasonable fast, adding something like another 10 µF of simulated capacitance in the CC mode.

There is a second class of lab supplies, that use a current controlling output stage. These generally need a considerably higher output capacitance. So here the 100 µF / A is somewhat realistic. One point here is that they need a certain ESR for that capacitance and the old electrolytic caps just need so much capacitance to reach low enough ESR values. So lower caps could work with modern low ESR types.

[Powermax]

It appears in the simulation, with a current source that steps from 10mA to 100A (set up as as an "active load" I assume that means it draws the configured current until the voltage falls to 0V or below.) I get a huge current spike, reaching 60ish V in simulation (if I decrease the timestep I expect I will see a higher reading peaking at 100A) due to the small 1uF "ideal" capacitor, and a tail of current dropping the set current of 2A after 16 microseconds. (using the LT1007's).

If I remove the 10uF capacitor (and adjust the compensation capacitor on the CC error amp to avoid oscillation) then the peak current spike is a little less at 45A and falls to about 28A within 0.3uS and rock solid there for about 2.3uS. I set up the CV at 15V so the op amp has to slew from 15 down to zero. Once it gets within range then the current again exponentially falls to the set level.

I tacked on an additional BJT with the emitter at the low side of the current shunt with the high side connected to the ground (output ground). This limits the current to 6.8A with already existing 0.1A shunt. I'd expect if I used a 0.25 ohm shunt (my dale resistors) the current will end up being limited to a bit under 3A.

[Powermax]

OK, since I am not getting any more replies regarding improvements to the design, I'll go ahead and make the PCB layout. But before I do that I need to select parts.

Huge shout-out to Kleinstein, David Hess, and Yansi for helping me improve the design! Thank you guys!! I appreciate it! I'm sure it can be further improved but at this point I just want to have a power supply that works. I might respin the board in the future to make improvements. But currently I keep running into the problem of needing a power supply and always end up bodging together a temporary LM317 circuit zener shunt regulators, I'm tired of wasting time with that.


I did some research on the 0.25 ohm resistors I have, and as it turns out, they have a 90 PPM / ^OC  temp-co! I found a really cheap (30 cent) strip of metal on Mouser claiming to be a 5% tolerance 0.1 ohm resistor with a more reasonable 20 PPM / ^OC temp co. There is only one resistor that has an even lower temp-co (at an astonishing 0.05% 3 PPM / ^OC) but they are asking $40 frickin dollars for it!!!
http://www.mouser.com/Search/ProductDetail.aspx?R=OAR1R100FLFvirtualkey66200000virtualkey66-OAR1R100FLF

Oh, BTW, is there a way to perhaps "glue" a resistor of simalar temp-co to the shunt and use it to control the set current? My idea is that as the temperature of shunt rises, so will the temperature of the thermal sense resistor and if it rises at the same rate as R4, then perhaps I can get thermal compensation? I'll look into the calculations for that another day but I'm sure it could help a little.


For a DAC (to allow interfacing to arduino and possibly even an app!) I selected the classic CMP4725 as it was one of the cheapest options available, appears to be easy to interface, and there is a sparkfun breakout board featuring the chip, so clearly it should be arduino friendly (libraries should exist for easily interfacing it.)
http://www.mouser.com/Search/ProductDetail.aspx?R=MCP4725A0T-E%2fCHvirtualkey57940000virtualkey579-MCP4725A0TECH

Unfortunately this limits me to 2mA resolution (which I guess isn't too bad) and 5mV resolution (From 0 --15V) if I want round increments without the ugly voltages produced by using the full resolution. Also I think this design might allow me to have an output that can go up to 30V, with the right transformer with enough tabs! The op amps should be (barely) capable as they can handle +/- 20V, which equates to 40V. I might add a shunt across them incase a surge occurs and blows my $$$ LT1007 parts.
 
I found these EC12D1524403's to be the cheapest rotary encoders that are pushable and have 24 clicks of resolution:
http://www.mouser.com/Search/ProductDetail.aspx?R=EC12D1524403virtualkey68800000virtualkey688-EC12D1524403

Most of the resistors, op amps, arduino, tack buttons, IC sockets, BJTs, diodes, and other passives, I have plenty of these. I have a significant savings here, too. 


The 2 things stopping me now is my selection for a transformer and, out of all things, finding some good knobs. On mouser, although there are a lot of options in power transformers, none are particularly cheap or suitable. I thought I might be able to get 3 smaller 5V transformers but this turns out to be more costly than a single large transformer.

Does anyone know of a source to get a nice large 100VA transformer with 5 3V tabs, 3, 5V tabs, or 2 individual 12V tabs? Or maybe a 200VA transformer with the number of tabs doubled? I have heard of suggestions to rewind a MOT transformer, which I like the idea, but I would need to increase the turns on the primary as well to avoid core saturation. I don't want to have to force cool the transformer as well.

And lastly, I want a jog wheel for the voltage/current set. I plan to use a single encoder that can be pushed to change between CC adjust and CV adjust. (no reason to spend extra for 2 knobs given no time saving in setting the output.) I would also use velocity control which would allow an automatic coarse and fine adjust.  So I need a nice big knob!!! Anyone can link me to a good choice?

[David Hess]

Oh, BTW, is there a way to perhaps "glue" a resistor of simalar temp-co to the shunt and use it to control the set current? My idea is that as the temperature of shunt rises, so will the temperature of the thermal sense resistor and if it rises at the same rate as R4, then perhaps I can get thermal compensation? I'll look into the calculations for that another day but I'm sure it could help a little.

It is a difficult problem and I think your expectations of resolution and accuracy at high currents may be excessive.

Precision current shunts are already temperature compensated by using an alloy like constantan or manganin so just adding the opposite temperature coefficient and getting it to track is not easy.

A temperature sensor could be added to the shunt and calibration done.

The easiest thing to do is power derate the shunt so that temperature rise is minimized.  Multiple higher resistance 4-wire shunts in parallel might be required but this leads to a complex layout.

For higher resolution at low currents, switch in a separate higher resistance shunt.  In practice this usually means bypassing the low current shunt at high currents.

One thing I have never seen done in a power supply which is possible in theory is to apply a low frequency modulated current to the current shunt and synchronously demodulate the voltage change to determine the current shunt's resistance in real time.  The voltage control loop keeps the low frequency signal out of the output.  Some multimeters can do this to make in circuit current measurements.

Unfortunately this limits me to 2mA resolution (which I guess isn't too bad) and 5mV resolution (From 0 --15V) if I want round increments without the ugly voltages produced by using the full resolution. Also I think this design might allow me to have an output that can go up to 30V, with the right transformer with enough tabs! The op amps should be (barely) capable as they can handle +/- 20V, which equates to 40V. I might add a shunt across them in case a surge occurs and blows my $$$ LT1007 parts.

I have blown out a few operational amplifiers this way.  Absolute maximum ratings are not to be ignored.

Some old but cheap operational amplifiers like versions of the 741 have a 44 volt maximum supply voltage.

Does anyone know of a source to get a nice large 100VA transformer with 5 3V tabs, 3, 5V tabs, or 2 individual 12V tabs? Or maybe a 200VA transformer with the number of tabs doubled? I have heard of suggestions to rewind a MOT transformer, which I like the idea, but I would need to increase the turns on the primary as well to avoid core saturation. I don't want to have to force cool the transformer as well.

Transformers are expensive so I usually make do with transformers salvaged out of surplus equipment or find them used.

A common design in older power supplies is to switch a dual secondary between series and parallel so the low voltage range has twice the current capacity as the high voltage range.

[ZeTeX]

OK, since I am not getting any more replies regarding improvements to the design, I'll go ahead and make the PCB layout. But before I do that I need to select parts.

Huge shout-out to Kleinstein, David Hess, and Yansi for helping me improve the design! Thank you guys!! I appreciate it! I'm sure it can be further improved but at this point I just want to have a power supply that works. I might respin the board in the future to make improvements. But currently I keep running into the problem of needing a power supply and always end up bodging together a temporary LM317 circuit zener shunt regulators, I'm tired of wasting time with that.


I did some research on the 0.25 ohm resistors I have, and as it turns out, they have a 90 PPM / ^OC  temp-co! I found a really cheap (30 cent) strip of metal on Mouser claiming to be a 5% tolerance 0.1 ohm resistor with a more reasonable 20 PPM / ^OC temp co. There is only one resistor that has an even lower temp-co (at an astonishing 0.05% 3 PPM / ^OC) but they are asking $40 frickin dollars for it!!!
http://www.mouser.com/Search/ProductDetail.aspx?R=OAR1R100FLFvirtualkey66200000virtualkey66-OAR1R100FLF

Oh, BTW, is there a way to perhaps "glue" a resistor of simalar temp-co to the shunt and use it to control the set current? My idea is that as the temperature of shunt rises, so will the temperature of the thermal sense resistor and if it rises at the same rate as R4, then perhaps I can get thermal compensation? I'll look into the calculations for that another day but I'm sure it could help a little.


For a DAC (to allow interfacing to arduino and possibly even an app!) I selected the classic CMP4725 as it was one of the cheapest options available, appears to be easy to interface, and there is a sparkfun breakout board featuring the chip, so clearly it should be arduino friendly (libraries should exist for easily interfacing it.)
http://www.mouser.com/Search/ProductDetail.aspx?R=MCP4725A0T-E%2fCHvirtualkey57940000virtualkey579-MCP4725A0TECH

Unfortunately this limits me to 2mA resolution (which I guess isn't too bad) and 5mV resolution (From 0 --15V) if I want round increments without the ugly voltages produced by using the full resolution. Also I think this design might allow me to have an output that can go up to 30V, with the right transformer with enough tabs! The op amps should be (barely) capable as they can handle +/- 20V, which equates to 40V. I might add a shunt across them incase a surge occurs and blows my $$$ LT1007 parts.
 
I found these EC12D1524403's to be the cheapest rotary encoders that are pushable and have 24 clicks of resolution:
http://www.mouser.com/Search/ProductDetail.aspx?R=EC12D1524403virtualkey68800000virtualkey688-EC12D1524403

Most of the resistors, op amps, arduino, tack buttons, IC sockets, BJTs, diodes, and other passives, I have plenty of these. I have a significant savings here, too. 


The 2 things stopping me now is my selection for a transformer and, out of all things, finding some good knobs. On mouser, although there are a lot of options in power transformers, none are particularly cheap or suitable. I thought I might be able to get 3 smaller 5V transformers but this turns out to be more costly than a single large transformer.

Does anyone know of a source to get a nice large 100VA transformer with 5 3V tabs, 3, 5V tabs, or 2 individual 12V tabs? Or maybe a 200VA transformer with the number of tabs doubled? I have heard of suggestions to rewind a MOT transformer, which I like the idea, but I would need to increase the turns on the primary as well to avoid core saturation. I don't want to have to force cool the transformer as well.

And lastly, I want a jog wheel for the voltage/current set. I plan to use a single encoder that can be pushed to change between CC adjust and CV adjust. (no reason to spend extra for 2 knobs given no time saving in setting the output.) I would also use velocity control which would allow an automatic coarse and fine adjust.  So I need a nice big knob!!! Anyone can link me to a good choice?

It's better to parallel 10 or more resistors, this way achieve higher accuracy and distributed power dissipation, so for 10 1 ohm resistor 1% its 0.1ohm 0.1% resistor.
MCP4725 has max +- 14.5 INL (Relative Accuracy) this is hopeless for power supply, you need something like +-4INL max, but in the graphs I don't see it reaching even close to 14.5INL. weird, I would use MCP4922 anyways.

Make sure that the encoder can be mounted on front panel if you are going to put in the a box, also the encoder you choose has only 15 clicks of resolution.
Maybe this one:
http://www.mouser.co.il/ProductDetail/Bourns/PEC11R-4015K-S0024/?qs=sGAEpiMZZMsWp46O%252bq11WUFm4YUtE9euaZTpksE9hoM%3d

For transformer, look here maybe:
http://spiratronics.com/toroidal-transformer-dual-primary-secondary-160va-0-12v.html
https://www.toroidal-transformer.com/shop/toroidal-transformers/160va.html

[Kleinstein]

From a 100 VA transformer you get about 70 W out on the DC side. AT this power level using just 2 transformer taps is usually enough. Only with higher power you might want more steps, to keep the peak currents when switching low, as the main filter caps have to load to the new possibly higher voltage.

For the DAC, there is also the possibility to use PWM and filtering for the set-points. One can also combine this with an DAC and thus extend the resolution of the DAC by a kind of dithering. Another option is using a low accuracy, but high resolution R2R chain as an DAC and use an ADC (e.g. the one for the display) for corrections.

90 ppm/K TC for the shunt is not that bad. Many cheap resistors are in the 300 ppm range. Correction is difficult as there are delays. One could have a second (lower resistance, more precise) shunt for the measurement / display, as here noise is not that important, because the BW is much lower.

[Powermax]

From a 100 VA transformer you get about 70 W out on the DC side. AT this power level using just 2 transformer taps is usually enough. Only with higher power you might want more steps, to keep the peak currents when switching low, as the main filter caps have to load to the new possibly higher voltage.

Ugh, I figured. VA ? W. I don't entirely understand VA either. I know it's apparent power vs real power and power factor relates the 2 to each other, or something. But I don't know how that works when considering such a nonlinear load as a rectifier and capacitor. Simulations like to show peak currents in the hundreds of amps! Would it help to add a resonant capacitor to the transformer tabs output to help power through those surges? Also I have a few Lamba linear power supplies and they use bolts for diodes (the diodes that look like bolts) but they have small electrolytic capacitors directly across each one. One is the purpose of those?

For the DAC, there is also the possibility to use PWM and filtering for the set-points. One can also combine this with an DAC and thus extend the resolution of the DAC by a kind of dithering. Another option is using a low accuracy, but high resolution R2R chain as an DAC and use an ADC (e.g. the one for the display) for corrections.

How clean could a PWM signal possibly get? It seems to me that it would destroy the noise and ripple characteristics, and enough filtering would cause a very sluggish response. Also the PWM from the arduino is only 10 bits resolution. Achieving higher would require software PWM, which has it's own problems.

90 ppm/K TC for the shunt is not that bad. Many cheap resistors are in the 300 ppm range. Correction is difficult as there are delays. One could have a second (lower resistance, more precise) shunt for the measurement / display, as here noise is not that important, because the BW is much lower.

I know lol  I wanted to get 3.5 to 4 sig figs resolution/accuracy for both current and voltage. That's what I'm aiming for.

It's better to parallel 10 or more resistors, this way achieve higher accuracy and distributed power dissipation, so for 10 1 ohm resistor 1% its 0.1ohm 0.1% resistor.
MCP4725 has max +- 14.5 INL (Relative Accuracy) this is hopeless for power supply, you need something like +-4INL max, but in the graphs I don't see it reaching even close to 14.5INL. weird, I would use MCP4922 anyways.

INL? because I am using an LM317 as the voltage output, I can tune precise output voltage. I am concerned more about relative accuracy. (like how linear the output is) I know nothing about DACs though. I do plan to connect an ADC to the output for the arduino to display voltage set and voltage output.

[Powermax]

I'd imagine the temp-co is mostly or entirely dependant on the resistive material. Why would anything else matter? How can one ni-chrome wirewound resistor differ from another?

If I cannot get a transformer supply for cheap, I guess I could settle for a variable switch mode supply, but I'll need good EMI sheilding and loads of filtering

[David Hess]

MCP4725 has max +- 14.5 INL (Relative Accuracy) this is hopeless for power supply, you need something like +-4INL max, but in the graphs I don't see it reaching even close to 14.5INL. weird, I would use MCP4922 anyways.

Graphs in datasheets usually show typical rather than maximum errors.

The DAC error is an important consideration.  The typical error of 4 INL out of 4000 counts is 1000ppm so a 20ppm/C current shunt would have to change by 50C which is almost reasonable.  With a maximum error of 14.5 INL, this becomes 3500ppm and a change of 175C.  So a 20ppm/C current shunt is better than really needed for this DAC.

The specifications for a Rigol DP700 are 200ppm/C for current and 100ppm/C for voltage and an accuracy of 2000ppm for current and 500ppm for voltage so the above errors seem reasonable.

This is a power supply and not a source meter.

[VEGETA]

Do you really need the 12-bit? just use the 10-bit adc or dac in the microcontroller, or do you really need more accuracy? what is your minimum set current or voltage? I guess a good PIC MCU can have 10-bit ADC and DAC. This may give you a 10mV step for your voltage and maybe the same for current. Do you need more?

[Kleinstein]

To simulate the rectifier one should include parasitic resistance of the transformer and caps too. Output resistance is about twice the DC resistance of the secondary. Also the fuse helps here a little. Usually one ends up at about 0.5-0.6 A DC for 1 A_RMS on the AC side. There is not very much one can easily do about this. An inductor would help, but this is bulky and the voltage drops under load. One can use just transformer - fuse - rectifier and filter cap and just accept that the power factor is not near 1 but more like 0.7. So for 1 amp of DC ouput current one needs something like an 1.8 - 2 A AC rating for the transformer.

With the slow Arduino SW PWM it is no practical to use PWM to set the voltage. With the much faster hardware PWM (up to 16 bit resolution) of the AVR it is practical. The Response time of a 3rd or 5th order filter is not that long - more like in the 10s of ms range, depending on the resolution. So the other options are usually faster, but for a power supply this should be fast enough.

The resistor materials are alloys and there TC depends on the exact composition and heat treatment. So it depends on the quality how low they can guarantee the TC to be. There are also a few different alloys used. Not all resistors need a low TC - so there is a market for low cost and higher TC ones too.

[Powermax]

Do you really need the 12-bit? just use the 10-bit adc or dac in the microcontroller, or do you really need more accuracy? what is your minimum set current or voltage? I guess a good PIC MCU can have 10-bit ADC and DAC. This may give you a 10mV step for your voltage and maybe the same for current. Do you need more?

I don't think it's possible to achieve 10mV resolution with a 10 bit DAC. 10 bits = 1024 discrete levels. The resolution will be 0.014648438 V. I could toss out 24 of those levels and have 15mV resolution, however. With a 12 bit DAC, I have 4096 levels, but only 3000 usable at a nice 5mV resolution. Much more wasteful. If I go for a 14 bit DAC, I can just achieve 1mV resolution throwing out 1384 out of the 16384 total bits. What I could do is interpolate a single more $$$ DAC using analog switches although I am unsure how much error would result from using these switches.



I do only care about resolution but only for the lower voltage settings, so it might be possible to get away with 10 or even 8 bits of resolution if I make the output voltage range selectable. (like 0 --  5V with 5mV resolution with 10 bits, or 0 -- 1A with 1mA precision) so this may be an option. Resolution becomes less important at higher voltage and power levels.

Make sure that the encoder can be mounted on front panel if you are going to put in the a box, also the encoder you choose has only 15 clicks of resolution.


[quote from: David Hess][Quote from: Powermax on Yesterday at 04:58:13 PM]
Oh, BTW, is there a way to perhaps "glue" a resistor of simalar temp-co to the shunt and use it to control the set current? My idea is that as the temperature of shunt rises, so will the temperature of the thermal sense resistor and if it rises at the same rate as R4, then perhaps I can get thermal compensation? I'll look into the calculations for that another day but I'm sure it could help a little.
[/quote]
It is a difficult problem and I think your expectations of resolution and accuracy at high currents may be excessive.[/quote]

I figured if it is easy to do (I can literally stick a resistor inside my wirewound noodle resistor without much problem.) then why not? But I was assuming the temperature coefficients were negative and linear. I don't know if that's the case or not. If it's not that easy then I won't bother.

Now I am starting to question whether a LM317 will be stable with input voltage and more variably, temperature. I might move over to a DAC which supports a proper voltage reference. My chosen DAC uses the noisy 5V rail which is no good. I see a few offerings of voltage references tuned suspiciously to 1024V, 2048V, and 4096V. These do cost over a dollar each though. Guess you pay for convenience. Found this one that appears to be good:
http://www.mouser.com/ProductDetail/Maxim-Integrated/MAX6043CAUT10TG16/?qs=sGAEpiMZZMuBck1X%252b7j9fADMRbLaMGMSpKa%2f1OuhCQ8%3d
and if I trade the temp co, precision, and cost for the convenience of a 4096mV source, I can get this part:
http://www.mouser.com/ProductDetail/Maxim-Integrated/MAX6198CESA+/?qs=sGAEpiMZZMuBck1X%252b7j9fADMRbLaMGMShqETZIS3RAM%3d

The rotary encoder are only 15PPM, but they have 30 indents. 15 refers to the number of cycles per revolution. But this can be 4 times higher (60) since there are 4 discrete pulses which can be detected per rotation, right? That's why I choose that one. It's the cheapest and I think 60 PPM with velocity control would be precise enough. What do you all think?

[VEGETA]

for my initial design idea, I wanted to use a 2.048v voltage reference to power the DAC\PWM\ADC stuff which are 10-bits of resolution. So 2.048v/1024 steps = 0.002v = 2mV per step. Since CV op-amp outputs 1v per 10v or 0.1v for 1v, the 2mV gives an output of 20mV which is less than 10mV steps that I want... perhaps I should reconsider. As for current, it is 0.1V per 1A so that 2mV = 20mA, not too good TBH.

If 12-bit is chosen, 2.048v/4096 = 0.0005V which means 0.5mV per step. This translates to 5mA and 5mV steps which is nice for voltage but not so good for current. maybe a solution is to get a 200mV voltage reference for current but this would mess the MCU a lot.

So this leaves us with 2 options: use a normal cheap MCU like PIC16F family and get external DAC\ADC. Or, get a very powerful MCU which has more than 12-bits of ADC\DAC in it.

Notice that resolution must be the same for your dac\adc because if your set accuracy is high (12-bit) while your reading is not (10-bit), you won't be able to sense the voltage accurately so your extra 2-bits of resolution will go in vain.

[Powermax]

for my initial design idea, I wanted to use a 2.048v voltage reference to power the DAC\PWM\ADC stuff which are 10-bits of resolution. So 2.048v/1024 steps = 0.002v = 2mV per step. Since CV op-amp outputs 1v per 10v or 0.1v for 1v, the 2mV gives an output of 20mV which is less than 10mV steps that I want... perhaps I should reconsider. As for current, it is 0.1V per 1A so that 2mV = 20mA, not too good TBH.

If 12-bit is chosen, 2.048v/4096 = 0.0005V which means 0.5mV per step. This translates to 5mA and 5mV steps which is nice for voltage but not so good for current. maybe a solution is to get a 200mV voltage reference for current but this would mess the MCU a lot.

So this leaves us with 2 options: use a normal cheap MCU like PIC16F family and get external DAC\ADC. Or, get a very powerful MCU which has more than 12-bits of ADC\DAC in it.

Notice that resolution must be the same for your dac\adc because if your set accuracy is high (12-bit) while you're reading is not (10-bit), you won't be able to sense the voltage accurately so your extra 2-bits of resolution will go in vain.

I'm not that fond of using the DAC or ADC inside a micro, those tend to be mediocre at best. I have 5 arduino pro's (ATmega328p) so the marginal cost of using is zero. I also have an LCD module, but it appears that it is being problematic. It refuses to work properly. I think dust and dirt worked it's way in between the LCD, zebra strips, and circuit board.

[VEGETA]

So what is your solution? and how it is affects your minimum set voltage and current?

[Powermax]

I am now looking into the feasibility of configuring my arduino to have 12 bit PWM outputs, I can actually make some measurements and get a feel for how accurate it's gonna be. I think the arduino has it's own 1.1V internal reference, but I am not sure how good it is compared to a proper voltage reference.

I do have some of these epic AD574 chips (12 bit ADC) and Burr Brown DAC71 (16 bit DAC) chips! They are moderately easy to interface (even with discrete logic!), just parallel. Loads of wires. The analog output is a bit more of a hassle to configure, they are not rail to rail and require a 5V logic supply and a dual rail supply.They are big and clunky and I think are not well suited for this application. They are a "successive approximation" DAC, but I guess that doesn't matter too much. What type of ADC is best?

Now of course I could easily set up another PWM pin and an error amp and more arduino code to basically force the PWM output to approach the value of the voltage and current output, essentially making my own successive approximation DAC. Although I feel this is a bit of a hacky solution. I might instead look into a I²C ADC.