EEVblog #567 - Precision 1A Current Source

[jesuscf]

If I take the same model and then add a small step, only 10mV at time = 1ms, the circuit bursts into oscillations:
Jay_Diddy_B

I agree.   The problem I was having is that the same LTspice circuit was not oscillating when simulated using Circuitmaker or Multisim.  If you simplify the current source to the circuit shown in "simple_ideal_circuit.png" you'll get the unrealistic oscillations shown in "simple_ideal_wave.png".

After I changed the ideal opamp with the LT1006A then I got the results as shown in "simple_LT1006A_circuit.png" and "simple_LT1006A_wave.png".  The results are very similar to the simulations with Circuitmaker and Multisim.  So there may be something going on in the way ideal opamps are modelled in LTspice.

I included the two LTSpice circuits in "current_source.zip".  By the way I changed the values of the components because my next step would have been breadboarding the circuit with parts I have at hand an see what is going on.  I may still assemble and test the circuit even though I have used this current source many times in the past (no inductor though) without any problems.

Jesus

[mkwired]

That distorted waveform bug on the Rigol scope occurs on earlier scopes as well.  I see it all the time on my DS1102E among other problems.

[calexanian]

This is how I would do it if I had the precision 1.25 ohm resistors.

[jesuscf]

I breadboarded the simple current source I simulated with LTspice (and Circuitmaker and Multisim) as shown in the post above.  I used a LM385-1.2 voltage reference.  Since I don't have 12.5 ohms resistors I made one by putting four 10 ohms resistors in parallel (2.5 ohms) with this parallel connected in series with a 10 ohm resistor.  I used an LM358 because I don't have a LT1006A opamp.  The big choke inductor is around 1 mH.  The transistor is a 2N3904.  The 5V comes out of a LM7805.  Here is a picture of the assembled circuit:



Here is a picture of the oscilloscope showing the voltage at the load after power on.  It flattens at approximately 1V.



Here is the same voltage from the Circuitmaker simulation.



Here is the same voltage from the LTspice simulation.



Once again, always be suspicious of simulations that oscillate just like that.

Jesus

[EEVblog]

This is how I would do it if I had the precision 1.25 ohm resistors.

That will have too much error. There is 2mA flowing in the sense pin, that's 0.2% error at 1A

[Jay_Diddy_B]

Hi,
I have put a designed a version of the current source. I have the parts on order. At this time the design is untested.

LTspice Model

Normally, I would do some LTspice modelling. Unfortunately, there is no model for the LTC6655 reference chip, so I have  gone straight to hardware.

Hardware Design

Schematic:



Board Design:

 The board is designed to replace the lid on a Hammond 1591B box. I will make this board on my LPKF Protomat.





4-Wire Resistor

The sense resistor is a critical component in this circuit. I have designed the board to take a Vishay VPR221Z series part. These are expensive. So I have designed a work around for the prototype. I will epoxy this resistor to a heatsink:






I will keep the group informed with progress, good or bad.

Jay_Diddy_B

[robrenz]

I have found that when the resistance of the connections used to parallel very low ohm resistors is not several orders of magnitude lower than the resistor values it pays to model the connection resistances also. IMO you need to model that resistor layout on spice with the full current and the all the actual trace resistances. When you look at the power dissipated in the trace resistances and the fact that they are copper with a terrible tempco I think it is going to give you poor precision.

[Rufus]

I will keep the group informed with progress, good or bad.

It will be bad.

You should add pads for capacitors between the chip OUT_F and chip GND and chip VIN and chip GND.

[Jay_Diddy_B]

Rufus,

I understand what you are saying. It certainly makes sense to add some extra pads for parts.

By doing the design in the open, I can bring the experience of the group to the problem.

My concern is that a capacitor from chip Vin to Chip ground, presents U2 with a capacitive load via C1. C1 and the capacitor from chip Vin to Chip ground would be in series. This may take some additional parts to make U2 work nicely with a capacitive load. I will check the datasheet.

With my LPKF machine I can make new boards very quickly.

This circuit is quite tricky....

Jay_Diddy_B

[Galaxyrise]

Since I only have experience with the Rigol scope, I thought that preserving the look of the screen when you hit run/stop was a feature.  If you want the result of just one waveform, use the Single button.  If you want to pause the screen, virtual phosphors and all, hit run/stop. 

Do other scope brands with virtual phosphors provide no way to stop the screen with the virtual phosphor effect still active?

[Rufus]

My concern is that a capacitor from chip Vin to Chip ground, presents U2 with a capacitive load via C1.

To be stable the LTC6655 requires a large low ESR capacitor on its output to ground. In normal circuits OUT_F and OUT_S are shorted, a capacitor between them isn't going to make it stable.

+ve currents into such a capacitor have to flow through the op-amp output and so the capacitor has the output impedance of the op-amp in series (and it is the output impedance at probably around 100kHz which matters). A capacitor from chip VIN to chip GND bypasses that but you are right it presents a large capacitive load to the op-amp. You might be able to feed the chip VIN through maybe 100R to present a load the op-amp can drive.

Taking the output capacitor all the way to power ground makes -ve currents flow through the op-amp and has the same problem + additional feedback from the load voltage.

I am doubtful that jacking up the ground of this voltage reference to make a current source like this is going to work out.

[Bored@Work]

Can the LTC2050 sink several mA? If not ...

[Rufus]

This with an LT6650 seems to behave and with some value adjustments (and 1 less diode) would probably work with an LTC6655.

[Jay_Diddy_B]

Rufus,

That will almost certainly work. There is also opportunity for controlling the loop gain.

Jay_Diddy_B

[cosmos]

This is how I would do it if I had the precision 1.25 ohm resistors.

That will have too much error. There is 2mA flowing in the sense pin, that's 0.2% error at 1A

I like calexanians proposal, perhaps one 1.25ohm resistor would be enough.
How about you buffer the feedback to OUT_S with one of your MAX4239s (or other low offset low noise op-amp) and let it handle the 2mAs?

Also, will there not be problems with the high side regulation when disconnecting the load?
Leading to the LT6655 floating with too low Vcc to operate just barely below the VIN rails? (can it start again when connecting the load?)

[calexanian]

This is how I would do it if I had the precision 1.25 ohm resistors.

That will have too much error. There is 2mA flowing in the sense pin, that's 0.2% error at 1A

I like calexanians proposal, perhaps one 1.25ohm resistor would be enough.
How about you buffer the feedback to OUT_S with one of your MAX4239s (or other low offset low noise op-amp) and let it handle the 2mAs?

Also, will there not be problems with the high side regulation when disconnecting the load?
Leading to the LT6655 floating with too low Vcc to operate just barely below the VIN rails? (can it start again when connecting the load?)

WOW!! 2ma draw on the sense pin? haven't they ever heard of a fet input? May as well use a 741 and a zener while they are at it!

[dac]

If you have an oscillator you need to reduce the amplification or add some phase shift! Add some RC filter in the amp input and/or output. In the input it will also protect the amplifier.

Which direction does the current flow in Australia? Here in Europe current flows from + to minus and electrons the reverse. Old US videos seem to flow the current reverse?!

Here is an informative video about how to make an oscillator (LC). You just do the reverse to get rid of it if an oscillator is not wanted.

--> http://youtu.be/a8casNPDvI4

[lewis]

WOW!! 2ma draw on the sense pin? haven't they ever heard of a fet input? May as well use a 741 and a zener while they are at it!

They probably use very low value feedback resistors internally to keep Johnson noise to a minimum.

[calexanian]

True.. Ground carbon shunt time! 

[EEVblog]

How about you buffer the feedback to OUT_S with one of your MAX4239s (or other low offset low noise op-amp) and let it handle the 2mAs?

Yes, that would be required to make this configuration a precision source.

[Rerouter]

and if you went one step further and made that buffer a differential amplifier configuration, bam your right back with what i suggested 2 pages ago :/

[nitro2k01]

Which direction does the current flow in Australia? Here in Europe current flows from + to minus and electrons the reverse. Old US videos seem to flow the current reverse?!

My theory is that we're converging toward a world where there's a relevant xkcd comic for every situation.

[Jay_Diddy_B]

Hi,

I think you are getting close the answer. It would be much easier to design and stabilize a current sink.

But, because this is going to be used in the Southern Hemisphere we need a current source.

Everybody knows that conventional current flows from the north pole to the south pole. Electrons must flow from the south pole to the north pole.

Or did I miss something when I was at skool? 

Jay_Diddy_B

[Napalm2002]

Did I miss something or am I just stupid. Why would it matter where the said item would be used. Or are you guys just messing around. Why would any electrical item care what part of the world it was used in. Besides the difference in mains voltage and frequency.

[EEVblog]

This is how I would do it if I had the precision 1.25 ohm resistors.

FYI, that solution works a treat, it you add a gate resistor, some decent capacitance across the shunt, and buffer the sense current.
Already shot a video on this, coming in due course. Supporters have already seen it.