DIY Scalable Bench Power Supply Design

[void_error]

First of all, since I'm new to this forum, I'd like to say hello to everyone. 

Off to the topic now... 

I've been digging into the internet for a good (read suitable for my needs) bench power supply design and I haven't found anything that suits my needs so I decided to design my own and I need a bit of help and a lot of opinions and/or suggestions. I won't post a schematic just yet because I don't have it, just the basic idea. Also, this is an El Cheapo build, so budget is limited.

Here are the specs:

--- 0-20V adjustable voltage (in 1V/ 100mV steps, gonna use a microcontroller with a DAC for that  )
--- max 3A current limit (also adjustable, haven't decided on the details so far but since I'm using a micro and a DAC I have plenty of options)
--- front panel volt meter (using the ICL7107 just because I have a few lying around)
--- front panel amp meter (ICL7107 again, with an INA168 as the current monitor - datasheet here http://www.ti.com.cn/cn/lit/ds/symlink/ina138.pdf)
--- last but not least - IDIOT-PROOF   

So far I have a 230V / 24V @ 4.17A mains transformer and I'm planning to use it (obviously).

It's going to have the output rectified and filtered and fed into the following:

--- +5V regulator (at something like 1A) using the LM2596-ADJ for the ICL7107 meters and micro (datasheet here http://www.ti.com/lit/ds/symlink/lm2596.pdf)
--- +12V regulator using the LM2596-ADJ for the fans (which draws about 100mA max, so 3A max is overkill) followed by a 78L05 (100mA) for the DACs, because I want a clean supply for my reference voltages
--- the 0-20V adjustable power supply using an opamp based linear regulator which will be fed from (you might have guessed already) another LM2596 configured as a step-down tracking pregulator which will (should) keep the linear regulator's input voltage 4V higher than the output (or that's how I want it to be).

On with the boring details...

--- the microcontroller will be a PIC and the DACs will connect on the SPI interface (I'm lazy and SPI is easy to program)
--- will use two temperature controlled fans (el cheapo computer fans; no details so far as I haven't thought about it too much)
--- box? damn it... why do I have to?! I'll probably buy one if it's cheap enough, otherwise I'll build it myself

Now the questions:

--- should I use the SMD or THT version of the main LM2596 for the tracking preregulator? I'd go with the THT one and slap it onto a heatsink, just for good measure.
--- could I use a LM317 for the linear regulator with an external PNP transistor to boost its output current? I know I'm going to need a negative supply of at least -1.25V to get down to 0V, so I might use an inverting charge pump off the +5V rail to get around -3V (the ICL7107 boards, already designed by the way, have an inverting charge pump using a MC14584 hex schmitt trigger inverter chip and I measured the output to be around -3.3V after I breadboarded the whole thing)

Thanks in advance,
void

[sfiber]

you should check to lm2676.I am working with this regulator for a few days and I strongly recommend to use this chip with a to220 package.

[kizzap]

wow, this sounds similar to what I am doing atm.

Fyi, to get to 0V output, you are going to need a negative voltage rail, so don't forget to account for that. Added to that, some op-amps won't go rail to rail, so the negative rail is crucial if you want the output to go to zero.

What you feed your voltage reference from counts just as much as what you power your DAC from. Linear Regs are probably the safest bet to get a low input ripple, However, check the datasheet for the ripple rejection ratio for the switching frequency you are interested in. Most of the specs are listed at 120Hz, or double mains, for where they are mainly used.

for measuring current, I suggest using the MAX4081. Caveat: ensure it's ground is at your -5V, as both inputs need to be 5V above GND to work. thankfully the MAX4081 provides external pins for you to be able to reference your output voltage to.

The switching pre-regulator will dictate the filtering that you need to put in place on the output to try reduce noise on the input to the linear regulator. As I stated before, The ripple rejection can fall to as low as 40dB at higher frequencies.

As for the package types for all the regulators, It will depend on the power dissipated through the device. The datasheet will tell you how to calculate it.

Also google the LM350 ;-)

[void_error]

you should check to lm2676.I am working with this regulator for a few days and I strongly recommend to use this chip with a to220 package.

Looked it up, the difference is that it runs at 260kHz compared to the LM2596's 150kHz, also its max duty cycle is only 91% compared to 100% for the LM2596 which means the max output voltage will be lower and I'm going to need every single volt if I want 24V at the output. Note that about 1.5V is lost on the internal switching transistor (NPN bipolar) of the LM2596 according to the datasheet. The 2676 uses a N channel MOSFET with a charge pump so it can't go to 100%.

wow, this sounds similar to what I am doing atm.

Fyi, to get to 0V output, you are going to need a negative voltage rail, so don't forget to account for that. Added to that, some op-amps won't go rail to rail, so the negative rail is crucial if you want the output to go to zero.

I'm going to use a negative supply from voltage inverting charge pump from the +5V rail as I mentioned in the first post. The opamps are LM324 for the regulator and probably MCP617 (low offset) to buffer the DAC outputs

What you feed your voltage reference from counts just as much as what you power your DAC from. Linear Regs are probably the safest bet to get a low input ripple, However, check the datasheet for the ripple rejection ratio for the switching frequency you are interested in. Most of the specs are listed at 120Hz, or double mains, for where they are mainly used.

If the high frequency ripple rejection is a problem then I'll just put a low pass RC filter in front of the 78L05 and it should clean everything up.

for measuring current, I suggest using the MAX4081. Caveat: ensure it's ground is at your -5V, as both inputs need to be 5V above GND to work. thankfully the MAX4081 provides external pins for you to be able to reference your output voltage to.

The switching pre-regulator will dictate the filtering that you need to put in place on the output to try reduce noise on the input to the linear regulator. As I stated before, The ripple rejection can fall to as low as 40dB at higher frequencies.

As for the package types for all the regulators, It will depend on the power dissipated through the device. The datasheet will tell you how to calculate it.

Also google the LM350 ;-)

I'm sticking with the INA168 mainly because it comes in a tiny SOT23-5 package and it's pretty easy to configure. I'll probably add an adjustable gain low offset opamp after it so I can calibrate the whole thing.

The dissipation for the 12V regulator is insignificant so I'll use the SMD versions for convenience since I'll be buiding this power supply at home (read 'I hate drilling holes. It's tedious and boring.'). As for the 5V one... at 1A output and 25V input (minimum) the duty cycle will be around 25% (taking switch and diode losses into account) which translates into 1A*1.5V*0.25 = 375mW. No problem using SMD. Temperature will rise to 0.375 x 50C + 50C = 68.75C which is reasonable but can be lowered with careful PCB design.

The preregulator however will dissipate slightly more than 1.5V x 3A = 4.5W (let's say 6W) and with a junction to ambient thermal resistance of 50K/W it means it needs a small heatsink. Junction to case thermal resistance is 2K/W so for an ambient temperature of 50C I'll need a heatsink of max 10K/W just to be sure, assuming the thermal interface material has 1K/W. That would give me a maximum junction temperature of 130C which is below 150C so I'll stick with the THT version.

Looked at the LM350 and at the LM338. LM350 has much better ripple rejection at 100kHz, although only 40dB, so I'll use that. No idea how I've missed it while searching for parts.

Thanks a lot for the suggestions and I'll keep you posted and stick around, I'll probably have a schematic up later this week.

[kizzap]

Looked it up, the difference is that it runs at 260kHz compared to the LM2596's 150kHz, also its max duty cycle is only 91% compared to 100% for the LM2596 which means the max output voltage will be lower and I'm going to need every single volt if I want 24V at the output. Note that about 1.5V is lost on the internal switching transistor (NPN bipolar) of the LM2596 according to the datasheet. The 2676 uses a N channel MOSFET with a charge pump so it can't go to 100%.

Is your transformer outputting 24V? They are generally rated as RMS, so your peak will be RMS * SQRT(2) = 33.94V approx. This may rise and fall depending on the line regulation from your wall outlet. In Australia for instance, our mains voltage can go -6%/+10% out of the 230V. Obviously don't forget to include diode drops, and you should have enough overhead for your circuit.

I'm going to use a negative supply from voltage inverting charge pump from the +5V rail as I mentioned in the first post. The opamps are LM324 for the regulator and probably MCP617 (low offset) to buffer the DAC outputs

Have a read of this Application Note from TI. With a simple change to a dual winding inductor and adding another diode and some capacitors, you get both rails, for quite cheap

[dannyf]

I would suggest a gradual approach.

1) Minimum output voltage > 0v: so you can use a 3-terminal voltage regulator as the base;
2) Maximum output current < 1.5v: 3-terminal regulator again;
3) MCU-based: no for now;
4) Current limiter vs. current "fuse": do you want to maintain the maximum current output or just shut off the output?
5) display: get one of those ebay led displays, for simplicity.

Such a device can be fairly easily constructed. Once you get it going, you can add / refine features.

The issue with mcu-based controller is that its ability is difficult to main, and it has poor transient responses.

[void_error]

Is your transformer outputting 24V? They are generally rated as RMS, so your peak will be RMS * SQRT(2) = 33.94V approx. This may rise and fall depending on the line regulation from your wall outlet. In Australia for instance, our mains voltage can go -6%/+10% out of the 230V. Obviously don't forget to include diode drops, and you should have enough overhead for your circuit.

It's outputting 24V RMS with no load and a 222V mains input (it's evening and everyone has their lights on). I actually did a test a while back with a 3A load connected (8 ohm heater wire resistor) across the secondary and after doing the math it turns out that the transformer's internal resistance is about 0.5 ohm. That means the output will be 22V at 4A. The bridge rectifier I'm going to use is a KBPC1010 http://pdf.datasheetcatalog.com/datasheet/wte/KBPC1004W.pdf (1kV overkill, 10A, but's that's what I found at the local store) which has a 1.2V diode voltage drop at 5A, so that makes 2.4V for two conducting every half cycle. If the output voltage of the rectifier + filter caps is not enough I could change the power supply's output voltage to 18V to ensure regulation.

Now the output voltage after rectifying and filtering should be 22 x 1.414 - 2.4 = 28.708V which is not too bad while ignoring the ripple which is another can of worms since a bridge rectifier with filter caps is not a linear load and the peak currents will be much higher than 3A... I could use some help here... 

Have a read of this Application Note from TI. With a simple change to a dual winding inductor and adding another diode and some capacitors, you get both rails, for quite cheap

Might actually go that way, I've seen this in a MC34063 app note, thanks for the tip. 
edit: I found I possible problem with that: the input voltage is variable so the duty cycle of the regulator will be variable and since the second output has no feedback it won't stay at a fixed value... am I right?
If I am right then I can only use the negative rail unregulated derived from the +5V supply and use a low voltage negative linear regulator. I'll still need three LM2596s anyway because the pregulator has to be separate and the +5V and +12V rails must be regulated, especially the +5V one. Or maybe I could get a 5V fan and ditch the +12V rail?! Decisions, decisions...

I would suggest a gradual approach.

1) Minimum output voltage > 0v: so you can use a 3-terminal voltage regulator as the base;
2) Maximum output current < 1.5v: 3-terminal regulator again;
3) MCU-based: no for now;
4) Current limiter vs. current "fuse": do you want to maintain the maximum current output or just shut off the output?
5) display: get one of those ebay led displays, for simplicity.

Such a device can be fairly easily constructed. Once you get it going, you can add / refine features.

The issue with mcu-based controller is that its ability is difficult to main, and it has poor transient responses.

I could indeed slap a smaller power supply together since I have all the parts lying around but that's not the point. I want a higher power bench PSU which would be easy to use. That's why I plan on using a microcontroller which will probably be last one to be implemented as I can replace the DACs with a TL431 set to 4V and a pot.

I'm not using the micro for anything else than setting the output voltage and output current limit, basically using some buttons or rotary encoders (haven't decided yet) to set two voltage references.

As for the current limiting I want constant current, but I might also implement fuse type limiting or hiccup current limiting, the latter requiring only a schmitt trigger, a cap and some resistors, or it could even be done with a 555 timer (I actually did that for experimental purposes and worked really well).

[dannyf]

I'm not using the micro for anything else than setting the output voltage and output current limit

It depends on what you meant by that.

For example, using the mcu to read the output voltage setting and output a steady reference voltage (ie, letting the regulator maintain a constant output voltage) is fairly straight forward.

Using the mcu to maintain the output voltage is actually fairly difficult - not impossible to do.

Using the mcu to maintain the output current is even harder - they are inherently unstable.

If you want, I posted a constant voltage + current limiter design using a mcu somewhere here. The interesting about it is that the opamps utilize floating rail design.

[void_error]

The microcontroller just sets two DAC outputs used as voltage references and the analog circuitry does the rest. That's why I need a really stable supply for the DACs. The microcontroller doesn't interfere with any way with the feedback loop of the power supply.

As for the floating rail design, it sounds interesting and I was actually thinking about it. I'd be grateful if you provided a link. 

[dannyf]

If that's the case, you actually could use pwm output as the DACs.

I will find out the link for you later. That design utilizes the mcu to stablize the output voltage and maintains the current limit. So it is a little bit different.

edit: here it is.

https://www.eevblog.com/forum/projects/el-cheapo-mcu-controlled-power-supply/

[void_error]

I'll avoid using PWM instead of DACs just because of the delay the RC filter at the PWM output will have.

After I do all the math for the analog section of the PSU I'll start working on the code for the microcontroller. I'll be using a PIC btw (no idea which one yet) and I'll use an LCD display with it for the set voltage, set current limit, actual voltage and actual current and ditch the ICL7107 meters (I'll find another use for them). All the code will be written in C (i know assembly as well, that's how I learned PIC programming).

Any idea on how to do the voltage adjustment and current limit on the LM350 with a bunch of opamps? The INA168 current sensor will output 1V/1A referenced to ground.

Also, how much will it matter if I supply the opamps from the tracking preregulator output (which is variable)?

[dannyf]

I'll avoid using PWM instead of DACs just because of the delay the RC filter at the PWM output will have.

Since it is an open loop from the mcu's perspective, delays here don't matter.

[void_error]

How about the accuracy? The PWM output from the mcu won't swing to the rails (just very close to) and it'll be dependent on the mcu's supply voltage.

In comparison, the DAC i'm planning to use, MCP4802 http://ww1.microchip.com/downloads/en/DeviceDoc/22244B.pdf has an internal voltage reference of 2.048V which will make programming a lot easier and I'll get precise 100mV steps at the output of the power supply (theoretically, practically this will be affected by component tolerances ).

[dannyf]

Since it is open loop, you have no hope at even moderately respectable accuracy - take a look at your regulator and you will see that the output typically go +/- 0.1v around the spec.

In terms of resolution, it depends on the pwm mode or the dac used but 8-10 bits are fairly common, 16 bits are doable.

[kizzap]

Now the output voltage after rectifying and filtering should be 22 x 1.414 - 2.4 = 28.708V which is not too bad while ignoring the ripple which is another can of worms since a bridge rectifier with filter caps is not a linear load and the peak currents will be much higher than 3A... I could use some help here... 

Take a read of this website. Your ripple will be the peak voltage minus the lowest voltage that you can allow. It gives the rest of the information you need there, and gives you an idea of the minimum capacitance you need to ensure you have enough overhead.

Might actually go that way, I've seen this in a MC34063 app note, thanks for the tip. 
edit: I found I possible problem with that: the input voltage is variable so the duty cycle of the regulator will be variable and since the second output has no feedback it won't stay at a fixed value... am I right?
If I am right then I can only use the negative rail unregulated derived from the +5V supply and use a low voltage negative linear regulator. I'll still need three LM2596s anyway because the pregulator has to be separate and the +5V and +12V rails must be regulated, especially the +5V one. Or maybe I could get a 5V fan and ditch the +12V rail?! Decisions, decisions...

The cores share the Ferrite core, the change of flux through one coil will be linked to the change in flux in the second coil. when the inductor takes the rail to the right level, the switcher collapses the field, thus dropping the output voltage. The net effect is, ignoring extreme mismatching loads, and losses in the inductor coils, the outputs track each other fairly well.

I'll avoid using PWM instead of DACs just because of the delay the RC filter at the PWM output will have.

Any idea on how to do the voltage adjustment and current limit on the LM350 with a bunch of opamps? The INA168 current sensor will output 1V/1A referenced to ground.

Also, how much will it matter if I supply the opamps from the tracking preregulator output (which is variable)?

Use a PNP transistor driven from the output of your op-amp, connect the output of the linear reg to the adjust pin via a 120ohm resistor (the regulator maintains a ~1.25V difference between those pins - the 120 ohm resistance provides your minimum output load ), and then connect the adj pin through the transistor to your negative supply.

I looked at using the pre-regulator as well for powering the op-amps. Pros: if you have a decent rail-to-rail op-amp it should be capable of doing what you need to do. Cons: Whatever noise you don't filter out of the rails could be coupled into the adj pin, and show on the output, thus destroying the ripple rejection.

As for current measurement, do it just prior to the input of the linear regulator. The current both sides of it will be the same, however as you have overhead on the input, the voltage drop over your current sense resistor will not impact the output regulation.

As an aside, be sure to have a look at the INL and DNL of your DACs. If you are not careful, you might not be able to get the full output range you want.

[IanJ]

Hi,

Very similar to yours, my own design uses an LM2576 switcher for the pre-regulator. It's quite hard to completely remove all hint of HF noise getting through to the main psu outputs. You'll need everything from a good pcb layout, filtering and ferrites......albeit depends on just how far you want to go.
Will be interesting to see how you intend to control the switcher output so that it tracks. It ain't so easy without causing some extra noise issues (I found anyways).

DAC control of CV & CC (setting only) is great fun.......for info 10-bit is workable, but preferrably more. You always need more!

Ian.

[dannyf]

Any idea on how to do the voltage adjustment and current limit on the LM350 with a bunch of opamps?

There are many ways to do it, but they typically follow the topology I used in that schematic linked above. The lm350 here functions like the mosfet in my design, with the control signal applied to the adj. pin of the lm350 / gate of the mosfet.

The diodes / leds in my design are really the key. They effectively OR active low control signals from either the CC loop or the CV loop.

In your case, you will have 2 - 3 loops, depending on your implementation:

1) the resistive devider for LM350. As long as they are of sufficiently high resistance, you don't need to OR such signal. This is the default control signal - they will always be there.
2) the CC loop: once the CC condition is reached, this signal goes low. It can be digital or analog;
3) the CV loop: once the CV conditions are reached, this signal goes low. This can be digital or analog.

2) or 3) can be combined. A digital implementation would be fairly simple, and if done via two LEDs, those LEDs also function as CC/CV indicators -> they light up when the corresponding conditions are reached / active.

You will likely face challenges combining the resistive divider with the CV loop; and getting the CC loop to not oscillate too much.

[void_error]

Thanks for the tips, kizzap 

Hi,

Very similar to yours, my own design uses an LM2576 switcher for the pre-regulator. It's quite hard to completely remove all hint of HF noise getting through to the main psu outputs. You'll need everything from a good pcb layout, filtering and ferrites......albeit depends on just how far you want to go.
Will be interesting to see how you intend to control the switcher output so that it tracks. It ain't so easy without causing some extra noise issues (I found anyways).

DAC control of CV & CC (setting only) is great fun.......for info 10-bit is workable, but preferrably more. You always need more!

Ian.

I did actually find your design just after I started this thread and borrowed a few regulator side things from it. However, I've done the tracking a bit differently.

You will likely face challenges combining the resistive divider with the CV loop; and getting the CC loop to not oscillate too much.

I'll probably have to place a cap somewhere in the constant current loop, although in simulation it seems stable.

Here's the schematic of the main regulator. The values of L2 & C4 might have to be tweaked. The opamps are powered from an extra filter (R12 & C5) at pregulator's output. Hopefully this should get rid of some noise.

The LM350 is set to 2.5V output. I'll probably use a PNP transistor to buffer the output of U4.1 as kizzap suggested. U4.2 handles the current limiting. The INA168 outputs 1V for each amp, so for 3A there will be 3V across R11.

Can I make this any simpler?

[dannyf]

Can I make this any simpler?

I think so.

1) I would build the thing without the pre-regulator first.
2) LM350 can be replaced with a npn/n-ch device for higher current capability.
3) u4.2 can be used to pull down the adj. pin directly - this isolates the CC loop from the CV loop.
4) I would add a diode / led to u4.1's output. and use a led for d4 too.
5) you will need to experiment this a little: I would reference the pre-regulator's ground to the output of the lm350. This makes the pre-regulator a floating design. See one of Jung's papers on this.

[void_error]

I updated the schematic (and it's also bigger this time). It'll probably be the final version.

The next thing that should be done is to figure out all the connectors going to the MCU board (pretty easy), but first the auxiliary power supply and there's a change here...

It'll supply +12V @ 500mA (or thereabouts), -12V @ 50mA from another winding on the inductor which is fed into a -5V linear regulator (most likely a 7905).

For the MCU and DACs (I also plan to use external ADCs to get the voltage and current readings) I'll probably use 7805s fed from +12V. I'd like to keep the analog and digital stuff separate.

[dannyf]

I updated the schematic (and it's also bigger this time).

I am not sure if we are talking about the same thing. This is closer to what I was talking about.

U3 and associated parts form your current sense amplifier.

U2 (in the middle) is your CC control loop.

U1 (on the bottom) is your CV control loop.

The output from U1/U2 are OR'd by the diodes and applied to the regulator (M1) - it can be your LM350 if you want but I don't know why you would want that.

V1/V4 are the power supplies.

V2 controls the output voltage and V3 controls the current limit.

The DC sweep on the bottom is for V2. As you can see, the minimum output voltage for the power supply is about 3v. As V2 goes up, the output voltage goes up linearly (determined by 1 + R3/R4), until such a point where the current limit kicks in (the current limit runs at 1V/1A, in the current setting).

Room for improvement:

1) A pre-regulator: a floating pre-regulator, preferrably similarly to Jung's implementation but with a DC/DC converter would be quite helpful here in reducing the power dissipation on M1.
2) current sense amplifier: it needs an opamp with wide common voltage range and preferrably R2RI operation.
3) error amps: needs R2RO opamps to extend the output voltage to near zero.
...

[dannyf]

Forgot this: R1 can be replaced with a CCS.

The LEDs ORing the signals from U1/U2 also double as indicator. They lite up indicating if the power supply is in CV or CC mode.

The plot on the bottom is for V2 (CV source). As V2 increases, the output voltage increases until the current limit is hit (1amp). At that point, the power supply maintains a steady output voltage even if V2 continues to increase.

[void_error]

Now I get it and how come I didn't think about that in the first place? Oh well...

Using the LM350 because it's convenient. Also, the LM358 doesn't swing to the positive rail (only about 2V below, but that's good enough). It doesn't even have to swing to the regulator's output voltage (4V higher than the opamp supply, or above that if I used a NPN darlington instead of the LM350), only to V_OUT - 1.25V or VCC - 2.75V.

The only problem might be how close the CC loop opamp swings to the negative rail at about 8mA sink current... the maximum output voltage should be 0.6 + 1.25 + V_LED (around 2V I think), but I haven't tested anything yet and haven't bought any of the parts. Could use a LM317 for testing purposes.

Anyway, here's yet another schematic. 

edit: the dual LED idea is a good one. Might use a bicolor common anode one.
another edit: another schematic, this time for the part that outputs the +12V, +5V & -5V needed. Let me know if there's something wrong.

[void_error]

Here are some updates on the project (for anyone interested  ):

---> Almost finished the digital part including programming:
      ---> using a PIC16F887 (44pin TQFP44 package, dirt cheap, less than $3)
      ---> MCP3208 12bit 8 channel ADC with SPI interface for the reading the linear regulator output voltage, linear regulator output current,  linear regulator input voltage and heatsink temperature
      ---> 2x MCP4921 12bit DACs with SPI interface as the linear regulator's voltage references, their Vref set to 4.096V, TL431 voltage regulator
      ---> MAX7221 8 digit multiplexed 7 segment LED display driver with SPI interface for displaying the output voltage and output current, each of them on 4 digits and/or some stupid message if I wish
      ---> 20x4 character LCD display for all the other stats like set voltage, set current limit, output power, linear regulator power dissipation and heatsink temperature
      ---> 2x 2 bit Gray Code rotary encoders for setting the output voltage and curent limit
      ---> two pushbuttons, one sends the settings to the DACs, the other one cancels the new settings and reverts to the previous ones
      ---> output enable switch (probably independent of the micro)

If you're wondering why I haven't used the LCD to display everything it's because the characters are not readable at a glance if I'm further than one meter.

---> Design changes to the regulator part
      ---> swapped the LM350 for a NPN darlington pair fed by a current source
      ---> ditched the -5V rail
      ---> using an LT1013 instead of the LM358 because it has lower offset
      ---> since the +5V rail will have to output the highest current now (most of it used to light up the LED displays) it'll be using a LM2596
      ---> +12V fan supply will be a 7812
      ---> 78L05 fed from the 7812's output will power the  DACs and ADC

I'll be left with 4 unused channels on the ADC so I have an idea...
Why not use them to measure external voltages via some probes? Might be handy. I'll obviously use an opamp buffer between the input and ADC with some additional protection circuitry.

Post any opinions or suggestions below. 

[robimarko]

Can you post new schematics