Electronics > Projects, Designs, and Technical Stuff

Building an Electronic Load inspired by Louis Scully

<< < (4/5) > >>

OM222O:
A major improvement suggestion:
Get a dual channel DAC, use one channel to control 3 fets and one channel to control 1 fet. It allows you to have "coarse" and "fine" adjustment. Set the channel that controls 3 fets to a rough value and use a PID loop to control the "fine adjust" fet.

In order to get fine adjust, you can either divide the voltage coming from the second channel and buffer it with a unity gain amplifier, or use a larger source resistor.

pqass:

--- Quote ---
--- Quote from: pqass on August 19, 2020, 04:25:09 am ---One way to compensate for the typical opamp (eg LM358) offset voltage may be to first pipe your microcontroller's +DAC output control voltage through a unity gain inverting op amp mirrored about a level slightly below the half way point of your Vref, and then onto the usual opamp-mosfet-shunt load circuit.

--- End quote ---
Thanks for the tip. I see that it works, but doesn't it provide exactly the same level of solution as the one by Scully? I.e. a small negative offset is added to the control voltage. Effectively, I we get something like   \$I_{set} = V_{control}-V_{offset}\$. The problem with this in my opinion is that you get a coupling between what the hardware offset is set to, and what the software needs to do to compensate for it. I mean, \$V_{control}\$ must be set such that \$I_{set}\$ get's the desired value. Perhaps if the hardware offset is a constant very well defined stable value, like -0.1mV or something like that, then the computation in the software can simply assume that value and maintain its own calibration quantities.
Perhaps I am misunderstanding and that is exactly what you proposed? :) But wouldn't a simple offset added to a normal non-inverting buffer work just as well?

--- End quote ---

A negative opamp power supply  (-1V as I proposed) is needed because for the output to swing to or slightly below 0V, the +Vin of the op amp has to be below ground by its offset voltage; which for a LM358 is ~2mA.  I proposed an inverting opamp with an offset tweeked below +Vref/2 point because it allows the use of only a single +Vref.  I'm not sure where Scully got his negative reference but it should be from a true voltage reference chip/zener; not a power rail. The opamp power supplies (+ and -) can swing a bit (say, +/- 100mV) but as long as the +Vref is solid, your opamp outputs should be solid too.  Using a non-inverting buffer instead would need a -Vref to dial in a -2mV at the non-inverting buffer's R1 to the -Vin pin; an additional complexity.

By tweaking the low adj. pot, you can set, say, any DAC output value above 999 to be the point where the mosfet is completely off.  Below 1000, it turns on by 10A/1000 = 10mA increments.  If you can't quite reach full 10A at 0 DAC output (like in my falstad png) you can always adjust the inverting buffer's feedback resistor to raise or lower the gain a bit.  Once the low adj. pot and feedback resistor is tweeked, your software can be completely oblivious of a configuration value. The offset voltages of both opamps should be fairly* constant with time and temperature.  *Too small to matter (crosses fingers).

However, the current monitoring opamp (summing opamp in the last attachment of my last post) will have this offset anyway that you'll need to account for in software. Oh well!

FYI:I've built my own simple analog CC load (potentiometer + LM358 + IRF640 + 0.250ohm shunt) and its lowest setting (full counterclockwise) is 10mA. Do you really need to go down to 0A on yours?

For the sake of circuit simplicity, it may just be better to spring for a more expensive low offset rail-to-rail opamp.

Vovk_Z:
Q2: I would prefer OAR5 series resistors. It is hard to get them 100 mOhm, so I would use 50 mOhm ones (OAR5R050). They have TCR of about 25 ppm - the lowest one. And the price is lowest too at the same time.

Q3: it has to be a reason to put series resistor in series with a cap.  The simplest way is to use only a cap - the opamp circuit becames simplest integrator cirquit. It was usually stable at my experience.

I would lower gate resistors at least two times or more. It is better if they are 10-47 Ohm (again - it is from my experience). With a too high value you'll have one more integrator in a cirquit, and this won't help with stability.

kalj:

--- Quote from: MarkF on August 19, 2020, 05:28:12 pm ---[Q1]  The op-amp I used is:
      https://www.digikey.com/product-detail/en/texas-instruments/TLC272ACP/296-7346-5-ND/374889
      A lot of people just use the LM358.

[Q2]  Sense resistors:
      3 watt (biggest you need for 2.5A per MOSFET)
      https://www.digikey.com/product-detail/en/vishay-dale/LVR03R1000FE70/LVRB-10RCT-ND/1166532
      5 watt
      https://www.digikey.com/product-detail/en/vishay-dale/LVR05R1000FE73/LVRC-10RCT-ND/1166540
      I used 200mΩ 3W sense resistors which yielded 2.5A @ 500mV across each.

[Q3]  I only have a capacitor in my feedback circuit.
      You will probably find that you will need at least a 10nF for stability.
      Later, you can experiment with smaller values to increase the switching speed response (say faster than 5KHz).

Notes:
      - I think you will want to add 1KΩ resistors in series with each op-amp (+) input.
      - You probably don't need two op-amps (U2) in the current monitor.  Unless, you need extra gain.
      - You might also want to monitor the DUT voltage.  A voltage divider, limiter zener and buffer to an ADC.

--- End quote ---

This is just part of the final circuit. I will use a dual-channel 18-bit ADC to get accurate voltage and current readings. The reason for the dual op amps in the current monitor is that they are both inverting (a non-inverting adder is more involved, and possibly less accurate). I will play around with different feedback circuits once I have the first prototype built.


--- Quote from: OM222O on August 19, 2020, 05:51:05 pm ---A major improvement suggestion:
Get a dual channel DAC, use one channel to control 3 fets and one channel to control 1 fet. It allows you to have "coarse" and "fine" adjustment. Set the channel that controls 3 fets to a rough value and use a PID loop to control the "fine adjust" fet.

In order to get fine adjust, you can either divide the voltage coming from the second channel and buffer it with a unity gain amplifier, or use a larger source resistor.

--- End quote ---

That is quite a good suggestion. I'm already going for a 1-channel 16-bit DAC (MAX5217). Hopefully the high resolution will give enough accuracy. Else I will try the dual channel idea. A 2-channel 12-bit DAC for instance is definitely way cheaper too.


--- Quote from: Vovk_Z on August 19, 2020, 07:45:23 pm ---Q2: I would prefer OAR5 series resistors. It is hard to get them 100 mOhm, so I would use 50 mOhm ones (OAR5R050). They have TCR of about 25 ppm - the lowest one. And the price is lowest too at the same time.

Q3: it has to be a reason to put series resistor in series with a cap.  The simplest way is to use only a cap - the opamp circuit becames simplest integrator cirquit. It was usually stable at my experience.

I would lower gate resistors at least two times or more. It is better if they are 10-47 Ohm (again - it is from my experience). With a too high value you'll have one more integrator in a cirquit, and this won't help with stability.

--- End quote ---

I did manage to find a similar low TCR resistor with 100mOhm, which was quite cheap too: https://www.mouser.com/ProductDetail/605-RMCJ3U000R1FS
I'll play around with gate resistors and the feedback circuit once I have something built up.f

kalj:

--- Quote from: pqass on August 19, 2020, 06:08:57 pm ---
A negative opamp power supply  (-1V as I proposed) is needed because for the output to swing to or slightly below 0V, the +Vin of the op amp has to be below ground by its offset voltage; which for a LM358 is ~2mA.  I proposed an inverting opamp with an offset tweeked below +Vref/2 point because it allows the use of only a single +Vref.  I'm not sure where Scully got his negative reference but it should be from a true voltage reference chip/zener; not a power rail. The opamp power supplies (+ and -) can swing a bit (say, +/- 100mV) but as long as the +Vref is solid, your opamp outputs should be solid too.  Using a non-inverting buffer instead would need a -Vref to dial in a -2mV at the non-inverting buffer's R1 to the -Vin pin; an additional complexity.

By tweaking the low adj. pot, you can set, say, any DAC output value above 999 to be the point where the mosfet is completely off.  Below 1000, it turns on by 10A/1000 = 10mA increments.  If you can't quite reach full 10A at 0 DAC output (like in my falstad png) you can always adjust the inverting buffer's feedback resistor to raise or lower the gain a bit.  Once the low adj. pot and feedback resistor is tweeked, your software can be completely oblivious of a configuration value. The offset voltages of both opamps should be fairly* constant with time and temperature.  *Too small to matter (crosses fingers).

However, the current monitoring opamp (summing opamp in the last attachment of my last post) will have this offset anyway that you'll need to account for in software. Oh well!

FYI:I've built my own simple analog CC load (potentiometer + LM358 + IRF640 + 0.250ohm shunt) and its lowest setting (full counterclockwise) is 10mA. Do you really need to go down to 0A on yours?

For the sake of circuit simplicity, it may just be better to spring for a more expensive low offset rail-to-rail opamp.


--- End quote ---

Okay, that actually made a bit of sense. So just to confirm that I understood properly; the precision, stability, and the exact value of the negative rail is not crucial, just that it is well below any value we want to pass through, i.e. -2mV in your case. On the other hand, it is crucial that we can produce accurate and stable values at the input to the op amp, including all the way down to say -10mV. With an accurate DAC, and a proper calibration procedure, the exact value of the negative offset should also not be of importance, only that it is rock stable and reproducible, since the values put out through the DAC are going to be adjusted based on calibration anyways.

Correct?

Regarding applying an offset to a non-inverting buffer, I cooked up this:



Does that not make sense? It seems simpler than the inverting one.

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod