Electronics > Projects, Designs, and Technical Stuff
Building an Electronic Load inspired by Louis Scully
Vovk_Z:
--- Quote from: kalj ---
--- Quote from: Vovk_Z on August 18, 2020, 08:38:35 pm ---Q2 is typical elecgronic load problem and typical decision.
--- End quote ---
Just to confirm, are you saying that using, say ±9V rails would solve the problem?
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- no, there is no need in +- rails. They only give a possibility to use any opamp instead of only several ones with 'input gees to ground' capability. The problem is an opamp offset which every opamp has, which leads to a DC current offset at electronic load input.
To get rid of it we add some small voltage to inverting input to 'close' opamp (so regulating Mosfet was closed with no input and control voltage).
kalj:
--- Quote from: MarkF on August 19, 2020, 01:44:35 am ---Yes. You will NOT want to drive the op-amps directly from the microcontroller.
You will want a voltage divider to lower the DAC output to the desired range for the op-amps.
You will end up with finer current control.
With an op-amp buffer, you are assured the resistor divider is not disturbed by the op-amps.
I'm guessing 0 to 500mV for the op-amps which would provide a maximum of 5A from each MOSFET with the 0R1Ω.
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Yeah, something like that is what I'm envisioning too.
--- Quote from: MarkF on August 19, 2020, 01:44:35 am ---What is the max power and current you wish to obtain?
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Power is still unclear. It all depends on how good a heatsink I can get my hands on. In the first prototyping stages I am fine with <50W. Voltage and current-wise, I am aiming for 0-30V and 0-10A.
--- Quote from: MarkF on August 19, 2020, 01:44:35 am ---
If you're going to use +/- power rails, Jay_Diddy_B's Load project would be a better design than Louis Scully's
I watched the Scully Series a while back and thought it was a mess.
Read Jay's analysis and see the circuit:
https://www.eevblog.com/forum/projects/dynamic-electronic-load-project/
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Thanks for the tip! I am definitely not limited to the design of Scully. I am now looking quite a lot at that HP/Agilent/Keysight 6060B schematic. Quite a treasure trove that one!
--- 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.
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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?
kalj:
--- Quote from: Vovk_Z on August 19, 2020, 09:01:13 am ---- no, there is no need in +- rails. They only give a possibility to use any opamp instead of only several ones with 'input gees to ground' capability. The problem is an opamp offset which every opamp has, which leads to a DC current offset at electronic load input.
To get rid of it we add some small voltage to inverting input to 'close' opamp (so regulating Mosfet was closed with no input and control voltage).
--- End quote ---
Okay, but then the software needs to be aware of what offset to apply (or subtract) in order for it to set the true value accurately. Is it simply naive to hope to get these things to align by just minimizing errors in the circuit? Perhaps some kind of calibration where a set of software offsets/coefficients are tuned is always necessary...
kalj:
Eventually, the plan is to control all the load parameters from a MCU using a DAC and an ADC. However, in order to avoid too many complications at once, the first prototype will only contain the MOSFETs and their feedback circuitry.
Here is what I have currently:
Here, "Control" is a voltage which sets the current of each MOSFET. With the present sense resistors, we get that Control voltage = [0, 250mV] --> MOSFET current = [0, 2.5A] --> Total current = [0,10A].
"Sense" is an output voltage measuring the total current through all the MOSFETs, i.e. Total current = [0, 10A] --> Sense voltage = [0, 1V].
Questions:
[Q1] What are suitable op amps for the feedback and the current sense, i.e. U1 and U2, respectively? Looking at the HP6060 schematic, we see that the current sense uses the expensive precision op amp OP270, whereas the feedback uses the cheaper MC34072. Is this general, that the feedback loop is not overly sensitive to the quality of the op amp? On the other hand, Scully uses the AD8630 precision op amp for this...
[Q2] My current shunt resistors will need to graded for at least 0.625W = 0.1 \$\Omega\$ * (2.5 A)^2 . Are there any other critical parameters for them? Can I simply use something like this: https://www.mouser.se/ProductDetail/IRC-TT-Electronics/LOB3R100FLF?qs=sGAEpiMZZMtlubZbdhIBII4g82wonVNopo8C%252Bg3hgbY%3D
[Q3] My feedback circuit is copied from the one from Scully, with a 1k\$\Omega\$ resistor and a 1nF capacitor in series from the op amp output to the negative input. I have also seen other variants, e.g. a capacitor and resistor in parallel, and just a single capacitor. What is the motivation for these different variants? Is there a single optimal one, does it depend on the op amp/other circuit elements, or is it something where almost anything will do?
MarkF:
[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.
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