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ET5410 Electronic Load mod: add external sense

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cybermaus:
Small mod, just mentioning it.

The ET5410 already has small internal sense wires from the board directly connected to the banana lugs, so they were easily intercepted.
I did not even cut them, but instead took out and rerouted the small 2-pin plug on the PCB, and connected those via a slider switch, see photo.
BTW: I forgot to take pictures of the inside, and I do not want to open it yet again, but its simple enough:

The sense wires each go via 2-pin plug a pair of 100K precision resistors (so 200K each) to a TL072 OpAmp with a 30K /4K resistor over it to make it into a voltage scaler;
30K or 4K switched by DPST relay, which is controlled by the range setting of the device, high 150V range presumably choses the 4K.
Alas no reverse polarity protection on the now separate sense wires, I am now wondering if I should have shorted the middle of the 4 100K resistors with a diode. After all, the TL072 can only stand -0.3V


Originally, I had some worry on how the device would respond if there was accidentally no voltage on the sense wires. I worried about software hanging with "divide by zero errors", but it is behaving perfectly:
- CC mode: it does not care, just keep drawing the current, but V and W display go to 0, and back up when reconnect
- CP, CR and CV mode : the device just stops drawing current, but does not hang, Reconnect, and it starts up again.
Even repeatedly switching from internal to external sense seems to not bother the device.

Oh, and while you are in there, put a  felt pad on he support screws of the main heatsink. They protrude a little, and if someone dents the sides, they may make contact to the V+ input.


Comparing to video's of the Kunkin I have seen, I like very much that I can change the CC setting while the load is on.
Also I did not observe any startup instabilities, and the load seems to work properly (on my supplies) down to 0.2V

Hydron:
Good to see someone having done this successfully - I'm looking at buying the 500V version of this load (or alternatively the similar looking DCL6104A) and doing the same.

Have you got any resistance between the main and sense terminals? A couple of hundred ohms or so would ensure that even if the switch was in the wrong position the supply will still see the voltage at the terminals with minimal error (200/100k = 0.2%) but should still be large compared to wiring resistance if the sense function is used (if the wire is 100 milliohms 0.1/200 = 0.05% error, not sure if that's a completely correct equation but it will be small regardless). Could put a parallel diode to clamp any voltage on said resistor too (in case the main connection goes open) though a polyfuse in series would be advisable as well to protect the diode if high currents are to be used.

Finally, do you have any pictures of the inside of this unit, especially the sense section? Would be interested in seeing them. And have you checked any of the more advanced features (e.g. the Dynamic load) - does it meet or at least let you set it according the datasheet specs (e.g. 1.5A/us slew rate - I recall someone complaining about this being unavailable on this or a similar load)?

cybermaus:
On the resistors, I considered that at first, expected to do so.

However, I  started rethinking this when noticed these were high-precision resistors, and the OpAmp was setup as a voltage divider.
So any additional resistor, or any resistor shunting between main and sense, would bring this voltage divider off balance, and affect precision.

I did some serious thinking on how to make a resistor network or even considered adding a 2nd opamp as voltage buffer.
But between the wanted precision, the fact I could not re-calibrate, and the fact it needs to work over the entire 0-150V, I could not work it out

So I decided to go for the low-tech solution. A DPDT slider switch. And as stated, it works well.
Also, the ET5410 matches my "best" multimeter precisely. Both before and after I added the switch.


On the slew rate specs, I did not test.
But I did at one point use it as a CR to test how some IBGT were working and managed to keep up on the emulated resistor quite well on a 20KHz PMW signal.


Edit: Oh, and as to a very low resistance like you state (100 ohm. Which would make the OpAmp 30K/230K turn into 30K/230K1 so only 0.04%
Yes, but. if I would cross over my lines, I'd have the full voltage over 100 Ohm, theoretically 150V and thus 225W, practically for me 24V so 6W
The resistor would evaporate. So rejected low ohm. And high ohm would as mentioned, would affect measurement.

Hydron:

--- Quote from: cybermaus on September 25, 2020, 12:04:11 pm ---Edit: Oh, and as to a very low resistance like you state (100 ohm. Which would make the OpAmp 30K/230K turn into 30K/230K1 so only 0.04%
Yes, but. if I would cross over my lines, I'd have the full voltage over 100 Ohm, theoretically 150V and thus 225W, practically for me 24V so 6W
The resistor would evaporate. So rejected low ohm. And high ohm would as mentioned, would affect measurement.

--- End quote ---
Yes this is indeed an issue, which is why I suggested a diode clamp and a polyfuse - unfortunately this is extra complication so fair enough if you'd prefer just to rely on remembering to have the switch correct.

The main issue here is the fact that the voltage and current range is so high - a power supply with sense may only supply say 5A or 30V - this can be covered either with a low value resistor (bypassed by a diode) or a higher value resistor with no diode (accepting a moderate accuracy loss if the sense switch is on but no sense leads connected). 150V/30A is trickier though, a resistor big enough in value to not smoke would cause major inaccuracy if the switch was in the wrong position, while a clamp diode would need to also deal with a big power dissipation if a lot of current were drawn.

I might have a quick look at how the itech supplies do it as I've got one handy at work (hopefully it's obvious).

Thanks for the IGBT testing info - if it handles 20kHz OK that's better than expected! Did you take any pics btw when doing the mod?

cybermaus:
Well, yeah. Though the rate cannot be adjusted (I did search for that) but was better than I expected.
20KHz worked, though I did see a little slow responce in the square wave. If I remember, at 10KHz the resulting wave was pretty much square, on about 3 Amps.
But this is in no way precise measureing


Anyway, I would just go with the slider switch.
Only, as stated, next time I open it, I will add a reverse protection diode, in the middle of the 4 100K resistors.
So it is 2x 100K away from input, no need for a fuse. And also still 2x 100K away from OpAmp, so reverse leakage current has less impact.


Edit: Simulation shows I should just add a very generic diode like 1n4148  as protection  completely on the inside, directly between + and - of OpAmp. While it is closer to the OpAmp, the voltage over there is so low, leakage is in the pA range (in simulation) so that it would not affect the measurement. Where is in the middle of the 2 100k resistors, it gets actual double digit voltages, and leakage does affect outcome (only a little, but more).

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