Electronics > Projects, Designs, and Technical Stuff
DC load using a CPU cooler
microbug:
@metacollin, OK. I'd probably have to buy a better heat gun if I connect the heat spreader to the cooler with solder, since the Atten 858D 'only' does 700W.
Kevin.D:
--- Quote from: Joenuh on September 25, 2014, 07:38:25 am ---
That's a problem that I'm having. I'm thinking of having multiple gain options on the opamp so that a switch can switch between different feedback resistors of the opamp. That switch could be something like a ADG884.
--- End quote ---
Theres a couple of ways you can have a multiple range load without any switching .
I did one a year or so ago it's got 4 decade ranges 12A ,1.2 A ,120mA ,and 12mA
on the lowest range you get 10uA adjustment precision . I designed it to be used as a current sink/source as well as use as a eload hence the four lower decade ranges (and it has true reverse polarity protection which is a must to protect the DUT) .It's also fully protable so I can take it to the garage outside where I use it on large lead acid batt capacity tests.
Search forum for 'eload' and you should find it
microbug:
I had some trouble finding it, so here's the link:
https://www.eevblog.com/forum/projects/fully-portable-150w12a-e-loadcurrent-reg-with-multiple-ranges/
EDIT: I think multiple ranges are a good idea. The end project will be digitally controlled, so I'll use analog switches (such as AD884) to control the range. I have dropped the INA194; the input offset voltage was too high. The 6uV of the OPA4188 should be much better :).
EDIT 2: I have been looking into the DAC some more. The DAC8411 uses the supply as the reference, and as it only uses 160uA (max), I'll power it from a 4.096V reference.
EDIT 3: This is better because I can make the current ranges 409.6mA, 4.096A and 40.96A (that one will be software and fuse-limited to 20A) and have adjustment with a resolution of exactly 6.25uA / 62.5uA / 0.625mA (instead of a long decimal number). Hardware constant-voltage ranges will probably be double the values of the current ranges (819.2mV, 8.192V, 81.92V, maybe 819.2V (software limited to 100V, of course!)).
EDIT 4: I'm going to use 100mOhm shunt resistors. With 21mOhm MOSFETs, that's a 61.5mOhms impedance altogether (MOSFETs and shunts are in parallel). If I assume the actual impedance will be 70mOhms (accounting for connections, traces etc), then the minimum power dissipation (I^2*R) is around 30W at max current. Graph attached - x is current, y is minimum power dissipation.
microbug:
Bump (I often edit my messages rather than replying, so see the last message I posted).
What should I do to generate a small (250-300uV) but stable negative offset for the current/voltage set line (do I have to use a negative voltage reference or can it be something cheaper)?
I'll make the constant current ranges 400mA, 4A and 40(20)A so I have some headroom to trim the DAC in software.
The minimum constant current will still be 1mA (or maybe 500uA), even on the 400mA range, because of the offset.
rob77:
i strongly suggest a negative rail for your op-amps in the design and strongly suggest to use op-amps with offset compensation pins (all old-school precision op-amps have those available - pins 1+8 if it's a 8 pin package) and trim down the offset to zero.
otherwise forget the accuracy you're trying to achieve ;)
btw... i'm always nulling the offset before i solder the op-amp in - setup a non-invert amp with gain of at least 5000 (non invert input grounded) and tweak the offset to zero. then solder the op-amp into the final board and either solder the 20turn trimpot with it or replace the trimpot with resistors for the final board.
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