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DC load using a CPU cooler

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Joenuh:
I'm planning on building something similar, also with the 1mA resolution. But thinking about that, wouldn't it be easier to use a 1 ohm shunt resistor instead of 0.1 ohm?
A 1mV error on a 0.1 ohm shunt would already be 10mA if I'm not wrong. A 0.1 ohm shunt would dissipate less power but it still seems like a 1 ohm resistor would make more sense, or is that just me?

rob77:

--- Quote from: Joenuh on September 24, 2014, 03:19:11 pm ---I'm planning on building something similar, also with the 1mA resolution. But thinking about that, wouldn't it be easier to use a 1 ohm shunt resistor instead of 0.1 ohm?
A 1mV error on a 0.1 ohm shunt would already be 10mA if I'm not wrong. A 0.1 ohm shunt would dissipate less power but it still seems like a 1 ohm resistor would make more sense, or is that just me?

--- End quote ---

i built one with a 1 Ohm shunt (50W 1R resistor) and even that is not good enough for 1mA with a jellybean opamp (LM324). so i decided to go for a 10mA resolution for my high power load and in addition going to build a new low power load with high resolution.
btw.. frankly speaking - i don't care if the load is 4.0 Amp or 4.003 Amp - for high currents the 1 mA resolution is useless.

Joenuh:
Well as you mentioned your opamp, a high precision opamp would probably fix that problem. And I agree that for high currents the 1mA resolution is useless. But if it doesn't get to expensive, I would like to build an all-in-one solution that accurately can go from small currents, as 1mA, to higher currents like 5-10 Amps.

Edit: You also mentioned a 50W 1ohm resistor, I hope that is realised by putting multiple resistors in parallel. If not, that also may be part of your accuracy problem. As power resistors are not really accurate/low tolerance. 

microbug:
The op-amp I am using is the OPA4188 which has a 6uV (25uV max) offset. The INA194 which is measuring current has a max 2mV offset voltage and typical +/- 500uV. I'm thinking about removing it though.

EDIT: Welcome to the forum, Joenuh!

rob77:

--- Quote from: Joenuh on September 24, 2014, 04:32:51 pm ---Edit: You also mentioned a 50W 1ohm resistor, I hope that is realised by putting multiple resistors in parallel. If not, that also may be part of your accuracy problem. As power resistors are not really accurate/low tolerance.

--- End quote ---

i compensate for inaccuracy of the resistor in hardware - both set voltage coming from a DAC (well... PWM with low-pass) and the readout for the ADC are tunable (voltage divider followed by a opamp buffer with tunable gain), so the exact resistance of the shunt is not relevant. and yes, it's a single big-ass 50W heat-sink mounted resistor ;)
the smaller brother of the DC load will have a spec of 0-250mA and will be built with a 4R7 (or 10R) shunt and OP07 op-amps.

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