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DC load using a CPU cooler

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Joenuh:

--- Quote from: rob77 on September 24, 2014, 05:10:43 pm ---
--- 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.

--- End quote ---

Yes ofcourse that would fix that problem :)

Maybe it's my own stupidity but why are people using ADCs in these projects? I know, for measurements.. But if you can accurately set the current you don't need to measure it right? Or are you using it to measure the input voltage in some way? To be able to limit in power?

microbug:
ADCs are useful for software constant resistance / power / voltage modes. For all of these modes, you'll want to easily see the current and voltage flow.

EDIT: Yes, over power limits come into it as well - software controlling a relay, for example.

timb:

--- Quote from: microbug on September 24, 2014, 06:18:24 am ---Won't the transient response be much worse? I through that was what C2000s (DSPs) were for.

The PSoC 4200 has a 12 bit ADC and a 12 bit IDAC (current DAC), which is not very useful. I chose 16 bit resolution with external parts because they aren't inhibitingly expensive so I might as well; it will let me get closer to my 1mA resolution ideal.

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Nah, your transient response will be just fine. Personally, I’d use the built in ADC and comparator to control an external DAC. Hook your INA up to a single to differential amplifier and use the built-in 12bit differential ADC.

That PSoC is 48MHz with single cycle multiply, so it should be more than fast enough for your needs. You should be able to get mA resolution with a 12-bit ADC and software calibration.

microbug:
20A/2^12 = 4.88mA

Like rob77, at higher loads I don't care much about milliamp resolution, so are you suggesting a PGA on the DAC (for multiple ranges)? Unless there's a reason not to, I want to use a 16 bit ADC/DAC as I already have the parts, and if I do there's no need for a PGA or anything. I was thinking of doing away with the INA194s altogether and running the sense voltages from the shunts through a couple of resistors, then scaling them for the ADC with a resistor divider.

Schematic / example to come.

EDIT: The other reason that I'd prefer not to do it in software is that the PSoC will be driving a graphic LCD and sensing a rotary encoder / keypad. This will definitely worsen the transient response!

Joenuh:

--- Quote from: microbug on September 24, 2014, 09:21:38 pm ---20A/2^12 = 4.88mA

--- End quote ---

Is that the right way of calculating it? Maybe it depends on how your output is. But the way I see it is that which voltage you set on the output of your DAC is the current that will flow, if you use a 1R resistor ofcourse. So wouldn't that be the Vref of the DAC divided by 2^12? So a Vref of 4.096V with a 12 DAC should give you 1mA steps. The problem is now that you can ''only'' go to 4.096A.

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.

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