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| Electronic load control feedback loop - Use ACS712? |
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| T3sl4co1l:
Oh. Well, that won't take long then. Tim |
| SpottedDick:
--- Quote from: T3sl4co1l on June 21, 2022, 11:27:22 pm ---Oh. Well, that won't take long then. Tim --- End quote --- It will be interesting anyway! With the cooling plate in place, I can't access the MOSFETs to read the voltage etc, but I can access the case through the perf board to insert thermocouples, so I can log termperature across all six to get an indication of how uneven it is. Also, that thermal image was used to decide where to place the NTC bead to detect over temperature which kicks in an external over temperature shutdown module, so hopefully it will just shutdown rather than thermal runaway as my max temp is set something like 20 degrees below where I believe Tj is in relation to Tc. |
| Kleinstein:
The control loops are rather simple. So there is no real need to do the extra effort (extra protection, selecting the MOSFETS, source resistors to drop some 500 mV) of trying MOSFETs directly in parallel. The main way to use MOSFETs in linear operation in parallel is to use extra amplifier to ensure current sharing. In priciple the ACS.... sensor could be used, but they are slow, noisy and drift quite a bit. One would get better performance with relatively low value bare wire shunts and a reasonable low noise op-amp. As a useful test instrument one probably want good performance and thus a high power shunt. There is anyway quite some costs for the heat sink and FETs. If const is an issue for special uses (e.g. battery testing) one can use extra series resistors to dissipate most of the power. Power resistors can run at the higher temperature then the FETs and are more reliable (MOSFETs tend to fail short, while wire wound resistors tend to fail open, which is often safer). |
| SpottedDick:
--- Quote from: Kleinstein on June 22, 2022, 08:34:55 am ---The control loops are rather simple. So there is no real need to do the extra effort (extra protection, selecting the MOSFETS, source resistors to drop some 500 mV) of trying MOSFETs directly in parallel. --- End quote --- I'm running them in raw parallel to see how it performs. I'd have no issue buying a few LM324 40c quad op amps and power resistors for a 'proper' build, but that's not the main goal of this project. My goal is an easily reproducible open-source design for anyone with a solder iron, a bit of perf board, and a few off the shelf components can build. Reproducing the feedback circuit on perf board 6 times is just too much. Going forward, I want to even try and remove the DAC from the project! However, it stays for now. The pulse to voltage RC circuit could be dodgy and I don't need that added variable this early in the design stage. --- Quote ---In priciple the ACS.... sensor could be used, but they are slow, noisy and drift quite a bit. --- End quote --- A high power and accurate shunt was the same cost as the ACS and the only additional components needed (as I have a spare input on the LM324) is four off the shelf resistors, so this is the route I'm going. (see post above for the resistor values used) |
| Kleinstein:
Paralleling MOSFETs in linear mode is known to not work well. Even if it does work in a few occasions it is by no way reproducible. It is more like the oppposite: usually not working except in a few exceptional cases with very well matched parts and close thermal coupling and even than usually only fo low voltage and not reliably. To make it reliable and repeatable one would need the extra control loops. This may be even in the version of cheap individual resistor and LM324 for the seprate control loops and than 1 extra look for the sum with better accuracy (e.g. higher grade shunt and low noise OP). The ACS712 does not compete with an accurate shunt - it's more like in league of a low cost shunt, maybe even just a piece of brass of bronze. Most hall sensors have quite some offset drift. They may be OK close to there rated current, but rather awful at 1% of the rated current. An electronic load as a test instrument should work over a large range - this may mean using only 1 channel for the lower currents. |
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