Ron H said:
Who else?
I was just wondering if you can buy them.
Don't mind me, that must have been a brain fart. I haven't looked for a .001 ohm 10 resistor yet - I just assumed one would be available (obviously not). If I can't find one at that power rating, I'll have to parallel two or more larger resistances to get the values I need.
I definitely like that idea about using an op-amp to control the voltage. Thanks Ron!
However, I see another problem. As I said, the batteries being tested are 1.2v NiMH/NiCD or 3.7v LiPo batteries. If testing a 1.2v battery, I'm sure the voltage will drop quite a bit at 100A. Assuming a worst case transistor scenario; if the shunt is dropping .1v, that means the tranistors will have to have a C-E saturation voltage of .7v or less (I figure I can stop the current test if the battery voltage drops below .8v since anything below that is unusable anyway). I can't seem to find a commonly available transistor that has a low sat voltage when passing any appreciable current - and the more collector current, the more the saturation voltage. So, I guess I'll have to use MOSFETs?
If so, do I still have to take the same precautions when paralleling MOSFETs as I do with transistors (using a balancing resistor)? I vaguely remember something in school (15 years ago) that MOSFETs conduct less when they heat up so they in effect self-regulate to keep one from going into thermal runaway. A random check of a datasheet showing Rdson vs temperature seems to confirm this. That said, can I simply parallel all the drains, sources, and gates together and use one shunt resistor?
When choosing a MOSFET, should I simply looking for one that has the lowest rds-on and largest drain voltage as possible? Of course, I will go through the various charts to make that device will work based on my other parameters, but I just need somewhere to start looking. Mouser and Digikey have about a bazillion choices to sift through! (OK maybe not that much, but you know what I mean
)
At first I thought this was going to be a relatively simple project, but I keep coming up with "gotcha's".