Hey folks, I'm working on a project to control some automotive motors with relays, but the one's I've been able to look at do not have markings telling how many amps to expect running through them to drive.
I've done a little looking around at auto parts suppliers, but haven't found the exact motors (Denso Aisin 85820-33020), but it does seem that many/most of these, such as in power windows, seats etc, seem to have a stall current as much as 25A. The operating currents are more in the range of 2-3A. Actually both of those sound like high numbers, maybe I'm just looking at the wrong motors?
The best thing would be to know exactly what my motors will require - I have a meter, but it maxes out at 250mA, I thought about using a shunt, but those seem to cost quite a bit.
On the other hand, I could just get some beefy relays - any problems you might foresee if I just get relays that can handle up to say 30A?
braddo_99:
I have a meter, but it maxes out at 250mA, I thought about using a shunt, but those seem to cost quite a bit.
Actually - unless your meter is a strange one - there should be a setting for up to 10A current measurement (typically one of the probes is put into a different jack on the meter, intended only for "high" current measurement). Still, this probably wouldn't be enough anyhow.
Regarding shunts: You can easily make your own shunt - just do a google search for "homemade current shunt"; many people make them in the "homebrew alternative power" community (read: homemade wind generator). Basically, the idea is to use thick solid copper wire (like is used for household wireing; bare copper grounding wire will work) of a known length and gauge; from there, you can find a table online that will tell you how many ohms per foot the wire will have, then you just cut off enough for say 0.1 ohms, bend it into a convenient shape to mount it on a board using mounts of your choice. Such shunts are useful only for quick measurements, of course, and they aren't temperature stable (unlike a real current shunt), but they are very cheap.
Thanks all! I think I will do all of the above. BTW, my meter is a toy. It's a tiny little thing I found at the shack and made an impulse buy. I have a better one in the basement but I haven't got the nerve to dig through the boxes yet. Will do though. Appreciate the help!