-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 > > Hello, > > > > This seems reasonable, but I have a few questions. First, I > > assume a small resistor is on the order of milli-ohms. Secondly, > > given such a small resistor value what power rating does it > > require? Theoretically, when power is first provided to the motor > > (i.e. the H-bridge is turned on) the motor resistance is 0. > > Thus, there is a very large current across this resistor. Now, > > the time must be very small (or there would be a short to ground > > without the current sense resistor). If I hope to run my > > controller at 24V and current limit at 20A, do I really need a > > sense resistor capable of 480W!? > There are other ways of handling this kind > of current, however. I don't know much about it, except that it's > possible. Something that uses the current amplification property > of a transistor, or something that uses the transconductance of a > MOSFET sound like what you want. Ah, yes, I remember now. The above is too unpredictable and temperature variable. Here's an easy way of doing it. Make a small (2-5) turn coil using heavy gauge wire (I think that should be a strong enough field), and place a hall effect sensor in the middle of it. Then compare the ouput of the sensor to some reference voltage that you can set with a potentiometer. That should work effectively, and reliably. Other than that, get a reference that tells you the resistance of PCB traces depending on various values, and make a PCB trace the right length for the resistance you want (Eg. the 5 milliohms that Sphero mentioned - --Brendan -----BEGIN PGP SIGNATURE----- Version: PGPfreeware 6.5.8 for non-commercial use iQA/AwUBPVGcLwVk8xtQuK+BEQL/+QCaAvRkPH/ZEdnv9xYF8WyI+8UVyX0AoNV0 gTEnk0efZzOM0oi28kc4Kp1Z =HhFg -----END PGP SIGNATURE----- -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.