On Thu, Apr 14, 2011 at 7:20 PM, V G wrote: > On Thu, Apr 14, 2011 at 4:32 PM, Olin Lathrop = wrote: > >> A bipolar would work "better" as a controlled current sink since that's >> what >> it does naturally. However, if absolute current accuracy is important, >> then >> the FET is better. With a bipolar, the current sensed by a low side sen= se >> resistor will be off from the battery current by the base current. This >> error goes down with gain, so with a high enough gain it doesn't matter. >> If >> you have the voltage headroom, you can use a darlington to effectively >> increase gain to the point where the error is immaterial for ordinary >> battery testing. > > > That's a good idea. Thanks. > > I'm looking to make a current source which will typically test batteries = at > around a 500mA - 2A drain. What gain values should I be looking at when > selecting a BJT? > > Related: If I do go the BJT route (seems like I will), do I need to put a resistive load on the transistor? Or can I just short it to ground and let the transistor dissipate all of the power? The BJT I selected is rated at 100V, 6A, and I'm at maximum going to load i= t at 4.2V, 4A (via a single Li-ion cell). I'm going to be using a heatsink for sure. --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .