Right, but if you measure the emitter current and try to control it to, say, 1 Amp, and the base needs 10mA drive current. Then, you will only be pulling 990mA from the battery so you will be off by 1%. That might be acceptable (or maybe not depending on the application). What's worse is that the current gain of many BJTs at currents of several amps may be very low, perhaps even as low as 10, so you could be 10% off in your measurement in that case. One thing going in your favor is that you are not operating the BJT as a "saturated switch" (i.e., full on) so you will not be in BJT saturation and the current gain will stay closer to the nominal value for that BJT. Also, you could use a Darlington which should easily get you up in the gain of >100 range even at high currents. Sean On Fri, Apr 15, 2011 at 10:35 AM, V G wrote: > On Fri, Apr 15, 2011 at 10:18 AM, Sean Breheny wrote: > >> Olin is assuming that you connect the battery to the collector of the >> BJT, control voltage to the base, and the emitter to ground through a >> sense resistor. The error comes from the fact that whatever current >> the base terminal draws will ALSO flow through the emitter. In other >> words, Ie=3DIc+Ib. >> >> The only way you could really avoid this (and still use a BJT) is to >> either use a BJT with a sufficiently-high current gain that the base >> current is negligible OR put the sense resistor in the collector lead. >> However, the latter option will require a differential amplifier to >> measure the sense resistor since it is no-longer ground referenced. >> >> Sean >> >> > Oh! > > The battery will only be providing the collector current. The rest of the > testing device will be powered by other means. > > My device will be something more like this: > http://postimage.org/image/1n89aa144/full/ > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .