The following simple single supply op-amp with transistor power booster won=92t go down to zero. With the input to the opamp at zero the output from the opamp is at 20mV and the emitter of the LM395 is at 0.6 volts.
(V+)---------------+ | | |\ C Vin----+ \ |/ (LM395) | >--5k--| +-- / |\ | |/ E +---10k-------+-------+ | | R=3D2.5k R load =3D 50 ohm heater. | | (V-)-+---------------------+
The LM395 is not a =91real=92 transistor. It=92s got overload and thermal shutdown circuitry, which I like. It=92s possible for the user to short out the load and with the LM395 I don=92t worry so much about something failing. When I replaced the LM395 with a real NPN. (TIP31C) the circuit was fine. (Base resistor reduced to 500 ohms, V out of opamp
0.43 volts, V emitter =3D 0.2mV.) The LM395 is really more like a Darlington which might be the root of the problem. Anyone know of a simple way to get this to turn off with the LM395? Oh the opamp is a LT1014.Thanks George H.