Will this one work? Take a look and please comment. The goal is to be able set a stable, constant current up to ~4A thru the load.
The LM317 is doing only the job of a voltage reference. I'm boxed in by a low supply voltage, so I need a low voltage at the emitter and thus a low control voltage at the base of the 2N3055. Also, a low shunt resistance at the emitter allows a higher current at a given wattage rating.
So I'm using the LM317 to give me just a volt or two, near it's bottom end. The diode is used to drop that an additional ~0.7v, allowing the base to fall below ~0.6, to allow near zero current to flow while still "on" at the FET and under control.
The biggest question I have is whether I can find a nice pot that operates at such a low resistance range to control the LM317 output. I know I don't have one in my parts cabinet. Thanks for looking.
PS: When the MOSFET is on, the ∆V across it is no more than 0.1V. I guess I haven't seen it handle a 4A load yet, so maybe it'll be a bit higher than that. Obviously not a problem in this circuit to get higher control voltage as needed. Maybe the pot should go to 150Ω.
The LM317 is doing only the job of a voltage reference. I'm boxed in by a low supply voltage, so I need a low voltage at the emitter and thus a low control voltage at the base of the 2N3055. Also, a low shunt resistance at the emitter allows a higher current at a given wattage rating.
So I'm using the LM317 to give me just a volt or two, near it's bottom end. The diode is used to drop that an additional ~0.7v, allowing the base to fall below ~0.6, to allow near zero current to flow while still "on" at the FET and under control.
The biggest question I have is whether I can find a nice pot that operates at such a low resistance range to control the LM317 output. I know I don't have one in my parts cabinet. Thanks for looking.
PS: When the MOSFET is on, the ∆V across it is no more than 0.1V. I guess I haven't seen it handle a 4A load yet, so maybe it'll be a bit higher than that. Obviously not a problem in this circuit to get higher control voltage as needed. Maybe the pot should go to 150Ω.
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