LM317 controls MJ2955 constant current source.


LM317 High Power Constant Current Source Circuit

by Lewis Loflin


A constant current source (CCS) in electronics is a device/circuit that produces a constant value of current regardless of source voltage or load resistance. A constant current circuit can also be used as a current limiter.

In this circuit we use a LM317 to control a MJ2955 15-amp PNP transistor. The above circuit is for testing only!

This is designed to limit circuit damage for developing other devices such as an H-Bridge. With a 5 or 10-ohm load adjust for desired maximum current.

Here I've used a LM317 as a constant current source to control the base current (Ib) through a MJ2955 15 amp 115 watt transistor. This supplies a far higher currant than the LM317 by itself.

In the diagram above taken from a live circuit test in the first video Q1's base current (Ib) is set by the 2K potentiometer and 10 ohm combination to produce an Ib of 8.62mA and Q1 with a DC gain (hfe) of 116 produces an output of 1 amp through a 10 ohm load.

See LM317 Constant Current Source Circuits

LM317 controls MJ2955 constant current source final.

Using a pass transistor does create a hfe drift problem as the transistor heats up - in my test higher hfe thus more current to the load. The LM317 current doesn't drop to zero (2-4mA) and even at the lowest setting Q1 is still turned on producing about 300mA so adding a 100 ohm resistor across Q1 B-E solves the problem.

Voltage regulation after the circuit is recommended and for the best efficiency use an input voltage near the desired voltage. 15-volts is good for a 12-volt regulator circuit if used.

Click here for full size schematic.

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