Adjustable current limiter (Ver 1.0) This circuit forms a very simple adjustable current limiter or constant current source. It can be used between 0-30A and the current limit can be adjusted by Vset voltage (0..10V). The Vset input can also be used to modulate the output current waveform. Note that no current sensing resistor is used, but the voltage drop across the FET (due to Rds) is monitored. [email protected] 3rd Sept 2005 Out+ D2 LED 14Vdc

D1 Out1N4148 R1 10k

Vset

D R2

R3 330

10k

G

Q2 N-fet BUZ11 S

C1

Q1 BC547

100n

The gate-source voltage (Vgs = Vset) of the Q2 FET sets the drain current (Id) as shown in the BUZ11 output characteristics below. If at a certain Vgs value the Id exceeds the desired value, the Vds voltage across the FET will rapidly increase and turn on Q1 through D1, R1, C1. This grounds the FET gate and turns it off to limit the current to the load. The time constant is set by R1 & C1, Q1 will turn on when the voltage stored in C1 exceeds ~0.7V. The led D2 turns on when the current limiting is active and Q1 conducts. D2 can be omitted if it is not needed. The FET needs to have low Rds and sufficient current capability. It also needs to be mounted on a large heatsink. The Q1 can be any NPN small-signal transistor.