Low Power Voltage Regulator with Current Limit

154

02:15:08

U_out = U_z - U_be
R_Z = U_in / I_Z
R_2 = U_BE2 / I_max

published 4 years ago

Robert_Kidd

4 years ago

The lower transistor is wrongly fitted. Swap collector and emitter.

Robert_Kidd

4 years ago

Reduce the 10k to 620 Ohms so you have sufficient zener current. In EC the zener knee current is at 13.6mA.

Robert_Kidd

4 years ago

Adding a 10000uF capacitor across the zener reduces mains ripple ( with your AC component reduced to 50 Hz to more closely represent mains fluctuations ).

Phrosume1

4 years ago

1. Done, works in both directions!? 2. Done, didn't knew that... makes the circuit less efficient :( 3. Changed to 100Hz due to 2 Pulse Ripple. 10mF is pretty much, but affordable

Robert_Kidd

4 years ago

1. No, doesn’t work properly in both directions. The volt drop across the low value resistor turns the lower transistor on when it reaches around 0.7V and in doing so turns top transistor off, thereby reducing output voltage and hence limiting the current supplied to the load.

Robert_Kidd

4 years ago

2. 13.6mA is just the EC set value. Fir real circuit look at data sheet for current required for knee since you don’t want to waste power or stress the zener any more than necessary.

Robert_Kidd

4 years ago

Capacitor value is biggish. It can come down in value or even omitted. If you put a switch in series with it you can compare ripple with/without it and different c values. Depends how much ripple you can live with.

Robert_Kidd

4 years ago

Finally, your 3V ripple is great for testing the circuit but in real life you hopefully can supply something rather smoother :-)

Robert_Kidd

4 years ago

Back to transistor - the base must be positive wrt the emitter for it to turn on. Say the transistor forward gain is 100, then since a transistor is somewhat symmetrical, if you connect in reverse it may work but with a forward gain of about 1. In most circuits that’s not going to work.

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