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# Current limiting circuit in AC-DC adapter?

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#### fireball003

##### Full Member level 3
Hi,
I made an AC to DC adapter using a (12v-0-12v)transformer, a 2200uF capacitor, a bridge diode and voltage regulators etc. But current out of the regulators are 1000 mA. I need 400 mA current.

I tried by using a 56 ohm resistor in parallel. But that becomes too hot and doesn't work.

What's wrong with the parallel resistor? How can I limit the current?

Thanks in advance

Below is a circuit that can be used to limit the output current to almost any required level ..
This circuit can be placed in series with the voltage regulator at both sides, but better option is before ..
The output current can be calculated as follows:
Ilimit=0.65V/Rsense
So, for 400mA Rsense ≈ 1.6Ω
The other resistor value is not critical and any 0.5W resistor between 100Ω - 1kΩ can be used ..
As most of the current flows through the top transistor, it should be rated for at least double the limited current ..

Regards,
IanP

### fireball003

Points: 2
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Thanks very much for your solution!

Would you please tell me what those transistors are? I mean what transistors would I use there? Would you please send me the full circuit (if available)?

Thanks.

Transistor types for the circuit from the previous post can be as follows:
Power transistor: TIP31, BD131, BD241 .. any transistor with Ic>2A and Vce>30V eill do ..
Currnet sense transistor: any general purpose NPN - 2N2222, N100, BC107, BC ..

Other option is to use voltage regulator (as current limitter) + voltage regulator (as voltage regulator); see picture below ..
Basically, idea is the same as with transistors, but realization is based on typical voltage regulators ..
In the example from the picture the output current is limited to 150mA ..
This current can be changed by varying the 8.2Ω resistor: Ilimit ≈ 1.25V/R ..

Regards,
IanP

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