# Current limiter help

Discussion in 'The Projects Forum' started by Morgs94, Mar 11, 2014.

1. ### Morgs94 Thread Starter New Member

Mar 11, 2014
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Hi,

I am working on a project where I have to create an AC to DC linear regulator in LtSpice, a modelling program. I have got all that to work but I have now been asked to add a current limiter, so if the load was taken out and the two wires short circuited the rest of the circuit will be safe.

The only available thing I have is a BJT and resistors.

In input voltage is 12rms-16rms at 50Hz-60Hz, and we need a stable 10V output, I'll post the schematic of the circuit below.

Any help will be greatly appreciated

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Last edited: Mar 11, 2014
2. ### #12 Expert

Nov 30, 2010
16,704
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School assignment?

We can't do much without a schematic and some specifications. 10 volts? 1000 volts? 100 milliamps? 30 amps?

3. ### Morgs94 Thread Starter New Member

Mar 11, 2014
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Ok, I updated the info

4. ### #12 Expert

Nov 30, 2010
16,704
7,351
You have to add a resistor between Q1 and R1 and use the voltage across that resistor to drive a transistor which defeats the drive current from the LM358. NPN, collector on the bottom of Rb2.

Got you started?

5. ### Morgs94 Thread Starter New Member

Mar 11, 2014
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Ahh ok, I understand that, what kind of resistor value? High or low? Wouldn't it still spike for a short amount of time through the transistors?

6. ### #12 Expert

Nov 30, 2010
16,704
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What would slow the current limiter down? Are you going to add a capacitor to keep it from responding immediately?

The sensing resistor is just enough to get enough voltage to trigger the transistor. The other resistors are higher. They depend on how much current you need to the base of the limiting transistor to get it to defeat the 1k resistor in series with the LM358.

7. ### #12 Expert

Nov 30, 2010
16,704
7,351
I just thought of another way. Put the current sense resistor in the ground line between the ground of the opamp and the power supply. Base and emitter across the resistor, collector to the bottom of Rb2. Presto! Constant current device.

Morgs94 likes this.
8. ### Morgs94 Thread Starter New Member

Mar 11, 2014
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I'm still getting 20A through the top BJT, is this what you meant?

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9. ### crutschow Expert

Mar 14, 2008
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Interchange the base and emitter of Q2.
Add a resistor in series with the output of U1.
Remove the ground connection from the input (The two ground connections short out R5).

500Ω for R5 will limit the current to about .7V/500 = 1.4mA. I assume you want more than that so reduce the value of R5 accordingly.

10. ### Morgs94 Thread Starter New Member

Mar 11, 2014
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With those changes made, there is no voltage across the load

11. ### ronv AAC Fanatic!

Nov 12, 2008
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It looks like you have 2 grounds on the same line which would bypass the resistor. But having said that your circuit needs to go in between R2 and the zener. Then the output to the top of Rb so it doesn't fight with the 358. If you are going to build it you might want to check the current in the 358 and the 547. The resistor values seem kind of small.

12. ### crutschow Expert

Mar 14, 2008
13,501
3,375
Did your reduce the value of R5?

Post your new circuit.