Transistor current limiter

I am playing around with this circuit in a simulator:



With 15V supply it works pretty much the same in shutting down at about 1A and there is a very sharp drop off in the voltage if the load current exceeds 1A. E.

But if the supply is 26V then I can't get the same sharp drop of when the current exceeds 1A:

When R12 (pot) is at 2% then the voltage drops from 26V to 19V, before cutting off completely if I move the slider to 0%.



Here is the 15V version of this circuit:


If R6 (pot) in this circuit is moved to 2% then the voltage is still close to 15V before it cuts off completely if I move the slider to 0%.

How do I get the 26V version of the above circuit to behave similarly?

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26V version of current limiter transitioning from >2% of 1k load to 2% of 1K load:

The transition is clearly visible as a drop in voltage.



15V version of current limiter transitioning from >2% of 1k load to 2% of 1K load:

The transition is not visible in the voltage output.

the maximum output current is about Vin/ R1 or R7 times the Hfe of the BD 140. So play about with these resistors.
Frank

The circuit doesn't provide reliable current limiting. The maximum output current depends on input voltage and transistor current gain.

FvM said:

The circuit doesn't provide reliable current limiting. The maximum output current depends on input voltage and transistor current gain.

Click to expand...

What do you mean that it doesn't provide reliable current limiting?

Do you mean the current gain of the transistor varies with the voltage and therefore the current limit will also vary with the voltage?

If so then I am trying to current limit a fixed voltage source not a variable one.

I made the circuit on a bread board and it seems to run a 12V PC fan quite well. And it shuts down nicely when I short it out.

But I am yet to do some current measurements to confirm the max current before it starts to cut out.

boylesg said:

What do you mean that it doesn't provide reliable current limiting?

Do you mean the current gain of the transistor varies with the voltage and therefore the current limit will also vary with the voltage?

Click to expand...

No, it means:

• The current limit depends on the current gain of the BD140 transistor.
• The current limit also depends on the supply voltage.

Since the current gain of transistors depend on temperature, the current limit will increase when the BD140 transistor gets hot, which is probably a bad idea too.

to make a half decent current limiter, stick a resistor R in series with the series control transistor's emitter. use a PNP transistor, emitter to incoming voltage, base to junction of R and power transistor. Now when the volt drop across R exceeds .7V, the transistor will conduct, so connect its transistor to the base of the power transistor and it will pull its base current down. Select R so that Imax = .7/R
Frank

Can some one tell me if this is correct about the below circuit:

I have pieced together what I think is happening while simulating the circuit with some additional ammeters in place.
Above the current limit the current through R2 and R3 increases such that voltage dropped across R2 increases, by virtue V=IR, while the voltage dropped across R3 decreases. The latter starts to shut off Q2.

There are no R2, R3, Q2 ...

FvM said:

There are no R2, R3, Q2 ...

Click to expand...

How about........

I have pieced together what I think is happening while simulating the circuit with some additional ammeters in place.
Above the current limit the current through R8 and R9 increases such that voltage dropped across R2 increases, by virtue V=IR, while the voltage dropped across R3 decreases. The latter starts to shut off Q4.

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Another question about this circuit.

How do you go about piecing together a formula that gives the rough relationship between R7, the input voltage and the desired current limit?

My only inconvenient option at present is to figure it out by trial and error on the bread board but it would be nice to get at least close to the correct values before resorting to trial and error for fine tuning.

boylesg said:

How do you go about piecing together a formula that gives the rough relationship between R7, the input voltage and the desired current limit?

Click to expand...

Current limit = hFE(BD140) * current through R7

Current through R7 = (supply voltage - Vbe(BD140) - Vfwd(LED) - Vsat(BC547)) / R7

So, roughly:
Current limit = hFE(BD140) * (supply voltage - 2.5) / R7