Current limiting circuit

[boylesg]

If I change R1 to 330R and R2 to 0.25R then I can get the current to limit at just under 3A in multisim 11.

Is this an efficient circuit and all that? Or would I be better off with something else?

I chose this one because it is one of the few that I have found with a good explanation of the components and workings.

Others with multiple transistors don't really give much explanation of the schematics and whether one can use different transistors, that you have available, to the ones specified.

I would be using this on the output of a high current linear voltage regulator arrangement (as per the datasheet for L7812C or others) in place of fuses and thermistors to protect the pass transistor and voltage regulator from over current. I would need to add a LED some where to indicate if and when the current limiter is active or not.

By the way, how does increased current through the scense resistor reduce the base current of the transistor and cause it to start turning off?

**broken link removed**

 

[barry]

There's no need to protect an 7812 regulator from overcurrent. It has built-in protection.

But, for a more general answer, I don't think there's any need to change R1, that just provides current to turn on D1 and D2. R2 is what controls the current limit (per the equation in the drawing).

The way this circuit works: Normally, the base-emitter voltage drop is about .7 volts when the transistor is on. The base is biased at 1.4 volts by the 2 diodes. As more current flows, the voltage at the emitter (the current times R2) increases. When the voltage at the emitter is greater than .7 volts (base-emitter voltage = 1.4-emitter voltage), the transistor starts to turn off, limiting current flow.

 

[boylesg]

There's no need to protect an 7812 regulator from overcurrent. It has built-in protection.

What else would cause a voltage regulator to suddenly cease working then? I seem to getting 0V out of the one I currently have on my circuit board. What does thermal shut down in these devices mean exactly? In the scense of a fuse or in the scense of a thermistor?

OK I have found the answer to this question myself - it is more like a thermistor. The only thing I can think of that might be causing this is that the fan (cools my mosfet on its heat sink) I have attached to my Vreg line has blown for some reason and preventing the voltage regulator from working. I did notice that when my circuit was driving the flyback transformer the speed of the fan was dropping, which might suggest that the limits of the voltage regulator were been being approached.

If I add a pass transistor to increase current capabilities of my voltage regulator then I will need this current limiter to protect the transistor. Either that or a fuse.

But, for a more general answer, I don't think there's any need to change R1, that just provides current to turn on D1 and D2. R2 is what controls the current limit (per the equation in the drawing).

In multisim, the greatest current I could get out of this current limiter, with a short circuit, was about 800mA with the 1k resistor. Only by reducing this resistor could I get just under 3A - the limit of the transistor I was intending to use.

The way this circuit works: Normally, the base-emitter voltage drop is about .7 volts when the transistor is on. The base is biased at 1.4 volts by the 2 diodes. As more current flows, the voltage at the emitter (the current times R2) increases. When the voltage at the emitter is greater than .7 volts (base-emitter voltage = 1.4-emitter voltage), the transistor starts to turn off, limiting current flow.

OK, remembering that current always flows from high potential to low potential and V=IR, when the collector emitter voltage drop approaches 0.7V, which equals the base emiiter voltage drop, then the current flow across base emitter starts shutting down thus shutting down the emitter collector current. Do I have it right?

 

[boylesg]

Actually I think I have probably again answered my own question about why a voltage regulator might fail.

I found some where that they can fail if the seals around the leads or not air tight or they have over heated over a long period in a poorly ventilated case,.......

And given that the voltage regulator in question is salvaged from a tv........