How to Build a Current Limiting Circuit

RichardO

Joined May 4, 2013
2,270
R1. As the current in R1 increases, the voltage at the base of Q1 decreases, turning it more "on" and pulling up the M2 gate, turning it more off.

ak
... and the 60 mV difference in the base to emitter voltages of the BVC62C transistors caused by the 10 to 1 ratio of their collector currents.
 

crutschow

Joined Mar 14, 2008
34,285
... and the 60 mV difference in the base to emitter voltages of the BVC62C transistors caused by the 10 to 1 ratio of their collector currents.
Okay, that makes sense.
But the two transistors should be a matched pair for that to work well in practice (as the BVC62C are).
 

AnalogKid

Joined Aug 1, 2013
10,987
will there be a voltage drop across the LM317 if I use it as current limiter?
1. Yes. See the datasheet.

2. Your PDF will not open.

3. This thread has been silent for over 1.5 years. The best way to reopen it is to start your own thread and reference this one in your first post.

ak
 

Veracohr

Joined Jan 3, 2011
772
I've been simulating this circuit to figure out how it works. The only thing I don't understand is: is it just a coincidence that the voltage across R1 at the point of current limiting is almost exactly 60mV? Or is there a calculation that can arrive at that?

I found that the threshold of the FET and beta of the BJT's make almost no difference to the limiting point. The values of the resistors make some difference.

Edit: never mind. I answered my own question.
 
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Bordodynov

Joined May 20, 2015
3,177
The operation of the circuit is based on the dependence of the base-emitter voltage on current. Transistor currents differ by a factor of 10.
As a result, the difference between these voltages will be
26mV*Ln(10)~60mV
 

topher217

Joined Oct 17, 2019
22
@crutschow I know this thread is years old, but I'm just curious where your simple design came from? Just out of your head, or do you have a recommended cookbook type resource for things like this?
 

JASL3241

Joined Oct 19, 2021
1
What is your load and why can't it control its own current draw? I mean, the capacity of the panel might be 4A but it's not going to provide that much current into a load that does not demand it.
Lots of loads draw more current than they and or their circuits are capable of handling. My guess here since it has to do with solar panels are batteries. They have a near 0 resistance so they can draw an almost unlimited amount of current which obviously will damage them. “Obviously not unlimited but for example an 18650 battery at 4.2 volts with an internal resistance of 0.1 ohm could draw 42 amps of current. Ideally you would want to charge at 0.5 - 1 amps.
 
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