# How to Build a Current Limiting Circuit

#### RichardO

Joined May 4, 2013
2,271
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
25,247
... 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).

#### ZimUnknown

Joined Oct 29, 2019
1
I might be late, but I need to ask, will there be a voltage drop across the LM317 if I use it as current limiter? I'm attempting to repurpose an old power supply from an old monitor to be a usb charger. Here's the schematics of it in PDF

#### Attachments

• 182.9 KB Views: 8

#### AnalogKid

Joined Aug 1, 2013
8,534
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

#### crutschow

Joined Mar 14, 2008
25,247
will there be a voltage drop across the LM317 if I use it as current limiter?

How much drop can you tolerate?

#### Veracohr

Joined Jan 3, 2011
720
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.

Last edited:

#### Bordodynov

Joined May 20, 2015
2,642
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

#### hornydiodes

Joined Sep 5, 2020
2
How does the legendary lm317 actually work ?

#### dl324

Joined Mar 30, 2015
11,231
How does the legendary lm317 actually work ?
This should give you a good idea:

Nat Semi application notes:

#### Attachments

• 287.2 KB Views: 0

#### topher217

Joined Oct 17, 2019
1
@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?