# limiting 2 amp current to flow through a transistor or mosfet

Discussion in 'General Electronics Chat' started by RodneyB, Sep 18, 2015.

1. ### RodneyB Thread Starter Active Member

Apr 28, 2012
652
14
I am trying to learn how to limit the current flow through a transistor or mosfet some direction will be greatly appreciated.

2. ### GopherT AAC Fanatic!

Nov 23, 2012
6,319
4,043
You can add a low value resistor to sense the current flow by ohm's law. For example, add a 1 ohm resistor in series with your load. That resistor will see a 2 volt difference across the leads when 2 amps is flowing through it.

That 2 volt differential can be used to trip a comparitor with a 2 volt reference. Once the comparitor is tripped, it drops the voltage on the MOSFET gate. You should add some hysteresis to insure the ohmic region of the mosfet is not used to limit the current as the comparator oscillates quickly between a triggered and un-triggered state.

Alternatively, you can feed the circuit with a 2 amp constant current source. That way, it cannot draw more than two amps (but will be allowed to draw 2 amps even if the load is very low resistance).

cmartinez and RodneyB like this.
3. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,066
Usually, the resistance of the load automatically limits the current through the transistor. Only in special cases do you need to have a secondary current limiter. Adding resistance in the emitter circuit of a transistor is one method.

Give a us a specific case?

4. ### AnalogKid Distinguished Member

Aug 1, 2013
4,701
1,300
A little more detail would help shape an answer to your needs. There are several standard methods ranging from a single resistor to a non-contact current sensor plus a signal conditioning circuit, and very many variations of them If you throw "current limit circuit diagram" into Yahoo, you'll get hundreds of schematics that show the techniques.

ak

RodneyB likes this.
5. ### dannyf Well-Known Member

Sep 13, 2015
2,196
417
Quite a few:

1. use a lateral mosfet;
2. use a fuse + beefy transistors;
3. use a crowbar + existing transistor;
4. use a puny power supply;
5. use a puny resistor in serial with the transistor;
...

the choices are limitless.

6. ### peter taylor Member

Apr 1, 2013
106
5

Here's something I used to limit the current from a car battery to an alternator come brush-less motor I was playing with.
High power transistors such as the 2N3055 often have low current gains and will act as a constant current source.
β= Ic / Ib
Ib = (Vs - Vb) / R
Then β= Ic . R / (Vs - Vb)
And R = β. (Vs - Vb) / Ic

If we throw in some values:
Vb = 0.6 V
Vs = 12 V
β= 50
Ic = 2 A
R = 50 x (12 - 0.6) / 2
= 285 Ω

A few things to remember:
The power rating of R:
P = V . V / R
= 11.4 x 11.4 / 285
= 0.46 W (use a ½ watt)
The heat dissipated by the transistor. Depending on what your load is doing, the collector-emitter voltage could go up to 12 V, meaning it has to dissipate 12 V x 2 A = 24 W. So mount it to a decent heat sink.

Have fun.

RodneyB likes this.
7. ### RodneyB Thread Starter Active Member

Apr 28, 2012
652
14
I am using a LM317 in my circuit to provide a regulated voltage of 18 volts to my circuit. The output goes to the base of a TIP41 which I hoped would regulate the current.

The TIP is getting so hot that the heatsink dis colours and burns the PCB

8. ### AnalogKid Distinguished Member

Aug 1, 2013
4,701
1,300
This idea has come up in other threads on small audio amplifier design. What you show is called dangle biasing, and produces inconsistent results. The current gain of a transistor changes based on base current, collector current, temperature, Vce, and of course from one device to another. While starving the base will indeed limit the collector current, it is the least consistent and repeatable way to do it.

ak

9. ### GopherT AAC Fanatic!

Nov 23, 2012
6,319
4,043
Don't do that. You cannot put all of that current through the base of that poor little transistor.

Do you need 2 amps or do you need 18 volts? Your LM317 can be wired as a constant current source. Check the datasheet. There is a 1.25v drop from out to adj pins. Add a o.68 ohm resistor across those two pins and take the power out from the adj pin (don't connect that pin to anything else. Obviously, you still need your input (+) to the in pin. The load connects between ADJ and (-) power (ground).

Note, the 0.68 ohm resistor should be 2 amps x 2 amps x 0.68 ohms = 2.8 watts (round up to 3 or 5 watts).

Cheers

RodneyB likes this.
10. ### RodneyB Thread Starter Active Member

Apr 28, 2012
652
14
How do I achieve the regulated voltage of 18volts and a current limit of 2 amps

11. ### GopherT AAC Fanatic!

Nov 23, 2012
6,319
4,043
To get both, you need to regulate the voltage to exactly 18 volts and then add a load of 9 ohms.

Normally, you want to set the voltage with a voltage regulator and then limit current draw to "no more than 2 amps", but will be something less than 2 amps if the load is more than 9 ohms.

- If your 18 volt supply must always Supply 2 amps, then you need a load that is less than 9 ohms (that would let more than 2 amps flow) and then you need to add some type of constant current source to limit it to two amps. Like a second lm317 in a configuration as I suggested.

RodneyB likes this.

Jan 17, 2007
3,698
2,791

13. ### GopherT AAC Fanatic!

Nov 23, 2012
6,319
4,043
This lets you set voltage regulation and current regulation with two zeners, a dual op amp and two darlington (high gain) transistors. The load is the resistor on bottom right.

PeterCoxSmith and cmartinez like this.

Apr 28, 2012
652
14

15. ### GopherT AAC Fanatic!

Nov 23, 2012
6,319
4,043
@RodneyB

So, in the circuit of post 13

The top circuit uses an op amp to limit current to R/1.25
That is, the Zener on that op amp is 1.25V and the resistor work together to set the current limiter. This only kicks in if the current is exceeded and the voltage across the resistor exceeds the voltage of the Zener. If the load is less than 9 ohms, the current will be limited to 2 amps (it will not shut off, it will be limited and supply voltage will appear to drop according to ohms law with 2 amps as the I and load as ohms).

The bottom section controls the voltage at 18V.