Adding current limiting to a pwm led circuit

Thread Starter

Michael Erbacher

Joined Nov 2, 2017
1
Hi All,

I have a 12v switched power supply running a pwm. I would like to add single leds to each channel of the pwm but need a way to limit curent to a maximum of around 1000mA. I'm hoping someone can provide me with suggestions that are a little more efficient and with less heat issues than adding a 12 ohm resistor.

Any help would be much appreciated
 

-live wire-

Joined Dec 22, 2017
907
Average current or peak current? And do you mean constant current, or just shutting it off if there is a short? The simplest way to limit it to 1 amp is a BJT configured with the right resistors. That allows for constant current of 1 amp. For one-time short-circuit protection, you can use a fuse. For overcurrent protection for peak current, have a shunt or hall effect sensors. For a shunt you need an op-amp. Then if that signal gets above a certain potentiometer-tuned threshold, it pulls a comparator high. This turns on a fet that can only be turned off again by a normally closed pushbutton. The fet then turns off another fet in series with you power supply. Like this:
For limiting the average current, use a low pass filter with a high R and low C. This filter goes after the raw instantaneous current signal. That is then what goes to the comparator.
 

danadak

Joined Mar 10, 2018
3,902
Simple current limiter using BJT


This is PNP case,for NPN Emitter to R1 to Load,
Zener from base to junction R1/Load, and R2
from base to power supply. I = (Vz - Vbe) / R1.
You trade off R1 power vs Vz Vs current source
compliance.

You can incorporate this into PWM power circuit,
with mods, and make pass transistor the power
element in PWM to LED interface. Using a switch
to force base on and off.

Something like attached.

Or using 3 term reg -



This approach may have issues with startup time for the regulator,
bench test it for applicability.

Regards, Dana.
 

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crutschow

Joined Mar 14, 2008
24,360
For maximum efficiency you would use current feedback to the PWM circuit to control the PWM duty-cycle and give the current you want.

Does your PWM power supply allow current-control (constant-current) operation?
 
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