I am working with LED drivers and compatible LED lights. They look somewhat like this:

led and driver

These come in two versions: (1) Constant voltage type and (2) Constant current type

I am implementing a circuit which is able to dim the light. It involves a mosfet switch which sits between the driver and the LED.

For the driver which outputs constant voltage, the circuit works fine. I am able to vary the brightness of the LED by varying the duty cycle of my PWM signal.

However it doesn't work for the driver which outputs a constant current.

What would be the equivalent method for dimming the LEDs in case of a constant current source?

  • 1
    \$\begingroup\$ I've written about one-type of dimmable LED power supplies here. \$\endgroup\$ – Transistor Mar 31 '18 at 20:00

Mains-powered constant current drivers will only work as current source inside a certain voltage range (see the label) and they will have a capacitor on the output, so you can't use a shorting FET to apply PWM dimming. They're more like constant voltage supplies whose voltage is adjusted (not very fast) to keep the current constant, which only works if the load isn't changing too fast...

You need a dimmable driver, with a 0-10V dimming input, which you can use to control it. Or something DMX compatible if you have lots of lights and a DMX controller.


Even it might seem odd, the switch should be parallel to the LED load so the overall current output to be constant otherwise the source maintain the average curent giving a higher current when the switch is on. PWM signal must be inverted. If the source has cap on the output then place an inductor large enough in series according to the PWM freqency before the switch and LED. You might need to limit the minimum off time to avoid reaching the minimum output voltage of the source.

  • \$\begingroup\$ IF the driver can take a compliance voltage of zero, which most commercial LED drivers can’t. OP is in a bad spot here. \$\endgroup\$ – winny Mar 31 '18 at 16:34
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    \$\begingroup\$ also most CC drivers have a cap on the output which will be shorted by the FET, this won't work, you need a dimmable driver \$\endgroup\$ – peufeu Mar 31 '18 at 16:42

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