next topic »

[on:]

Hi, I have a high power led (3W) that already has a constant current power supply(700mA). I've tested that power supply and it's fast, it can be turned on and off in some miliseconds, and it works with any voltage from 4 to 15V. How can I drive it with a digital PWM signal? I need something like a relay but faster. 

Thanks and sorry for my English

[Reply #1 on:]

You can use a MOSFET or a transistor -- they're much faster than relays. Here is one circuit idea using a transistor:

http://ruggedcircuits.com/html/circuit__19.html

and another using a MOSFET:

http://ruggedcircuits.com/html/circuit__11.html

The details will differ depending upon your circuit (15V, 700mA) but the overall configuration shows you what to expect.

--
The Rugged Audio Shield: Line In, Mic In, Headphone Out, microSD socket, potentiometer, play/record WAV files

[Reply #2 on:]

You can use a MOSFET or a transistor -- they're much faster than relays. Here is one circuit idea using a transistor:

http://ruggedcircuits.com/html/circuit__19.html

and another using a MOSFET:

http://ruggedcircuits.com/html/circuit__11.html

The details will differ depending upon your circuit (15V, 700mA) but the overall configuration shows you what to expect.

--
Thank you so much, I've found that site http://www.electronics-tutorials.ws/transistor/tran_7.html that shows the same that you says. I think I need a P channel mosfet if I want to place my led between something and ground, so I will use the NDP6020P I hope I found it in Argentina.

Thanks 
The Rugged Audio Shield: Line In, Mic In, Headphone Out, microSD socket, potentiometer, play/record WAV files

[Reply #3 on:]

If you are switching the constant-current regulator on and off fast you should have minimal decoupling on it - ideally you want it to switch in some microseconds, then the standard PWM rates of 1kHz or so will do the right thing.  The decoupling should be on the supply that's _input_ to the PWM MOSFET/transistor.  It you find its not really dimming till zero PWM then that implies too much decoupling after the PWM stage, and the decoupling capacitors are stretching the PWM pulses.  If you are lucky it will just work nicely.

You could reduce the PWM frequency by reprogramming the timer's clock pre-scaler, but that would be likely to cause annoying flicker on the LEDs.  Any switching transistor or MOSFET that can handle 700mA easily (ie not have more than a fraction of a volt across it when on) should suffice.