I am trying to control a 3W RGB LED (total 9W) by using a Mosfet on each channel, where the supply voltage is 12V. Each channel is supposed to be good for 700mA continuous. If I put my multimeter in the circuit and measure current, do I look at DC or AC current?
I am switching the mosfet using an Arduino Uno using analogWrite. I found if I ramp up to 40 (40 out of 256 = 16%) I read ~ 650mA AC, and about 300mA DC. I know the real current is pulsing all around but I'm assuming the average current is what is important to the LED, is the DC value a realistic measure of the average current into the LED?
Thanks
but I'm assuming the average current is what is important to the LED,
You assume incorrectly it is he peak current that matters as well.
A meter will not give a sensible reading for pulsed current, which is why you have.
What are you using to limit the current? You can not connect a power led up directly to a power source even through a FET. Normally some sort of constant current drive is needed.
I needed to set up something quickly and I didn't have a 700mA driver. I found it has worked pretty well so far. I'll scope the voltages/current later when the project is not in service. Nothing has burned out and it's not too hot so I think there's enough smoothing in the circuit to limit the peak voltage/current. Sorry I'm not too experienced with circuits like these. I had the PWM up to 130/256 = 51% which I slowly ramped up to incrementally until the brightness seemed right. PWM frequency I think is ~500 - 1000 Hz. I'll try a LPF.
Nothing has burned out and it's not too hot so I think there's enough smoothing in the circuit to limit the peak voltage/current.
So you think so do you, despite admitting:-
I'm not too experienced with circuits like these.
So what smoothing is this then?
I needed to set up something quickly and I didn't have a 700mA driver.
It is a well known fact that and excuse like this invalidates the laws of physics.
I can only advise you on doing things correctly.