PWM to control LED with use of mosfet

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Kjetil
Posts: 65
Joined: Dec 11th, 2010, 4:18 pm
Location: Norway

PWM to control LED with use of mosfet

Post by Kjetil »

I have build a light for my ROV with use of a total of 34 leds 3,2V 20 MA.

I`m planning to supply these with the onboard battery, and then I need to add resistors. Problem is that the resistors gets realt hot. Above 110 dg C (230dg F). So I planned to use a mosfet and PWM to try to get the mosfet to adjust the voltage from 0-3,2 volt (and to get dim function).

I made a shematic and tested this with adjusting the output PWM from the software and potensiometer.

Problem is that I`m not able to get the voltage lover that 4,23 volt (its going directly from 4,23 to 0 v). Its vorking fine from 4,23 to 13 volt.

Will it help to use an other type of mosfet? or is there anything else I can do to get the voltage down to 3,2 volt?
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thegadgetguy
Posts: 238
Joined: Feb 13th, 2011, 8:27 pm
Location: Pennsylvania

Re: PWM to control LED with use of mosfet

Post by thegadgetguy »

What type of voltmeter are you using? Digital voltmeters can get confused by PWM signals. Technically the voltage doesn't actually change, just the duty cycle of the signal. Have you considered wiring the LEDs in series? You would still be able to dim them, but your starting voltage could be 12 volts.
Kjetil
Posts: 65
Joined: Dec 11th, 2010, 4:18 pm
Location: Norway

Re: PWM to control LED with use of mosfet

Post by Kjetil »

I have tested with 4 led in series, that is working fine. I will make new PCB and build a new lamp.

Thanks
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