Hello, Im using Pulse Width Modulation on a 12volt resistive load(light bulb) using a Mosfet(and mosfet driver). Id like to reduce the amount of Noise it produces, as it interferes with other devices in the room.
What Id like to know is Where would I locate the inductor? Should I put it inline with the Loads Power line, or the PWM'ed Negative line? Also, How many mH should the inductor be? I currently have a Hi Current RF Choke 25UH. Im Running around 3Khz PWM I believe. Does a Faster Switching rate reduce noise?
Thanks a lot for your time, Im just a little lost with selecting the proper Inductor.
If you can accept a minimum light level and a lowered efficiency, then putting a suitable wattage, suitable value, resistor across the switching element may help. The lamp should be a dull red, at least, at all times, even with the mosfet disconnected.
You need to determine whether the noise is radiating from the lamp or travelling into the supply. Stick the lamp (temporarily!) into an earthed metal box. If that significantly reduces the noise then a lot of line filtering isn't going to help much.
Putting an inductor in the load power line is a good way of increasing noise and blowing up the mosfet.
A faster switching speed is better at reducing the amplitude of noise generated in the lamp, but generally radiates more efficiently - making the apparent noise greater.
Thinking of putting a *filter* into the supply lines is a good idea. This is a combination of inductor and capacitor(s). Look up filters, particularly pi filters and choose values for L and C for the frequency you wish to reject. But only fit one if the test above shows that the noise isn't being radiated from the load.
If it is radiated noise, one solution to consider is to use an old microwave and put the lamp in that, shining out through the door window. Don't forget to earth the microwave.
Another is to abandon pwm and simply put the lamp as the load of a suitable wattage audio amplifier, eg from a car radio. Then connect a variable amplitude sine wave generator to the input.
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