Q:

Vary voltage using relay. Is this possible?

I am planning to control a RGB lamp using Arduino. The lamp is 24V and I want to be able to control the brightness of the lamp. I found a relay that will let me control the 3 channels (red green and blue), but will only let me turn it on and off (Not control brightness)
I am looking for a way to control the lamp both on/off and brightness. What component will do the same as my relay, but also let me use the varying voltage from the arduino to control my 24V lamp? (Just like a transistor)
I have an external 24V powersupply along with 5V for the arduino
Thanks!

A:

No, you can't vary the voltage with a relay. The only way you could do it with a relay would be to have a selection of different voltages available, and then use multiple relays to select which voltage is used. Less than ideal.
You need to use PWM to create a square wave with varying duty cycle. The duty cycle (the percentage of time the power is on within a given period) defines the average voltage of the output.  That PWM needs to be fed into a MOSFET which will in turn control the 24V power to the lamp.
It's exactly the same as driving RGB LED strips, except it's in the shape of a lamp not a long strip.

