Variable resistor separate from control circuit?

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1

Good morning,

I am making a simple AC chopper for a dimmer circuit for a table lamp. It consists of a simple Triac which is fired using a RC circuit with a variable resistor. Here’s the schematic.

http://i.imgur.com/hWtdzQZ.png

My problem is that instead of using a potentiometer like in the circuit, I now want to control this circuit using a microcontroller to vary the resistance. The problem is that there is 220vAC flowing through the variable resistor and I obviously have a control signal of 5vDC coming from the microcontroller.

I have controlled AC switches before using DC control circuits by using simple relays which have 2 completely isolated coils. What I’m asking is, is there a similar way to control a variable resistor?

I’ve been researching photocouplers, looking for a solution. But it seems photocouplers behave similar to relays (in that there are only 2 states, either on or off). Any help would be appreciated.

Thanks !

2

The problem is that you are trying to extract a single component and find a microcontrollable version of it. A better approach would be to step back and see how the circuit works as a whole and how a microcontroller could emulate that.

The pot varies the portion of the cycle that the triac will be on. This is what your modified circuit must do, and how automated/digital versions of the circuit work. Detect the zero cross time of the signal and using that as a time reference, decide when to switch the triac on.

Alternately, you could buy a motor-controlled pot from SF, but that’s a suboptimal way to solve the problem.

3

Image search for “optocoupler AC dimmer circuit” to see lots of useful examples.

I hate to refer people to Instructables, because most of them are just awful, but the circuit presented in this one doesn’t seem too bad: http://www.instructables.com/id/Arduino … e-circuit/