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Variable resistor separate from control circuit?

biggerB

Oct 4, 2015
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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. I've attached the schematic.
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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 !
 

GPG

Sep 18, 2015
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For phase cut control you could use a opto triac driver, but your micro would need to have a zero crossing signal to set the phase. How is the micro being powered?
 

biggerB

Oct 4, 2015
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For phase cut control you could use a opto triac driver, but your micro would need to have a zero crossing signal to set the phase. How is the micro being powered?

Ok I looked into opto triac drivers. My micro is being powered by the same power source as in the input, but it is being passed though an on board transformer + rectifier + filter + linear regulator combination to get a steady 5 vDC supply.

I think understand what you are proposing, instead of using an RC circuit to fire a triac, use an opto-triac to fire it directly from the microcontroller. Correct me if I'm wrong there.

But then again, the problem would be that I would have to have some way to detect the zero crossing of the input AC voltage and then fire the triac after introducing a small delay. What would you recommend I use to detect the zero crossing?
 

Alec_t

Jul 7, 2015
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Here's a possibility, using an optoisolator to provide a zero-crossing reference to the micro.
MCU-controlled-dimmer3.PNG
 

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GPG

Sep 18, 2015
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What IC?
Can be done with a couple of 1N4148 3 resistors and a NPN transistor
 

Alec_t

Jul 7, 2015
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Oops. A reverse-biased diode should be across the optoisolator's LED. I'll edit the schematic in post #4
 

GPG

Sep 18, 2015
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like
 

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GPG

Sep 18, 2015
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Red, junction of 1N4148s Green Collector zero pulse. The bridge is existing supply.
 

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