Variable resistor separate from control circuit?

[biggerB]

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.

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 !

 

[Nigel Goodwin]

A micro-controller dimmer is nothing like the crude analogue one you've shown - it's all done in software, you detect the zero-crossing point, and trigger the TRIAC a suitable time after that in order to set the dimming level you want.

If you wanted a crude analogue dimmer, then use a servo (or simple geared motor, if only want relative control and not absolute) to turn the pot, and a micro-controller to control the servo/motor.

 

[biggerB]

A micro-controller dimmer is nothing like the crude analogue one you've shown - it's all done in software, you detect the zero-crossing point, and trigger the TRIAC a suitable time after that in order to set the dimming level you want.

If you wanted a crude analogue dimmer, then use a servo (or simple geared motor, if only want relative control and not absolute) to turn the pot, and a micro-controller to control the servo/motor.

Thankyou very much for your reply Nigel.

I am very new to analog circuitry and triacs, so forgive me for proposing a preposterous analog design

From what I understand, you are proposing that I use a triac which would be fired directly by my microcontroller through the coding. Correct me if I'm wrong there.

A few things that pop into my head if this is the case then.

1. My microcontroller is powered by the same power source, except it passes through a transformer + rectifier + filter + linear regulator combination to obtain a steady 5v DC voltage. How would I go about using the output 5v of the microcontroller to control a triac? Though I imagine this would be done using an optocoupler, what are your thoughts?
2. How would I go about detecting the zero crossing point of the input voltage? How would I translate that into an interrupt for my microscontroller?

Thankyou very much!

 

[Mikebits]

You could probably use this part, and have mcu control the opto coupler.

The part has the zero crossing detector built in. The link for the data sheet: https://www.fairchildsemi.com/datasheets/FO/FODM3063.pdf

This will keep you away from the mains more than building the zero cross detector from separate parts. Nigel may have a better solution?

 

[Nigel Goodwin]

From what I understand, you are proposing that I use a triac which would be fired directly by my microcontroller through the coding. Correct me if I'm wrong there.

Nope, you're quite correct - that's the way it's done

A few things that pop into my head if this is the case then.

1. My microcontroller is powered by the same power source, except it passes through a transformer + rectifier + filter + linear regulator combination to obtain a steady 5v DC voltage. How would I go about using the output 5v of the microcontroller to control a triac? Though I imagine this would be done using an optocoupler, what are your thoughts?
2. How would I go about detecting the zero crossing point of the input voltage? How would I translate that into an interrupt for my microscontroller?

If you google for 'PIC light dimmer' there's loads of info out there - such as this example:

broken link removed

As you can see, it's quite simple (or at least the hardware is ).

 

[biggerB]

Nope, you're quite correct - that's the way it's done



If you google for 'PIC light dimmer' there's loads of info out there - such as this example:

broken link removed

As you can see, it's quite simple (or at least the hardware is ).

Thankyou for the reply.

I understand the circuit you gave in the link you posted, but it is connected to the mains grounds though a high resistance. Wouldn't it be better to just isolate the controller side of things completely?

For now this is what I think I'm going for, as this circuit completely isolates the AC Mains via 2 opto couples. However I may also look into opto-triacs with built in zero cross sensors like Mikebits said as they seem to be much easier to work with.

Thanks for everything guys !

 

[Nigel Goodwin]

However I may also look into opto-triacs with built in zero cross sensors like Mikebits said as they seem to be much easier to work with.

Except you were wanting dimming, the zero-crossing TRIAC's can only switch ON and OFF, they can't dim.

 

[Mikebits]

oops :{
I totally forgot about the dimmer part.