How to dim a 12V AC halogen

Question

I have a couple of lamps, halogen I think. I bought them decades ago. When new they had a nice dimmer knob that led down to a transformer and I could dim the lights or, more usually, turn them off and on with the dimmer.

The transformer in both died after a long time and I replaced it with a simpler transformer that accepts 220V and outputs 11.7V AC. And I can turn the lamps on and off at the wall.

But I got wondering if I could build a circuit to support the dimmer function. The original transformer is gone but the VR that controlled the dimming is there. I'm okay with 5V DC circuits, and I've done a bit of Arduino stuff so I tried wiring up the 11.7V through the VR using the middle pin and one of the others to just add a resistor to the circuit. The VR is a 470K and is the original that came with the lamp. That just meant the lamp never comes on no matter how I twiddle the VR.

Next try was to try and control it with a TIP120, so the VR feeds into the base of the TIP120 and, hopefully, triggers the output off and on. That gave me smoke which surprised me because I was following this circuit, though not too closely. I was, as I said, using a TIP120 which is rated for 60V and the same VR as above (470k).

But maybe this is because AC behaves differently and I am used to DC. Anyway, before I produce any more smoke I thought I'd better ask for advice.

To be clear, I am more interested in just turning it off and on rather than dimming.

Also I'm getting weird readings on my multi meter when I check the 11.7V. As I said, I'm more used to AC but I switched the mm over to AC and it gives me something like 3V (though that varies a lot). I tried another multi meter and got similar but different results. Maybe that's part of the problem. Whatever the voltage is the lamp is happy with it.

Answer 1

The bulbs are AC or DC they do not care which. You can use a triac as a simple dimmer circuit. Sometimes even the cheap ones from the box store will work on 12V AC. Look at landscape dimmers, they are not normally that expensive and are designed for AC. You can also purchase PWM dimmers that will work with 12V DC but you will need a DC power supply capable of driving the light. I would highly recommend you use LEDs as they will be more energy efficient and run a lot cooler if you can.

A a bit of caution with halogen lamps, they must be hot for the halogen cycle to work. Dimming them to much will cause a decrease in life. What happens is basically as the light is on tungsten from the filament is vaporized and then recombines with the filament. If it is not hot enough this cycle will not work and they will have a shorter life. The filaments are run at an extremely high temperature compared to standard light bulbs. The black you see on the inside of tungsten bulbs is tungsten from the filament.

The start up current for a light bulb is extremely high. When cold the resistance is at its lowest, as it warms up the resistance increases. Generally it is about 10X or so at about 25C. Your electronics must be able to withstand and support this surge as must your power source. This surge is non existent in LEDs making them much easier to work with in dimming applications.