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I have two halogen automobile headlights (12v, 65 watt) that I’m using to make a wall hanging. I want to be able to dim the lights with a dimmer that I can have on a side table. I have a transformer to go from wall outlet to 12v, but what type of dimmer do I buy so that I can dim both of these headlights at the same time?

Chris
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  • Perhaps check out some of those variable DC controllers. – Solar Mike Aug 15 '23 at 16:13
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    You could use a DC/DC converter powered off of the 12V but I think a 150+ watt adjustable DC power supply powered from the wall would be your best bet. Some of them have remote inputs that let you control the output from an external voltage or resistance so you could kinda hack the existing dimmer into it. Shouldn't be too expensive but I wouldn't go too cheap either since the continuous 11A draw at full power is a touch higher than most hobbyist grade stuff. – vir Aug 15 '23 at 17:42

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Expect short lamp life time when you dim Halogen bulbs. There is a minimum temperature they need for the Halogen cycle to operate properly. When operating properly the tungsten evaporated from the filament is deposited back on the filament however when to cold it gets deposited on the glass. This reduces not only brightness but the lamp life as well.

I would consider using tungsten lamps or LEDs, you will be happier in the long term as a lot less bulb changing. As you dim the tungsten lamps there life will increase I believe it is exponentially.

Gil
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    You're right, but note that halogen lamps are used in things like microscopy where they're dimmed routinely. Going down too low can cause permanent deposition of a metal film on the inside of the glass as well. Also "halogen" = "tungsten halogen" in the sense of lamps, so if buying you have to exclude "halogen" rather than include "tungsten" – Chris H Aug 16 '23 at 07:53
  • ... Of course they're also dimmed in cars that have [dim-dip](https://en.wikipedia.org/wiki/Automotive_lighting#Dim-dip_lights), with a big series resistor – Chris H Aug 16 '23 at 08:04
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A halogen lamp will draw far more than it's rated wattage when first powering up, as the filament will be a low resistance until it heats up. You will need a power source that can supply enough amperage to get the filament up to operating temperature before dimming it down. If powered up with the dimmer set to some lower output, the lamps may not light and may continue to draw a higher than normal current.

You mention having a transformer to go from wall outlet to 12 V, but you don't specify that you have a DC supply. Are you planning to power it from 12 VAC? That is going to make a difference in how you could dim it. For AC you might be able to use some type of triac based dimmer. Do you have the current or VA rating of the transformer?

One option might be to go with LED lamps, these would be easier to dim and use much less power. If you don't need the aesthetics of a halogen lamp I would look into LEDs.

GodJihyo
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  • You can soft start halogens perfectly well, in fact it extends the life. You'll still get to the same equilibrium temperature in the end even with quite a weak power supply, where "in the end" is within a second or so. – Chris H Aug 16 '23 at 07:55
  • That low resistance start-up behavior is characteristic of most incandescent lamps - a lab set up as part of one of our electrical subjects... – Solar Mike Aug 16 '23 at 08:09
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Low voltage Halogen lighting was very common but less hungry LEDs are replacing it .You can get 12VAC dimmable lamp transformer and use a normal cheap dimmer .Most of these transformers have overtemp cutout and saftey approvals

Autistic
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Dimmable 12V AC can in practice be done with a mains dimmer (triac-based) wired into a transformer. You can even do that if the transformer's output is rectified to give DC.

You're slightly mistreating both the dimmer and the transformer, and I'd over-rate both. Better than a dimmer might be a drill speed controller. That also uses a triac to chop up the mains waveform, but is designed for driving inductive loads

But most power supplies with DC outputs aren't simple transformers (heavy copper and iron) they're much lighter and more efficient switch mode power supplies which can't be abused in the same way. Unfortunately such power supplies are often called transformers, even when that's not an accurate description. And a transformer rated for 120W+ output (to drive 2 headlamp bulbs in parallel) will be pretty big to start with.

There are a couple of ways of getting two brightness levels:

  • You can wire a switch to connect them in parallel (bright) and in series (dim - half the voltage gives much less than half the light, and it will look very orange)
  • You can switch in a series diode to half-wave rectify the AC, blocking it half the time, so again having much less than half the brightness. This can be done independently for each lamp, so gives you more options
Chris H
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