Dimming Miniature 12v LES Lamp...

[Ben99]

Hi, I wonder if anyone with a better electronics brain can help me out...

 

I want to dim a very small LES 12v lamp via the usual desk/dimmer set up (dimmers are strandLD90s).

 

Here's the details of the lamp - http://uk.rs-online.com/web/search/searchB...t&R=0104581

 

I'd intended to run it from a birdie-style 12v transformer (eg http://uk.rs-online.com/web/search/searchB...&R=4196537), but of course this doesn't work, because the transformer requires a minimum load of 20W (rather than <1W) and as the lamp doesn't provide sufficient load it fails to light up at all.

 

A bit of experimentation shows that if I load the transformer with a birdie (so the LES lamp and the MR16/50W lamp are wired in parallel) both lamps dim fine.

 

Now, I could just run with this, and hide the MR16 lamp away in a corner... but that's a bit tricky given the set design, and need to avoid unnecessary light spill.

 

So - and here's the question - can I provide a substitute load to avoid using the the MR16 lamp? Some kind of resistor based circuit perhaps?

 

Or is there a better way to do this?

 

thanks!

[J Pearce]

Use a real transformer, and that won't require a minimum load.

 

However, you will still need a load lamp, as a triac probably won't hold open with 1W. However this can be at 240V, away from the set. A common lamp is a 500W parcan or flood gelled in a dark blue.

[Ben99]

Use a real transformer, and that won't require a minimum load.

 

However, you will still need a load lamp, as a triac probably won't hold open with 1W. However this can be at 240V, away from the set. A common lamp is a 500W parcan or flood gelled in a dark blue.

 

thanks, sounds good (I can defo use a par or whatever to load the dimmer) but can you explain what do you mean by "a real transformer"?!

[J Pearce]

An old fashioned wire wound transformer. Won't cost much to get one suitable for 1W loads.

[boatman]

At 12V a 6.8 ohm resistor will dissipate just over 21W which will be ideal. Just get one of these and wire it across the output of your transformer. It will get very hot, so make sure to bolt it to some sort of heat sink and fix it into an enclosure so that inquisitive fingers can't touch it.

[Ben99]

At 12V a 6.8 ohm resistor will dissipate just over 21W which will be ideal. Just get one of these and wire it across the output of your transformer. It will get very hot, so make sure to bolt it to some sort of heat sink and fix it into an enclosure so that inquisitive fingers can't touch it.

 

thanks boatman, that was the kind of thing I had in mind, just couldn't remember how to work it out. d'oh!

 

and thanks J Pearce (by the way, will a wire-wound transformer dim ok on a theatre dimmer, I'd always understood that electronic transfomers were required for this?)

[J Pearce]

Most decent dimmers will dim inductive loads, some require a load lamp.

I have used 250W of birdies on a big transformer on an LD90 with no problems.

A load lamp never does any harm though!

 

The resistor method will work well, as long as you can heatsink it and keep it away from fingers.

[Ben99]

great! thanks again!

[adam2]

I would advise against the load resistor, although it would undoubtably work, the heat and resultant risk of fire or burns suggests the alternative approach might be better.

 

Obtain a traditional wire wound transformer, not an electronic one. The output voltage of such transformers tends to rise a bit on light load and might therefore be nearer 13 volts than 12, which is unlikely to matter.

As others state, dimmers have a minimum load for correct operation, but a suitable load lamp may be placed wherever desired.

[aidso]

easy way to get round the amount of heat generated is increase the wattage of the resistor. try this, it cost more but would work better http://uk.rs-online.com/web/search/searchB...t&R=0136294

[boatman]

easy way to get round the amount of heat generated is increase the wattage of the resistor. try this, it cost more but would work better http://uk.rs-online.com/web/search/searchB...t&R=0136294

 

I don't think so. No matter how big the resistor, as long as it's 6.8 ohms it will dissipate 21W. Replacing a 50w wirewound resistor with a 300W equivalent will spread the heat slightly further but that's all, apart from a ten-fold increase in price of course. It will still need a heatsink and some protection from finger gepoken.

[adamharman]

Given a suitable heat sink, 21w of heat shouldn't be an issue.

After all, the lamp in a birdie is only around 5% efficient so is chucking out 47w of heat. I know a lot of this is projected out the front with the light, but the cans still get very hot.

[Ekij]

If you put a 230V 20W lamp in series with the 12V 1W lamp, (both lamps draw about 85mA)

Across the 20W lamp you would get 218V and across the 1W lamp 12V at about 85mA

 

Issues to be aware of

1] Heating up time might not be the same for the two lamps. Thus soft start would be advised

2] Dimming characteristice of the two lamps may not be the same.

3] The wiring and insulation of the 12V lamp is not rated for 230V (which would be OK unless the 12V lamp failed when all 230V would appear across the open cct and may arc).

 

To avoid issues 1&2, you could just wire 20 12V lamps in series (hiding the other 19 well away from the one you care about).

You'll still need a dummy load for the dimmer (as 20W isn't going to be enough) but you won't need a transformer.

[David A]

the best way to do this job is to find a competent technician to disconnect the 240v from the triac/scr's and feed the 12ac in instead, this works fine, I have done it often, but only if there is a short run from dimmer to load, but this is not an issue with such a small load.Option 2 if you have analogue converter use 1 output through a 1.2 gain 2watt amplifier circuit

[gyro_gearloose]

Because buying, wiring, and hiding 19 extra little lights is so much easier than getting a transformer and adding a load lamp. And if just one lamp fails, they will all go out. With 20 lamps in series, this is 20 times more likely than one lamp+transformer failing. And then you'd have to find out which lamp failed...

 

I know the OP asked about a solution which did not involve a transformer and load lamp, but I think that it will be the easiest solution. Another idea would be to use LEDs and a DMX LED driver board from Milford Instruments. I don't know for sure, but I see no reason why Milfords LED driver board couldn't drive the 12volt lamp the OP is using.