I need a simple way of dimming 12V Halogens

SNIP Take a look at model plane/car speed controllers for electric motors.... They are pwm devices and can supply several amps. Some go into the tens of amps.

Can't you get the same effect by just turning off selected lamps out of each bank?

First, get a professional to help you, mucking around with mains is dangerous, and get a budget from the boss

2) I assume you are using 230/240V ac in, the easiest way would be to use 230V dimmers, directly into a 12V out transformers, close to the dimmer pack. Then cable up to the 12v lamps. Choose a nice XLR that wont annoy the sound guys when they plug your connector into their stuff. I'm not in the biz anymore, but there probably is a "standard" 3) If you agree with point 2), you will need thicker cables

4) Many people here have studied for days, even weeks to become professional engineers, you wont do this overnight (safely anyway)

5) check your insurance policies

martin

This post should be to the basic electronics group............................

Search for Lot of circuits, or kits. A simple mosfet dimmer with the right mosfet(s) can control many amps.

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Hi Nik,

You might get some help from 'rec.arts.theatre.stagecraft'. Otherwise email me off list.

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John B

Isle of Arran

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Probably not the way to go. What is the total wattage at 12V?

I'd be inclined to step down to 15 VAC and use remote control dimmers to feed the lamps from there.

Look in your local library for a copy of the GE SCR manual or a GE hobby manual - old but if you can find one it will have useful ideas you can use.

** Groper alert:

** Are you using 12 volt batteries or AC power ?

** It is generally not practical to *directly dim* 12 volt lights used in stage performances. Any dimming is done at the much higher AC supply voltage level.

The problem is excessive "voltage drop" in the cables feeding the lights. A voltage drop ( due to cable resistance and dimmer losses) of 5 to 10 volts in a 230 volt system has very little consequence, but in a 12 volt system it would be a disaster.

This is why you always see 12 volt halogen lamps sited very close to their step down transformers.

....... Phil

Remember that halogen lamps work on an iodine-tungsten cycle. If you drop the voltage, hence reducing the power, you will interrupt the cycle and damage the bulbs. What does this mean? Probably a much shorter life for the bulbs.

If you can live with that, and you use AC, use a variac to control the AC to a transformer that drops the voltage to the 12AC that the lights use. So, if you drop the voltage to the transformer in half, than the voltage to the lights will drop to 6V. Will this mean a decrease in half of the illumination? Dunno, you have to experiment.

You will need some hefty transformers and variacs. It, of course, depends on the number of lights you want to control.

Al

. . .

This is helpful to understand the problem:

In our Scottish climate, it would probably be better to use 110VAC distribution and local dimmer boxes with DMX interfaces. Use a bridge rectifier and a power FET for each luminaire. Assuming 50W lamps you'll need 8A components to allow some headroom. Because of the losses in the semiconductors you need to start at around 15VAC. To minimise filament rattle each dimmer channel should switch on at the zero crossing and turn off in mid-phase.

There's a good starting point for home brew stuff here:

but I think the software only supports triacs which fire in mid-cycle.

If you want to buy then have a look here:

--
John B
In the Isle of Arran