Electrical calculation problem.

[The Warthog]

If I am setting up a 6v lamp with .5a in two bulbs, this calculates out to 12 ohms resistance in the circuit. I want to put a variable resistance in the line to use as a dimmer switch, giving a smooth dimming action over the range of the potentiometer. I have a feeling an inline dimmer made for a 110v, 100w circuit, would be too big, but I have no way of knowing what resistance I need to do the job. The potentiometers in the radio store vary from 1Kohm to 100Kohm.

What do I need? Anyone able to help?

[daz]

If my maths is correct, a 1K ohm pot would give you a current of 0.006 amps at 6V. This size should be more than enough to dim the lights, but you may find that the pot really has no effect until the last few degrees of the turning circle (simply not enough current to light the bulbs)

Of course, I may be talking gibberish here - if no-one else can help, I'll check with the engineers at work tomorrow

[The Warthog]

I took a different approach when I was coming home on the bus, and came up with 500ohm to 1Kohm, but if you can check with your engineers, I'd be interested in hearing what they think. It's been a long time since I had to pass any exams on this!

[daz]

Being done today WH - will let you know ASAP!

[Kaptain Klevtsov]

Lets see - you have 12 ohms in your bulb, so if you add another 12 ohms then the resistance doubles, so the current halves.

Power from the bulb then will be from only 3V (other 3V drops over resistor) and 0.25A. This will give 1/4 power as you have halved the volts AND halved the current. (Power = volts x amps.)

If you look for a wirewoumd type resistor used in audio stuff you migt get further. you should be looking for 0-20 ohms, nowhere near kilohms.

Another way round might be to switch the lamps, if they are in parallel you could arrange a switch to put them in series to drop the light level to 25% (it wont be 25% as the filament will cool, the emmision from it will drop off and other really clever stuff) which might get you where yuo want to be in a more simple way.

There is a way using op-amps if you want to get more involved though the parts might set you back a couple of bucks (yes really, not much more anyway).

Let me know if this route is useful and I'll do some playing.

Later dude

CC