I'm running a string of 240V AC lights on my Christmas tree but they are too bright for my liking (I think they're outdoor lights). I'm thinking of putting a potentiometer in the string to be able to dim them a little as it's cheaper than replacing them with a LED string. I've found this pot which I think will do the trick. By my calculations at maximum resistance it will only have to dissipate .23W. My questions are: Is it a suitable component? and How should I wire it up (as it's an AC application). I can't seem to find a circuit diagram for a pot in AC Before anyone asks, the lights aren't too hot for the tree. They ran all last Christmas on the tree just fine.
Resistors act very much the same in AC circuits as in DC circuits, so just use a standard diagram and use Phase and Neutral as Positive and Negative. As for the pot being suitable, I wouldn't know. What is the wattage of the bulbs and are they in parallel or series.
If the string uses normal bulbs, i highly doubt that that pot will be able to handle the effect (Wattage?). The current won't be very high, but the voltage drop can potentially be pretty high. Since effect is (Voltage drop) x (current), you can see that the effect will get high pretty quickly. I think you would be better of getting a normal dimmer that is made for the purpose. You can get one of the kind that is to be mounted directly on the wires. Any shop that sell lamps should have these. Just be sure about minimum effect. Normally they can handle about 150-300w (the small ones). But they won't work if the load gets under a certain limit (normally 30-60W). Just check the wattage on the light string, and find one that have a range in that area. Edit: I think i misread your post. How did you get to .23W? This will depend on how much you dim the lights. (Do you know what resistance will give you the brightness you want?)
The usual way of dimming high-powered lights is using a switching dimmer, i.e. if you want 50% power, the light is given power for 50% of the time. This is a much better way of doing things that using a resistor, as the power doesn't have to be dissipated as heat else where, reducing the wastage of the system. As far as I am aware domestic dimmable lights use the same system, which is probably the type of thing that Smilodon was talking about. I don't see why there should be any minimum load, but he probably knows more about it than me.
I realise I did the maths wrong. I was working on it from the other end (voltage and resistance of the pot). I hadn't taken into account current draw (I don't know why I didn't) The string is wired in series. I'm not sure of the wattage as I no longer have the packet (I've had them a couple of years now). I hadn't given consideration to a dimmer before but that's pretty much what I'm looking for. I'll have to cut the string and measure current draw before I go buy one.
I've seen a stat or two on how many fires and stuff like that are caused at xmas from people playing with lighting, but i can't remember where. here's one
A cheap and simple thing to try is a rectifier diode (eg, 1N4004 or other 400PIV diode to suit the current) in the string. They may flicker on half-wave 50Hz, but in Xmas lights that's not all bad. Triacs and thyristors need a minimum load current to stay on. We've a 5-lamp chandelier on a 500W dimmer, with only one 40W bulb in it flashes erratically at low light settings. Remember 230V can really spoil your Christmas...
It doesn't say fires. It says accidents. As long as the amount of lights are kept to a reasonable level, it shouldn't be any problem. I would guess the biggest problem is damaged wiring from a year in bad storage. Bad wires=sparks. Sparks+dry fir tree=fire. I THINK a normal bulb size is something like 0,1W-0,2W. Just multiply this number with number of bulbs, and you should have a rough number to go by. On the other hand, you still have to cut the wire to mount the dimmer. I'm not sure why there is a minimum load, but i believe it's because the triac needs load to fire. (Just like a transistor. If the collector is floating, a voltage on the base have no effect)
