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12VAC powered single 1watt led? resistor heat problem

I have a dead halogen 12vac, 20w bulb that I was trying to replace with 3.7 volts 1w led, so I added a reverse parallel diode, resistor, and a capacitor on the other side(in the picture below), and got smoke after few secs it was on (it was the resistor), nothing burned out, so I had 100 tries more- changed resistor values, and finally figured out that the brightness depends on the value of the capacitor. With 50v 4.7microfarads(looks like the one in the picture below, but different value) I got big smoke and bright light, with 50v 25microfarads everything was cool but led was glowing very weak
So my question is: what should I change in my circuit so my 1w led would light at full brightness and wouldn't start a fire?


Picture of 12VAC powered single 1watt led? resistor heat problem
led circut.JPG
1w led.JPG
50V-Capacitor.jpg
sammons6 years ago
Not sure what this circuit is supposed to achieve but it doesn't look like it should work. The first problem is the resistor value is too low. If you were working with DC you would need a 27 Ohm resistor. With a 3 Ohm, the maximum current you will get will be something like 4amps depending on the LED. Next problem is that the Capacitor is basically acting as a high pass filter and blocking any DC like current. So at 50HZ the capacitor is blocking most of it thus most likely not getting 4A out of it (thus dim LED). (If you were working with higher frequency AC then it would be a different story). Next, when current is flowing reverse, i.e. not lighting up the led, all the current is flowing through the resistor. so you have 12V (minus diode drop) across a 3 Ohm resistor. not good! The easiest way to get this to work it so buy a driver, they are as cheap these days. http://cgi.ebay.com.au/3-1W-330mA-LED-Driver-Power-12V-AC-Constant-Current-/270503194610?cmd=ViewItem&pt=LH_DefaultDomain_0&hash=item3efb3f2ff2#ht_751wt_911 or if you want more of them http://www.aliexpress.com/product-fm/304132279-led-constant-current-driver-for-MR16-lights-DC-AC-12V-input-3-1W-320ma-output--wholesalers.html It connects to 12ac and gives you up to 3W at 320mA Just what you are after. if you look carefully at the circuit, it has 4 diodes, (bridge rectifier), an inductor and a 220uF Cap. (for filtering). and a voltage regulator and resistor for current regulation.you could put something together like this easily enough but probably not as compact and not for the price. :o) Phil
Re-design7 years ago
I don't know why the capacitor is in the circuit.

I would just use a diode and an adjustable voltage regulator.

Or better yet just change the power supply to the proper size.