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converting low voltage AC to DC to power LED ? Answered

I have a circuit that is 5.5v AC used to power old incandescent power on light , I want to convert to DC and use an LED ...do I need to build a bridge rectifier for this ?what will 5.5V ac convert to in DC ?thanks?



7 years ago

I'm an electronics flunkie...and want to do the same thing - add an LED indicator that will lite when an AC circuit is on. The line/output is 24vAC at .5A according to the manual. This is an irrigation controller, which sends 24v to activate a water/sprinkler controller valve. I want an LED to lite up when the controller is ON.

I read 2 related 'led monitor lite' pages:
and a 3rd article that suggested using a Bi Polar LED.

I'm not sure what I need. Any suggestions appreciated. Thanks

Like lemone said, just put a resistor in series with it, but I would also put a diode in series, too. The reverse PIV of the LED might not be a high enough voltage to keep it from being damaged when the polarity reverses. The diode in series (e.g. 1N4002) will protect it in the reversing phase. Wire it cathode of the LED to anode of the diode.

If you connect your 5.5v ac supply to a bridge rectifier (4 diodes, all 1n4007), you get about 7.5v dc with out any load. At the bridge rectifier's out put, connect 1000 micro Farad 12V electrolytic capacitor and 1000 ohms 1watt carbon resistor. Now the DC voltatge will be around 6V. You can connect one LED bulb across the capacitor along with a series current limiting resistor of 330 ohms 1/2 watt to get fairly bright light form the LED. To get more bright light from the LED, you can replace this 330 ohms resistor with 47 ohms 1watt resistor. But donot connect the LED with out a current limiting resistor in series with it. G Gururaja, Scientist, NAL, Bangalore, India.

thanks you 2 ...while I was waiting I built a simple bridge rectifier and now the led seems bright and steady ...although there is no load on the PS as of yet , perhaps I will have to put a cap in if it does flicker ...what type of value would you recommend ? thanks

5.5V AC after a bridge rectifier will give you about 4.1V. Each half cycle passes through two diodes, which each drop the voltage ~.7V. However, it won't be what you're looking for yet. You'll need a decent capacitor to help level out the voltage first, then a resistor to lower the current to a level appropriate for the LED. If the AC frequency is low to begin with you may see the LED flicker slightly. If it does get a capacitor with a higher Farad rating until the flickering is gone or tolerable. On the other hand, lemonie's answer is definitely quicker, cheaper, and requires less room inside your device. Resistors alone are also much easier to obtain. Again, with a low frequency you may notice flickering. You could use a hybrid solution and use just a capacitor and resistor. That would the middle-ground of cost, availability, complexity, etc.

If you want to light an LED all you need is a suitable resistor. Try something around 400 Ohm for starters. L