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Old 7th July 2007, 11:54 AM   #1
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Default I need 12v DC- whats the best option?

Hi folks,
I need 12v DC to run an LED. Is there any straightforward way to do this. I am using a Dynaco PA060 transformer which has secondary windings of 5v, 6.3v and 720v! Can I use these somehow with a bridge rectifier or would I be better off adding a small seperate 240 - 12 v transformer with a rectifier circuit? I am a little concerned that adding another transformer might contribute to noise?
Cheers,
Rob
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Old 7th July 2007, 12:00 PM   #2
paulb is offline paulb  Canada
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Why do you need 12V to run the LED? Does it have a built-in dropping resistor?
Even if it does, you can probably run it from the 6.3V winding, with a diode and small capacitor to convert to DC.
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Old 7th July 2007, 01:21 PM   #3
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I once built a successful 555 delay circuit that used a 12v relay. I got the 12v it needed using a voltage doubler off the 6.3v heater winding. I used 2200uF caps in the doubler.
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Old 7th July 2007, 01:47 PM   #4
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Quote:
Originally posted by ray_moth
I once built a successful 555 delay circuit that used a 12v relay. I got the 12v it needed using a voltage doubler off the 6.3v heater winding. I used 2200uF caps in the doubler.

The voltage you get out of a doubler is a function of the storage capacity -- if you are just driving a string of LED's and need about 10mA -- the value of the caps can be much lower. About 100uF will work -- and these can be rated at 25V so they will be pretty small -- 2200uF is too "stiff" for a low current application and the output voltage will be closer to 15V. If you were driving a preamp chip the ripple with 100uF would be inadequate due to the ripple.
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Old 7th July 2007, 10:16 PM   #5
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Default 12v

Hey thanks guys - I need it for a single LED in an illuminated switch which is a sealed unit and rated at 12v - it must have a resistor built in. The voltage doubler looks like a g great option. I presume 0.5w resistors will be OK. Thanks for the advice - great idea.
Rob
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