Anyone good at circuit building? AC to DC for LEDs!

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Old 12-06-09, 05:56 AM
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Anyone good at circuit building? AC to DC for LEDs!

Ok, this is quite random but out of my scope of knowledge!

I have one of those old alarm clocks with the numbers that flip. I love the look much more than a digital but the orange neon bulb that lights the numbers is too dim.

I want to wire up two white LEDs to illuminate the numbers.

This clock has no DC inside; it's all electromechanical so all I have to work with is AC.

What exactly do I need to do here?
 
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Old 12-06-09, 07:26 AM
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is there only 120 volt power or is there some sort of stepped down voltage available within the clock?
 
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Old 12-06-09, 07:34 AM
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Originally Posted by nap View Post
is there only 120 volt power or is there some sort of stepped down voltage available within the clock?
OP wrote:
This clock has no DC inside; it's all electromechanical so all I have to work with is AC.


.......................
 
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Old 12-06-09, 07:46 AM
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Originally Posted by ray2047 View Post
OP wrote:

.......................
ah, ya but there may be varying levels of AC and I was asking if any were within the clock.

It's easier to design a circuit for LED's using 12 volt AC than it is uisng 120 volt AC.

I'll let you go ahead and take over on this Ray. You seem to understand what is going on.
 
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Old 12-06-09, 08:11 AM
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Originally Posted by nap View Post
ah, ya but there may be varying levels of AC and I was asking if any were within the clock.

It's easier to design a circuit for LED's using 12 volt AC than it is uisng 120 volt AC.

I'll let you go ahead and take over on this Ray. You seem to understand what is going on.
Heavens no. I get confused if it doesn't have vacuum tubes.
 
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Old 12-06-09, 08:19 AM
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I'm not sure about size, but I know they make/sell 120 volt LED lights. They have the limiting circuit built into the package. A lot easier than building a power supply for just one or two lamps.

Bud
 
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Old 12-06-09, 10:08 AM
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Perhaps you could get a 120 volt LED bulb, take it apart, and use the components from that?

The only thing is you would want to well protect all bare wires with heat shrink tubing.
 
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Old 12-06-09, 11:01 PM
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Neon dims with age. Why not just replace the existing neon with any 120VAC lamp?
 
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Old 12-07-09, 04:52 AM
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I have done exactly what you want to do--but I used a miniature base 120V/3Watt bulb. An LED will require a low-voltage DC source as from a small filament transformer with a rectifier/filter circuit. . . Hardly seems worth the trouble.
 
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Old 12-08-09, 06:53 AM
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LEDs like to be current controlled, not voltage controlled. A transformer will provide a lower voltage, but you will still need to limit the current, especially for white LEDs which are more picky to drive.

Another way to limit the current is with a capacitor. Let's say you wanted to limit the current to 10ma. Then a capacitor of about .22 uF in series with the LED would do it. The capacitor needs to be rated for the full 120 volts. And the LED needs to have another diode in parallel with it to pass current in the opposite direction. Or you could simply put an identical LED pointing the other way. That way each LED will conduct on opposite halves of the AC cycle. Of course the entire circuit has lethal voltages since it is connected directly to the AC line. If you cannot ensure this circuit is completely protected from human contact, then go with the low-voltage transformer instead.

Bob Scott
Ypsilanti, Michigan
 
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