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eschuen · Jan 24, 2008, 03:03 PM

Can anyone tell me what size resistor I need to run a 12volt led pilot light on a 24 volt system? The transformer is a 24v 40va its on a furnace. Thanks

Stratmando · Jan 24, 2008, 04:23 PM

Would work better with DC voltage, But still should work, Maybe not as bright.

You take the voltage in(24 Volts)subtract the voltage of the LED(1.7?)which equals 22.3 volts, then divide by the LED current(.02 amps(20 milliamps).
22.3/.02 = 1115 Ohms. Use the next closest resistor.

Stratmando · Jan 24, 2008, 04:26 PM

Sorry, Just saw, it was 12 volts, Use same math to figure out.
Another way would be to use 2 12 volt led's in series, Quick, easy, and cheap.
Be sure to have anode to cathode(series), if it doesn't light when it should, reverse polarity.

KISS · Jan 24, 2008, 05:11 PM

Strat:

Actually Add about 1/2 the resistor computed for a 24 V system. I'd probably use 1000 ohms based on a 10 mA current. Take the 12 V LED and throw in a diode and a 1K resistor in series. If it doesn't lite swap the diode.

Stratmando · Jan 24, 2008, 05:37 PM

KISS, think a full wave bridge, and capacitor is overkill?

KISS · Jan 24, 2008, 06:17 PM

I don't want to bother. No cap. Think 1/2 wave rectified or pulsating DC or 0V for 8.3 mS and a peak voltage of 24/2/.707 or 17 V p-p. Add (1.2+0.6) and get 1.8V. That's about 16 Volts peak or zero to peak.. Now lets go to the high side or about 40 mA and you get about 400 ohms.

So with an LED + Diode + 400 ohms will work fine.

But we can guess that the 12 V LED designers used R = (12-1.2)/(10 to 20 mA) or 540 to 1080 ohms. Better choice - Just use a diode with the 12V LED when you do the math.

I don't like using the LED as a rectifier.

For overkill, I'd use the LM 334 set to 10 mA with a resistor and a plain LED. That part will then work from about 3V to 40V p-p AC or DC

minhal110 · May 30, 2008, 02:47 AM

Hi I'm new here... I have the same problem.I need to charge my lamp which requires 6v and 1.2A max charging current... I have a dc power supply of 12 V and 3A max current... how can I reduce it to 6V and 1.2 A current??
Thanks in advance

KISS · May 30, 2008, 06:47 AM

You need a regulator. This will work fine. Jameco Electronics Education & Hobby: VELLEMAN INC. : K1823

Even though it says it needs an AC input and a transformer, DC will work fine. You will need a voltmeter to set the output voltage.

Stratmando · May 30, 2008, 07:01 AM

Excellent link above,
These may be other options:
6 Volt Battery Chargers: BatteryMart.com
Cliplight Manufacturing - OEM-Battery Charger - 6 Volt / 1.5 Amp Automatic Battery Charger

minhal110 · May 30, 2008, 12:11 PM

Thanks to all of you...