Help With LED Wiring

[steamnut]

Hi folks,

I hope someone can point out whatever stupid mistake I've made with a current project.

I'm assembling a power supply for a new kit-built throttle. Although the designer recommends just screwing the transformer to my benchwork, I insist that my power supplies be in an enclosure for safety reasons (although the kids are now grown, we still have a dog who wanders around under the layout).

So I've wired the transformer, which of course even a certified idiot should be able to do, with a fuse and a toggle switch. Although its technically unnecessary, I like to have an LED power-on indicator. So I wired a small full-wave bridge rectifier package and checked with my multimeter that it is indeed putting out DC voltge (in this case 20v). Then I put a smallish capacitor across the outputs to smooth the ripples, rechecked and - yep, still 20v out. Then I put a 1K resistor on one side of the DC output. Prior to hooking the output to an LED, I checked the voltage and - STILL 20V (which presumably would fry the LED).

What am I doing wrong?

[CSX Robert]

You're still getting 20 volts because you will not get a voltage drop across the resistor until you complete the circuit.

Depending on the voltage drop and current rating of the LED, you may want a higher value resistor - with a 1k you'll be close to 20 ma, which is the upper limit of some LEDs.

[richg1998]

  As CSX Robert said, you need some kind of load on the rectifier/capacitor. For 12 volts DC, I use a 1k resistor which allows 9 ma current. The LED is plenty bright enough for me.

Rich

[cacole]

For a 20 Volt input you're going to need a resistor of 1300 to 1500 Ohms on the LED, assuming that the LED draws 15 ma at 1.5 Volts, which is a typical value.

[steamnut]

Thanks everybody. I warned you that it ws a stupid question. I already knew the theory but my error was that I thought the multimeter itself would complete the circuit for the voltage reading. Obviously not as I went ahead and hooked up the LEDs and everything is good with voltage reading under load at 2.

 BTW (a) these LEDs  are 20 ma at 2.1 volts forward and also, when the transformers are under load the DC voltage should reduce to 16 to 18 so I think I'm OK there.

 Thanks again for the helpful comments.

[60YOKID]

steamnut

So I wired a small full-wave bridge rectifier package and checked with my multimeter that it is indeed putting out DC voltge (in this case 20v). Then I put a smallish capacitor across the outputs to smooth the ripples, rechecked and - yep, still 20v out.

 

Actually, you don't need the rectifier or the capacitor because the LED is a rectifier in itself.  However, the full wave bridge will reduct the "flicker" effect of the LED.

[CSX Robert]

60YOKID

Actually, you don't need the rectifier or the capacitor because the LED is a rectifier in itself.

LEDs can not stand the same amount of reverse voltage as a rectifier diode and 20 volts is more than enough blow many of them.

[richg1998]

 Get a 1000 ohm 1/2 watt resistor and use that for a load without any LED's, just the resistor. Check the voltage with your voltmeter and use DC voltage, not AC.

Rich