How to Safely Light an LED from 20V AC Using a Diode for Wireless Doorbell Indicator

Hi,
Would it be possible to power a LED from a 20 Volt supply with a diiode in reverse across the LED?
I have a Wireless doorbell that requires an AC supply. I have a transformer that will supply the 20 Voltsbut want a minimum component count as the space is limited inside the transformer case. I would like to be able to see at a glance if the supply is working.


Thanks,
Gerald

Hi Gerald-Yes, you can easily do that. You will need to add a current limiting resistor in series.  Maybe 2 to 4K ohms should work well- it's not critical at all as long as it limits the current to something below the maximum current rating of the LED. You can place the diode in inverse parallel across the LED, but you might want to consider placing it in series instead.  That way you will be dissipating half as much power in the current limiting resistor.Either way, you may notice that the LED is flickering very quickly, since it will only be illuminated of half of each cycle. It's unlikely that you will even notice the flicker unless you are moving your eyes past it. FYI- most through-hole LED's have a flat on one side of the package and have one lead shorter than the other.  The short lead and the side with the flat on it will be the negative (cathode) connection.-Rick

There's another technique you can use here.  If you want to do this with 240V AC (mains) your resistor would have to be R = V/I = 240/o.o1 for 10 mA, = 24 KOhms, but it would have to be rated at about 2.5 Watts.  However if you use a capacitor instead, then the capacitive reactance would have to be the same amount, ie 24 KOhms, giving a capacitor of about 0.1 uF.  Do the same thing at 20V and you'd need a capacitor of around 2 uF.  If you use this technique you MUST have the diode in reverse parallel with the LED so the capacitor can charge and discharge, a series diode will not work.The capacitor must be a non-polarised type and rated for the full AC voltage.

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