4 component circuit for powering standard LED's off of mains

[dumle29]

Hey there EEV. So I dug out an old box I made 4 years ago when I was 15. My teacher dug out his old electronics magazines when he heard I was looking for neons for some mains indicators, and found this circuit for powering a standard LED off of 220V mains, and it works like a charm

Here it is. As for how safe it is? The entire circuit is inside a plastic project box, and the 5mm LED domes are the only thing poking through the front

[David_AVD]

Pretty common arrangement that's been around for more than 30 years.   

The usual failure mode is the capacitor going low in value causing loss of current drive.

Note that the capacitor needs to be an X2 class type.

EDIT: The diode is usually a 1N4004 / 1N4007 type.

[dumle29]

Ah I guess my google-fu was weak Odd that my teacher told me to throw on a germanium diode back then

[mariush]

For those that wonder how that circuit works, watch these.. you'll also learn other useful things:

[richard.cs]

Ah I guess my google-fu was weak Odd that my teacher told me to throw on a germanium diode back then

The diode reverse voltage only needs to be sufficient to handle the forward voltage of the LED but the diode forward voltage directly corresponds to lost energy so a germanium diode would have an advantage here.

[fcb]

The diode can happily be as piddly as 1N4148.

Pay close attention to the capacitor though. If it fails short, it'll spit the top of the LED across the room. I've seen two used in series before, but an X2 rated part would be fine.

[Niklas]

It works for pure sine wave but I doubt it will survive a burst test. During that test, the capacitor is low impedance to all the pulses with high frequency energy, but the resistor is still there. 220 V over a 1.1k resistor will generate a lot of heat and probably also some magic smoke.

[HighVoltage]

I have seen it plenty of times that only a 120 K resistor was placed in line with the LED on a 230 V mains.
May be not safe but this instrument that I had opened up, was in service for many years and the LED worked still alright.

[David_AVD]

I have seen it plenty of times that only a 120 K resistor was placed in line with the LED on a 230 V mains.
May be not safe but this instrument that I had opened up, was in service for many years and the LED worked still alright.

I've seen failed LEDs on two different items that used this method.  I'd say that the violation of the Vr was to blame.  Even adding the inverse diode would be better.

[tszaboo]

ON semi give you a solution with 4 component, which is even constant current drive.
http://www.onsemi.com/pub_link/Collateral/AND8433-D.PDF
Just note that they have no idea how to draw a schematic on figure 1. This is constant current, there is no capacitor to be de-rated. Sadly it is only yank compatible.

[SeanB]

I just used a single appropriately rated resistor, a bridge rectifier and the LED, all potted in epoxy into a circuit breaker casing. Still working as fas as I know, I designed it to have 5mA through the LED with 440VAC supply, using a few 0.5W resistors in series to both keep surface temperature to barely above ambient and to get the voltage well below that which would cause stress on the carbon composition resistors I used. Diodes were 12V zener diodes, as I still have a box of 5000 of them, so use them as regular diodes if need be. At 230V the led was slightly dimmer, but still would throw a shadow across the room at night.

[fcb]

I can imagine the 25Hz flicker being a pain for those that can see 25Hz.

Use a full wave rectifier before the LED and then you won't need the reverse diode and you'll have 50Hz/60Hz flicker frequency (better than 25Hz/30Hz).

Very wasteful of energy though don't you think? .. Good for the odd simplistic use I spose.

You'll get 50/60Hz flicker actually. 100/120Hz if you use a bridge rectifier.

[Conrad Hoffman]

It's not a bad circuit, though susceptible to spikes. The resistor helps a bit. Use two LEDs instead of a diode. That will take care of the reverse voltage limit. It will also flash at 120 HZ (or 100), making it a perfect turntable strobe light! You can make the image crisper by including a pair of high value zeners so the LEDs are on over a shorter part of the cycle.

An interesting thing is to run it off an AC wall wart for safety, then look at the current by looking across the resistor. The current waveform is nothing like I would have expected.

[AG6QR]

The diode reverse voltage only needs to be sufficient to handle the forward voltage of the LED ...

That's true until the LED fails open circuit, which could happen if there's a stray spike or something.

I suppose it might not be a huge concern, if you can be sure that exceeding reverse voltage will cause the other diode to fail open circuit.  But a fuse or something similar is probably a worthwhile precaution.

[richard.cs]

That's true until the LED fails open circuit, which could happen if there's a stray spike or something.

I suppose it might not be a huge concern, if you can be sure that exceeding reverse voltage will cause the other diode to fail open circuit.  But a fuse or something similar is probably a worthwhile precaution.

If the LED's dead it's coming apart for repair anyway. You could add a fuse but it will only protect against a short circuit capacitor. If either diode fails short circuit you will get hardly any change in current. I tend to use antiparallel pairs of LEDs unless there's a good reason to only use one.

[MLXXXp]

I tend to use antiparallel pairs of LEDs unless there's a good reason to only use one.

Does anyone make antiparallel LEDs in the same package, similar to bicolour LEDs but with both the same colour, for this purpose?

[richard.cs]

They do, I haven't seen them for a while but I'd assume they're still around. They're a bit specialist and aimed at hobbyists along with the LEDs with the built-in resistor or flashing circuit.