4 component circuit for powering standard LED's off of mains - Page 1
Forum section: electronics / projects

author: dumle29
author_rank: Regular Contributor
timestamp: March 31, 2014, 11:06:02 pm
content: 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 charmHere 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

author: David_AVD
author_rank: Super Contributor
timestamp: March 31, 2014, 11:39:08 pm
content: 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.

author: dumle29
author_rank: Regular Contributor
timestamp: March 31, 2014, 11:48:27 pm
content: Ah I guess my google-fu was weakOdd that my teacher told me to throw on a germanium diode back then

author: mariush
author_rank: Super Contributor
timestamp: March 31, 2014, 11:58:10 pm
content: For those that wonder how that circuit works, watch these.. you'll also learn other useful things:

author: richard.cs
author_rank: Super Contributor
timestamp: April 01, 2014, 08:47:21 am
content: Quote from: dumle29 on March 31, 2014, 11:48:27 pmThe 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.

author: fcb
author_rank: Super Contributor
timestamp: April 01, 2014, 09:08:33 am
content: 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.

author: Niklas
author_rank: Frequent Contributor
timestamp: April 01, 2014, 09:15:20 am
content: 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.

author: HighVoltage
author_rank: Super Contributor
timestamp: April 01, 2014, 10:52:18 am
content: 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.

author: David_AVD
author_rank: Super Contributor
timestamp: April 01, 2014, 11:49:31 am
content: Quote from: HighVoltage on April 01, 2014, 10:52:18 amI'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.

author: tszaboo
author_rank: Super Contributor
timestamp: April 01, 2014, 12:35:33 pm
content: ON semi give you a solution with 4 component, which is even constant current drive.http://www.onsemi.com/pub_link/Collateral/AND8433-D.PDFJust 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.

author: SeanB
author_rank: Super Contributor
timestamp: April 01, 2014, 05:48:48 pm
content: 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.

author: fcb
author_rank: Super Contributor
timestamp: April 01, 2014, 06:13:38 pm
content: Quote from: AhhhChoo on April 01, 2014, 12:18:51 pmYou'll get 50/60Hz flicker actually. 100/120Hz if you use a bridge rectifier.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: April 01, 2014, 06:57:41 pm
content: 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.

author: AG6QR
author_rank: Frequent Contributor
timestamp: April 01, 2014, 08:14:34 pm
content: Quote from: richard.cs on April 01, 2014, 08:47:21 amThat'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.

author: richard.cs
author_rank: Super Contributor
timestamp: April 02, 2014, 11:35:12 am
content: Quote from: AG6QR on April 01, 2014, 08:14:34 pmIf 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.

author: MLXXXp
author_rank: Frequent Contributor
timestamp: April 02, 2014, 12:44:05 pm
content: Quote from: richard.cs on April 02, 2014, 11:35:12 amDoes anyone make antiparallel LEDs in the same package, similar to bicolour LEDs but with both the same colour, for this purpose?

author: richard.cs
author_rank: Super Contributor
timestamp: April 02, 2014, 01:15:19 pm
content: 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.