Mains LED

will

How much more? The peak will be 1.414 *230 = 325, but the RMS will still be 230, and the resistor will still be 230/.02 for RMS current of

-Bill

Well, you could just use a current limiting resistor. For 20 mA with 230 V, you get R = 230/0.02 = 11.5 k.

The power loss in the resistor is I^2 R, which is 0.02 * 0.02 * 11.5 k, which is only 4.6 Watts. Shouldn't be hard to find a 10 Watt (just to be safe) 11.5 k resistor. ;-)

You didn't say you wanted an efficient circuit, did you?

Good luck!

--Mac

It is something we overlook. When only small current levels are needed, a small transformer is fairly easy to come by. I once used a non-PC switching supply for something, and after thinking about a DC to DC converter to supply some low current negative voltage, I realized I'd just use that small transformer I pulled out of a clock radio.

Of course, a simpler solution may be to do away with the LEDs. Use some neon bulbs, right off the AC supply. Get an electroluminiscent panel, and again run it off the AC supply. It all depends on whether the poster is asking about LEDs because that's all they know, or if there is something that specifically requires LEDs.

Michael

I hope I didn't give you the impression that I thought wasting 4.6 Watts to light an LED was a good idea? I will try to make it more obvious the next time I am being sarcastic. Sorry.

As far as the voltage goes, the OP said 230 VDC. I decided not to challenge it. Anyway, where I live (USA), the nominal mains voltage is about half that.

--Mac

Consider using a series capacitor to limit the current and connect the LEDs two in sereis and another two in series back to back with the first two. Each pair will limit the reverse voltage for the other pair. The cap will limit the forward current. Use a cap conservativley rated, if the cap should fail short, the LEDs will disappear in a flash of smoke. I'd add a small resistor 100 Ohms and fuse in series with the cap just to limit the fault and high frequency surge current.

Maybe for more saftey use two caps, one on each power lead.

The total reactance of the caps should be on the order of (you figure out the exact value) 5kohms which is about 0.47 uF.

Mark

--- 120V For 120VRMS and 20mA LEDs, R = ------- = 6000 ohms, and since 0.02A

you're pretty close, but the fatal flaw with that scheme is that the reactance of the capacitor will be so small to the turn-on transient that it will allow the LEDs to be fried. maybe not the first time the circuit is turned on, and maybe not the second, but it _will_ happen. That 100 ohm resistor won't help much either, since if the thing gets turned on at the peak of a half-cycle, it'll let the LEDs see about

-- John Fields

As there is a bridge rectifier involved on 230VAC (nom.) mains, there will be significantly more than 230VDC. A *far* better thing to do here (ie. it's also safer) would be to get a miniature 1 or 2 VA transformer and step down to say 6V.

Ken

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You answered his question, which was a fair thing, so I wasn't criticising you, just pointing out things OP may not have considered or realised needed to be considered. OP is from UK, blessed as it is with a mains voltage suitable for domestic purposes, unlike your quaint, paler imitation. :-)

Cheers.

Ken

Or this one

------------------------------------------------------- Paul Taylor BSC (Hons) Electronics Technician School of Environmental Science University of East Anglia Norwich NR4 7TJ

Phone: +44 (0)1603 592502 Fax: +44 (0)1603 591327

Email: snipped-for-privacy@uea.ac.uk Web:

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The capacitor direct circuits work fine, but since they have no filtering you will see a strobe effect with moving objects in the illuminated area. This is not very noticeable while standing still or with stationary objects, but will cause moving objects to flicker or jitter about. The half-wave circuits will strobe at 60 PPS and the full wave will strobe at 120 PPS.

Early LED traffic signals with hundreds of LEDs were driven with similar circuits, some had half the panel running from each half-wave, and TV cameras would always catch only a band or stripes across the face due to the scan rate of the camera.

Just an alert to be aware of what you are trying to illuminate.

Pat

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Actually, it's 12K for 20 mA since the line is 230. That would be about 0.27uF at 400 volts, non-polarized. You also need a series resistor to limit the surge current to a couple hundred mils. Something around 2K should work and will waste about 1 watt. But if you just use a single resistor and no cap, the power wasted is only about 5 watts, so all you save with the cap is 3 or 4 watts.

-Bill

I suppose you're right. Here in the states, many houses are forced to run in a special 220 VAC circuit for things like air conditioners, electric heaters or clothes dryers, and electric ovens or ranges.

It probably would be a bit easier if we just used 220 for everything.

But you have to admit that 60 Hz is vastly better than 50 Hz. ;-)

--Mac

Yeah, I just don't know how we manage with 10 less than you guys. Still, we go for quality, not quantity, with everything. :-)

Ken

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Perhaps one of the reasons for not knowing how to cope with 10 less than a proper supply is the fact that it is 20 less.

OK thats a good point about the turn on surge, lets make it just a bit more complicated then......

How about a series cap feeding a beey zener at about 10 volts that can take the surge current. Then you can rectify and filter the output from the zener so that you also solve the strobe problem.

Mark

I read in sci.electronics.design that Mark wrote (in ) about 'Mains LED', on Fri, 18 Feb 2005:

Surely it's easier to include a series resistor to limit the inrush current?

Just download and use:

There your can select between multiple LED circuits for AC and it will also do all calculations for you.

also

The trouble with sites like this is that they are only any use for those with limited knowledge of what they are doing. Then it leads them down dangerous paths - exactly as in the above link. Has anyone been killed by it yet? Hopefully no-one has tried it with a few uF (for, say, a Luxeon emitter) without any discharge mechanism on the cap.

Very few are born with full knowledge of a field, most people have to use some time learning it. When learning it is usual nice to have some tools to help, some people only use pen, paper and books. Other are accepting that we now have something called computers and internet and using these tools also.

Following the advice in newgroups or electronic magazines are can be even more dangerous.

A few uF is not exactly deadly, and your get only shocked by it, if your touch it at the wrong places.