Mains, LEDs and Law

org/- A UK political party

About 30 years ago, I took apart a 9V "battery eliminator". It had the physical form of a standard 9V battery, but with an AC cord extending opposite the 9V battery terminals. I took it apart because I could not believe they could make a transformer so small, so imagine my shock when I found none. Instead of a transformer, there was a series electrolytic capacitor and a 2 Watt resistor (that got very hot when the device was operated). In addition there were a few diodes including a power zener and some filter capacitors. That was it -- no isolation and a series capacitor. This device was legally marketed.

Tom P.

For which country?

INAL. In the EU/UK it would depend on whether the led side ought to be part of the safe low voltage part of the equipment or could sensibly treated the same as an ordinary lamp and be part of the hazardous area.

Since I'm thinking about a mains lamp (think table lamp) I suppose it would be part of the hazardous area. I assume it would either have to be housed in plastic or earthed metal?

Housed in insulated or earthed conductive housing. Nothing to stop you using insulators other than plastic.

True. What do commercial mains LED lights use? This kind of simple circuit or neat stepdown xformers and constant current PSUs?

You are in the UK? I'm sure you can find the applicable codes for your area and follow them. Creepage distance, the path length along insulation surfaces from the mains to ground and to humans touching the product will likely be the biggest issue. This may be a problem given the small dimensions of an LED and it leads. It may require additional insulation to meet the requirements. In the US this requirement is often met by double insulation common in small appliances.

I do not think the use of a series capacitor assuming that it is an appropriate type rated for AC current will be a problem providing the creepage distance and insulation requirements are met. Many motors use series capacitors so it is done all of the time.

The only problem might be attaching the LED to a metallic heatsink, which would have to be insulated or shielded.

"Dirk Bruere at NeoPax"

If you are considering a cap of 6.8 uF to get 500mA of AC current - you are nuts.

But we all know that already.

..... Phil

So that would be like supply full mains to a light bulb eh? F****it moron. [See - I speak fluent Allison]

Just a cap will probably fry the LED, from line transients. Usually a series resistor is added to soften things up.

John

In my case a 120W PF corrected switcher with Supertex LED controllers. I don't know what people do in smaller stuff but look at Supertex for example. They do a range of off line control ICs for driving leds. There are several with 400V inputs driving up to 50mA with out needing an external fet.

Customer just rang. They misjudged the optical design and need 18000 lumens not 8000. In 300 x 300mm, 1 foot square. Can I fit 200 Golden Dragon leds in the space and up the PSU to 250 watts. The PSU is complete, done, finished. Customers! :(

Well, it's obviously not just a cap, but a bridge rectifier and a bit of smoothing as well. The cap just provides a cheap and convenient constant current source from mains.

I need about 1000lm in a 100mm square

Oh. But "obviously"? I have seen LEDs run off the line with just an RC driving a pair of back-to-back LEDs, or one LED with a reverse diode.

John

I know, but that's just horrible! I rather dislike the use of a mains cap, but it is certainly cheaper and simpler than an xformer and constant current PSU.

Which gets me thinking... Does anyone do constant current wallwarts?

But they keep me from stepping on the cats at night.

All you'd need is a heap of leakage inductance.

John

"tlbs101"

About 30 years ago, I took apart a 9V "battery eliminator". It had the physical form of a standard 9V battery, but with an AC cord extending opposite the 9V battery terminals. I took it apart because I could not believe they could make a transformer so small, so imagine my shock when I found none. Instead of a transformer, there was a series electrolytic capacitor and a 2 Watt resistor (that got very hot when the device was operated). In addition there were a few diodes including a power zener and some filter capacitors. That was it -- no isolation and a series capacitor. This device was legally marketed.

You must be talking abut the US back in the early 60s, where for example one could sell tube guitar practice amps to budding musicians which effectively had a live chassis.

Manufacturers back then took sole responsibility for the safety of electrical devices they sold and there was no requirement for agency approval or compliance with government regulations.

In most developed countries, one of the very first electronic devices to come under compulsory safety approval regulation were " battery eliminators " - cos so many of them were soooooo damn dangerous.

Your example is a classic and thankfully a relic.

..... Phil

I think you'll find that the thermal issues will be your major concern. With that qty of those LEDs is a 300x300mm luminaire design, the solder will melt unless your heatsink is approx 300x300x300mm too!

JB