240v ac dissipation challenge, can I do better than 0.2W ?

Assuming both AC sources share the same neutral, all you need is two diodes, a resistor and a flashing LED. Surely your flash-rate isn't THAT critical?

The only downside is that the whole circuit is 'live' and thus needs adequate insulation to be safe.

Oh, and power disipation would be less than 100 milliwatts.

"Craig Hart"

With R =33 kohms ( = 10 mA peak) = 0.87 watts in the resistor.

....... Phil

The LED is not illuminated constantly. The duty cycle of a flashing LED is about 10%, so divide your figure by 10, reaching a figure of less than 100 milliwatts, as stated.

Use a cap as the dominant current limiting device, for lower dissipation.

"h;hjk"

" ... led 250ms on/250ms off .... "

....... Phil

er no !, the led needs to be really noticeable and a 10% duty cycle doesnt cut it it terms of visibility, 50% duty cycle is the requirement. Since pf isnt an issue for the type of load I can get it down to 200mW with a 680KOhm resistor and "a couple of other bits"... The issue is can I get it down to 100mW and 50% duty cycle... ?

do they make flashing LEDS capable of switching 340V? did you have some other scheme in mind? (a larger resistor and a reservoir cap?)

I'm thinking something with small cap driving a diode pump providing a small constant current source charging a reservoir and a PUT or similar switching the LED on (which drains the reservoir below the PUT's threshold...

should be well below t00 mW Bye. Jasen

Obviously the first thing to try is finding the most efficient LEDs, there is a great variety of performance in this respect.

Another thing you could try is to get multiple LEDs of the same colour and connect them in series (e.g. agilent/avago surface mount LEDs are tiny and fairly efficient), but with a single "lens" or diffuser in front of them. Maybe you can buy something like this as a commercial product. This would get you more light for a given current. It would drop more voltage but that wouldn't bother you particularly.

Another poster mentioned using a capacitor to drop most of the voltage which is a good idea as the power consumed by the device will be less than the VA. It won't be a great power factor but this probably doesn't matter in such a low powered device (I don't know the standards but at least in some countries I think there is an exemption from the power factor requirements for low powered devices.) Also since it will be capacitive, it will probably improve the power factor of your house as a whole, by a miniscule amount. I would not drop all of the voltage with a capacitor but I would use a resistor to drop a few tens of volts in series with the capacitor. I would do that because if there is a mains spike, the high dV/dt would cause very high currents if there is no resistor in the circuit. I would consider putting a power zener diode across the series combination of the LEDs and half of the resistor, to protect against surges.

Chris

How about simply using a LED with a higher efficiency and run it at a lower current? Might be more expensive or more difficult to obtain, but if your main requirement is current consumption then LED efficiency would be the #1 thing to weak.

Dave :)

That's why I said "dominant". (It is a fairly standard commercial practice.)

Hi all,

Managed to get it down to just over 0.13W with a 3v flashing red led (yes 50% duty cycle), diode, cap, resistor, zener (just in case). This thing also works down to 110v 60Hz np, tested it on a HP6813A and its cheap as chips to build,

Interesting range of comments however, I can see why this forum is sometimes a bit quiet, some people are disuaded from commenting or asking what might appear to be newbie type questions because some people insist on abuse - I have no place for annoymous abuse, its the sign of an incompetent coward - to all the others thanks :)

Might have something a bit different later on, one thing I've been toying with is to use all those left over PC power supplies to make a low end mig, how big an inductor should I get, mmmmmm ?

Thats all folks,

Yes, I knew what you were on about, but wanted to make sure that the reason was clear for those who had not seen this before.

Chris