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

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Hi chaps,

Had a small design challenge come up the other day:-

Flash a red or green led 250ms on/250ms off from the
240v AC and provide an 'or' function so if mains switch A
or B are on then the little sucker is powered up and flashes
(rate doesnt change if both and brightness same etc)

Four main criteria are low cost <doh>, to minimise
dissipation, no hot spots exceed 80deg C when
ambient temp is 45 deg C and finally fit in 0.5 cubic inches.
This is for mounting around an induction exhaust fan recess.

I've managed to get it down to about 0.25W but wonder if that
is the limit asymptotically for the class of low voltage devices
needing to run off 240vac, ie Regardless of compenentry there is
going to be a limit vis a vis current to drive the led and then its
just a matter of dropping the voltage by the best method for
the very lowest dissipation.

Is it likely I can get it down to 0.1W to 0.15W ?

--
Regards
Mike
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Re: 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

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

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

"Craig Hart"

**  Really -  how ever did  you get that ?

240 AC half wave  =  170 volts rms.

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

.......  Phil

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

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.

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

"h;hjk"

**  The OP's requirement was for 50%   -   dickhead.

" ... led 250ms on/250ms off   .... "

.......  Phil

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

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... ?

--
Regards
Mike
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Re: 240v ac dissipation challenge, can I do better than 0.2W ?

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 :)

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

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

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

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

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

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

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

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

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

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

Re: Update & Thanks, 240v ac dissipation challenge, can I do better than 0.2W ?

snipped-for-privacy@iinet.net.au says...

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,

<shrug>

--
Regards
Mike
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