LEDs on A/C

I'm sure this has probably been discussed in the past but I'm not sure how best to search for the thread. If you know of one, point me in the right direction, please.

If not...

Whats the smallest, easiest way to run an LED off of 120v A/C? I took the time to research and find a good bathroom light/fan combo thats fluorescent and uses a high efficiency blower motor, but it still uses an inefficient, several watt night light. I'd like to stick a 1w in there, running at maybe half power (175ma) so I don't have to worry about heatsinking and I think that will provide plenty of light for the room at night. Any ideas on how to do this without getting overly complex?

I don't have tons of space but I can probably make something work.

Thanks.

Mark

Getting 175mA at about 4V from 120VAC is not so easy a task.
There are some special purpose transformerless power supply
chips and/or circuits, but they tend to max out at about 50mA,
and they're pretty expensive.

The EASIEST way is to buy a "wall wart" style power supply of
appropriate rating and use it. Or pull one from your junkpile.
An old nokia cell phone charger ought to be just about right!
(This is probably the cheapest way, too.)

A half-watt luxeon may be too bright for a nightlight. I have
some commercial LED nightlights with standard 5mm white LEDs, and
they're "almost" enough. (the ones I got have gotten dimmer or burnt
out distressingly rapidly, though. Grr.)

BillW

How about this...Home Depot has LED nightlights packaged with CFLs on clearance for 4 bucks. I picked one up the other day. Maybe take one apart and use the circuitry? its certainly smaller than a brick already and I can get rid of the photocell stuff.

Mark

There have been several threads. The CPF search is difficult for me to use. You will get some hits if you search subject only and search for +led +night +light.

Here's one thread:
LED night light

The idea is to use a capacitor in series to limit the current. Keep in mind the AC voltage is dangerous.

Hi there,

I would use a wall wart for this because the power demand
is a little greater than your average little white LED.
With the night lights, you only need 20ma average current
so you can use reasonable value caps to act as a voltage
drop without losing efficiency.

A wall wart would also add electrical shock safety to the
light, which in the bathroom is important. A series
cap would not.

If you're still interested in using a series cap though
even for a higher powered LED i'll go through the
calculations for you and come up with a circuit that
has the right value cap and correct rated resistors for
surge protection and a fast acting fuse too...let me know.

Wall warts can be found all over the place. All you need
is the right voltage output and a series resistor of the
correct value.


Take care,
Al

What about a series of led's, say for holiday lighting? I bought a string of 25 led's on clearance after the holidays but have not had a chance to "break" them yet. But from the looks I see no outward signs of a voltage dropping device. It has no walwart either, just a regular plug...

Well, as stated in that thread, I don't want to use a wall wart because of how inefficient they are. If I'm taking out a 5watt bulb, I don't want to use an LED that consumes 1/4 watt and a wall wart that uses 5 watts. /ubbthreads/images/graemlins/wink.gif

I'll have to take apart one of these nightlights and see what the circuitry is and if I can use it. I've rethought my white luxeon idea and I think I might go with a red luxeon. Then I wouldn't need to run much power at all because of how efficient they are. That 20ma or so might be enough...

Thanks guys.

Mark

Hi again,

Oh ok, if 20ma or so will be ok then you can use a small
white LED. There wont be any good reason to use red
but if you like that color better then by all means use it.

Here's the circuit:


You can use a bridge rectifier in place of the four
diodes. The zener is a 9v zener, but if you increase
to 12v you can run two whites in series for more light
output.
This circuit is tried and proven, as i've had one running
now for some 2 years i think. It's constantly on 24 hours
a day 7 days a week.
Power used is very very low, less than 1/2 watt.


Take care,
Al

That looks like a good circuit. What voltage rating on your capacitor? I understand the voltage transients on AC can be quite high. I guess the 100 ohm resistor in series would filter out the high frequency spikes.

Hi there eluminator,

The voltage rating i used was 400v, but you can probably get
away with a 250v cap. Yes, the first 100 ohm resistor helps
to reduce surge current for line spikes, or when plugging
the unit into the wall when the input sine wave happens
to be at a high peak (170v).

Take care,
Al

The ccrane company use to carry a 1 watt night light bulb that just screwed into a standard night light socket.