LED'S as indicator lights on 110 volt a.c.

spindown · Post by **spindown** » Tue Jun 10, 2008 1:41 am

Is it possible to use leds as indicator lights on ll0 volt a.c. circuits without rectification? I believe I saw some reference to this in a NUTS AND VOLTS Magazine but can't find it. If so how would you do it? Thanks Paul

k7elp60 · Post by **k7elp60** » Tue Jun 10, 2008 6:11 am

Here is a circuit that will work. The diode is necessary to prevent the reverse polarity damaging the LED

Externet · Post by **Externet** » Tue Jun 10, 2008 7:02 am

The diodes should be in counterparallel

The resistance, chose it according to half the maximun led current

Miguel

Robert Reed · Post by **Robert Reed** » Tue Jun 10, 2008 7:51 am

A small value capacitor can be substituted in place of the high wattage resistor to drop the line voltage (not in your configuration). There was an extensive thread on this about a year ago. If you have the patience, scroll back thru for more info.

Janitor Tzap · Post by **Janitor Tzap** » Tue Jun 10, 2008 11:32 am

Heres a similar design too k7elp60.

The only thing that could give you trouble.
Is finding the 91K resistors.

Signed: Janitor Tzap

Robert Reed · Post by **Robert Reed** » Tue Jun 10, 2008 4:58 pm

Jantrap

The 1N4001 ,I believe should be reversed

Janitor Tzap · Post by **Janitor Tzap** » Tue Jun 10, 2008 5:37 pm

Robert Reed wrote:Jantrap

The 1N4001 ,I believe should be reversed

Oops!
I got stop trying to do three things at once.
I went back, and fixed the drawing.

Good catch there Robert.

Signed: Janitor Tzap

MrAl · Post by **MrAl** » Tue Jun 10, 2008 7:41 pm

Hi there,

There are a number of solutions for an indicator light that runs
off of 120vac. One of the other desirable properties is that the
circuit doesnt use a lot of power. The main reason is so that
none of the components get hot or very warm.

Here are three solutions, all of which use under one quarter of
one watt, and none of the components get warm.
Two of them require high brightness LEDs, such as the high
brightness 5mm white LEDs, but blue would work nicely too,
or even green, as long as they are the high brightness type.

For the last solution in this drawing, a high brightness LED is
not required, but it's still a good idea.

spindown · Post by **spindown** » Wed Jun 11, 2008 3:06 am

Thanks for the helpful replies to my query. What is the electrical differences between an led and a high brightness led? Are there any markings or physical characteristics that would help in distinguishing an led from a high brightness led? Thanks Paul

MrAl · Post by **MrAl** » Wed Jun 11, 2008 4:07 am

Hi Paul,

Well, the main difference is that when you drive a 'regular' LED
with 10ma of current you get some brightness, but when you
run a 'high brightness' LED with 10ma it appears much brighter.
Other than that it is hard to tell the difference.

Usually the way you know which is which is when you order them
they will give the millicandle rating or some rating like that.
For example, 5000mcd is pretty high and will be quite bright,
while 100mcd is only good for an indicator lamp. When i say
indicator here though, i mean it's *only* good for an indicator.
With the prices of high brightness LEDs coming down so far over
the years it means they can be used for very low current indicators,
putting out as much light as a 'regular' LED but with much less
current.

To give you a better idea what this all means, for example, i use
one high brightness LED in an app that only draws 15ua of current,
yet it provides enough light to tell when something is on or off.
With a 'regular' LED i wouldnt even be able to see it at that current,
and would have to pump around 5ma at least just to see the thing.
Thus, high brightness LEDs make very good indicators when the
current is limited or you want to run on low power.

With the line voltage being 120v the current has to be limited, and
doing this with a resistor alone means some power has to be dissipated.
With the high brightness LED this can be made without any special
parts as shown by my diagrams previously posted in this thread,
and the resistor sizes can be kept small (1/4 watt).

Janitor Tzap · Post by **Janitor Tzap** » Wed Jun 11, 2008 11:30 am

Well since were talking about LED Lights.

American Science & Surplus got in a bunch of these LED lamps.
http://www.sciplus.com/category.cfm/sub ... maxrows/11

Signed: Janitor Tzap

Dean Huster · Post by **Dean Huster** » Wed Jun 11, 2008 4:37 pm

The nice thing about high brightness LEDs is that you can use a circuit to vary the current for all kinds of blinking options for mode indicators or warning signals. With a standard LED, you were pretty much limited to on and off. A high brightness allows you to blink on-off, blink low-high, blink off-low-high or whatever you want. Simply varying the current or using PWM to do the job is easy.

Dean

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