Using LEDs with AC Power

[LyleCaldwell]

Looking around the net, I've seen three suggested ways to power LEDs with an AC supply.

These two ways:



And "just add a diode to one leg of the LED and a resistor to the other".

So, advantages/disadvantages?  Is there a consensus on a "right" way to do it?

I have 9vac in a device that I'd like to add LEDs to, but don't want to deal with adding a separate DC supply if I can avoid it.

[Basstyra]

Well, the idea is simple. You just convert the AC voltage into a DC one using a single diode. You'll have oscillations, but also a continuous voltage. Add a big capacitor (electrolytic) if oscillations cause the led to blink.

The 1st schem seems ok, but the 2nd let the current go thought the diode and R without getting througth LED. It seems stupid to me. Or maybe it's a protection against leds that don't like high reverse voltage ?

In fact, maybe the LED alone would be enough. I think I would put a diode and a cap to obtain a decent DC voltage, then the usual led+resistor.

[gez]

In fact, maybe the LED alone would be enough. I think I would put a diode and a cap to obtain a decent DC voltage, then the usual led+resistor.

Problem with LEDs is they don't take too kindly to being reverse biased...not with any significant voltage across them at any rate, so the diode in series is a good thing.  The cap you mentioned is also a good idea.

[Joecool85]

I am using AC to light a LED...All I used was a resistor to cut down the voltage.  It works fine.

[LyleCaldwell]

Well I guess it only costs $2 to find out.  <g>

[Joecool85]

I've had mine like that for 2 months and no problems.  And its on the amp I have for my computer speakers.  Stereo lm1875 :-)  I ran the LED straight off the 18v coming off the transformer.

[xpaladin1313]

I think most LEDs will work fine reverse-biased at only 9V.  I mean, it won't be lit when reverse biased, but it won't blow up.  You could always check the data sheets if you aren't sure.

Also, if the frequency is 60hz (or more), it shouldn't blink.  It should look like it's always on, because our eyes can only see blinking around 48hz or so.

I'm pretty sure joecool's right.

[LyleCaldwell]

I think most LEDs will work fine reverse-biased at only 9V.  I mean, it won't be lit when reverse biased, but it won't blow up.  You could always check the data sheets if you aren't sure.

Also, if the frequency is 60hz (or more), it shouldn't blink.  It should look like it's always on, because our eyes can only see blinking around 48hz or so.

I'm pretty sure joecool's right.

I'll order an extra LED as a guinea pig.  I have enough 1N4001s lying around I can put them into service if needed.  These aren't going to be on a board, so I can wing it. Thanks all - still trying to learn when to follow the rules and when to bend them.

[R.G.]

It's generally accepted that it's quieter if you'll put a regular diode in anti-parallel with the LED.

Both ways work, as the series diode with the LED prevents the LED from having any reverse voltage either. The equal currents each way gets a sllight nod from some people.

Just don't reverse bias the LED - it'll break over and die.

[Joecool85]

Quieter?  I'm assuming this is just to light a LED to show the thing is on...how would it make noise?

[gez]

Quieter?  I'm assuming this is just to light a LED to show the thing is on...how would it make noise?

And what else does your AC supply power?

[Basstyra]

Also, if the frequency is 60hz (or more), it shouldn't blink.  It should look like it's always on, because our eyes can only see blinking around 48hz or so.

I'm pretty sure joecool's right.

I'm currenctly working on leds powered with PWM, and they do blink at frequency less than 100hz. Not when you stare at it, but just move your eyes and that's it, they blink. I PWM at 200hz to be sure, and they still blink when they move fast. But here it begins to be a detail !

[d95err]

Quieter?  I'm assuming this is just to light a LED to show the thing is on...how would it make noise?

In my Easyvibe, the oscillator is completely isolated from the signal path (by LED/LDRs). The oscillator only lights the LEDs. However, the ripple caused by the oscillator can cause clicking in effects several pedals *up* the chain...

[Joecool85]

Quieter?  I'm assuming this is just to light a LED to show the thing is on...how would it make noise?

And what else does your AC supply power?

Your amp or whatever.  But it doesn't seem to effect my amp.  My LED taps the AC power before it goes to the power supply board.  So it taps off the transformer, then the power goes through the rectifying diodes, then two 1500uF caps, then to the amp.  The little draw or ripple caused by a LED, in my setup, will not affect anything.

[Paul Perry (Frostwave)]

There is no oscillation in an ordinary 2 cent led. Almost all leds used in modern equipment (except stompboxes!) appear  to blink when you move your eyes, but that's because they are powered by microprocessors that are driving multiplexers that are turning the  diodes on for a few milliseconds each. I've never had a problem wiht a led having reverse voltage across it (but, I havn't gone beyond 30v).

[gez]

Your amp or whatever.  But it doesn't seem to effect my amp.  My LED taps the AC power before it goes to the power supply board.  So it taps off the transformer, then the power goes through the rectifying diodes, then two 1500uF caps, then to the amp.  The little draw or ripple caused by a LED, in my setup, will not affect anything.

If you have no problems, fine.  I think what RG was getting at (and this would be my feeling about the matter) is that with a constant current (which you get with the parallel diode, and the slightly poorer alternative of a smoothing cap) means no spikes at mains frequency.  These could present a problem.

Well, that's my 2p's worth...

[gez]

I've never had a problem wiht a led having reverse voltage across it (but, I havn't gone beyond 30v).

They're faulty, send them back!! 

Just looking at one of my supplier's catalogues, it seems a universal reverse voltage max is around the 5V mark.  I haven't done any testing on tolerances before something fails (why would I?), so I've always carried out basic precautions to prevent Max figures being exceeded.  I'll still play safe! 

[Joecool85]

Yeah, but LEDs are rated around 2V, so all you do is use a resistor to cut it down to that, and you are all set.

[gez]

Yeah, but LEDs are rated around 2V, so all you do is use a resistor to cut it down to that, and you are all set.

Thats the forward threshold, not the reverse rating.  When reversed, the comparatively small current limiting resistor will have little voltage across it and most of the supply will develop across the reverse biased LED (see RG's post to find out what happens to them).

[Joecool85]

Well, regardless what it says on paper, it works in practice.  And I'm just using a cheapo RS LED.