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AC LED Light Electronic Circuitry Question
By valterra, Section Newbies Posted on Sat Feb 09, 2008 at 06:28:16 PM MST
Question about resistor getting too hot - not sure which section to post it

I tried to make an AC LED light by doing the following:
Changing AC to DC with a Rectifier
Doing the math for 5 LEDs and then putting a resistor inline.
With a 3.3k resistor and 5 LEDs, I figured the math as follows:
110v - (5 x 3.6v) = 92 volts
92v / 3.3k resistor = 27.79mA current.
It all worked great, and the lights lit up reasonably bright - EXCEPT that the resistor got REAALLLLY hot! It started to turn brown, like it was burning up, so I unplugged the whole thing.
Obviously this resistor must be too small for this use. By small, I mean its wattage rating. Is that right? I believe I got a 1/2 watt resistor, and that's as "big" as they come, at least at Radio Shack.
Am I trying to limit too much current? In other words, do I need to use more LEDs? Or do I need to put more smaller-rated resistors in series? Or is there something else fundamentally wrong with my very simple circuitry?
Thanks in advance for the help!

AC LED Light Electronic Circuitry Question | 19 comments (19 topical)

Re: AC LED Light Electronic Circuitry Question (3.00 / 0) (#1)
by fungus on Sat Feb 09, 2008 at 11:36:50 AM MST

92v* 0.0279A = 2.66W .. I'm not surprised ..

'Two things are infinite: the universe and human stupidity; and I'm not sure about the universe.'-Albert Einstein
Fungus - www.reenergy.co.uk

Re: AC LED Light Electronic Circuitry Question (3.00 / 0) (#3)
by TomW on Sat Feb 09, 2008 at 11:43:00 AM MST

Gus;

Beat me by 3 minutes while I was previewing.

TomW

The Truth is the Truth, even if no one believes it; and a lie is a lie even if everyone believes it

[ Parent ]

Re: AC LED Light Electronic Circuitry Question (3.00 / 0) (#2)
by TomW on Sat Feb 09, 2008 at 11:39:36 AM MST

valterra;

Quick top of the head math tells me thats going to disipate 2.5 watts or so. I think the nearest common rating will be a 5 watt resistor. A 1/4 watt resistor is running about 6X its rating. Remember it is top of the head math so could be wrong.

TomW

The Truth is the Truth, even if no one believes it; and a lie is a lie even if everyone believes it

Re: AC LED Light Electronic Circuitry Question (3.00 / 0) (#4)
by valterra on Sat Feb 09, 2008 at 12:07:07 PM MST

Just some answers would be nice... no need for namecalling. didn't know if the wattage rating was for which part of the circuit.

So more leds would be one solution then... enough to drop the wattage down. Would a greater number of resistors also help?

I don't recall 5W resistors at the store... but radioshack isn't exactly the be all end all of electronic components.

Re: AC LED Light Electronic Circuitry Question (3.00 / 0) (#5)
by Flux on Sat Feb 09, 2008 at 12:24:05 PM MST

You could try 5 680ohm resistors in series or 5 15k resistors in parallel.

Dreadfully inefficient but if you are happy good enough. Not really going to be very efficient unless you use loads of leds.

Capacitor ballast would be more efficient but no way would I try it with components from Radio Shack. The existed here once as Tandy and it was a good day when the disappeared.

Flux

[ Parent ]

Re: AC LED Light Electronic Circuitry Question (3.00 / 0) (#8)
by valterra on Sat Feb 09, 2008 at 01:46:58 PM MST

I got ya. Of course, efficiency is what I am going for. I don't believe in LEDs at any cost! :)

But the only way an AC system is going to be more efficient is by "spending" my wattage using LEDs and generating light, instead of using more resistors and wasting it away as heat.

I got a whole bunchof white LEDs for Christmas, and don't have a bunch of money for other stuff... looking for a good way touse them. :)

[ Parent ]

Re: AC LED Light Electronic Circuitry Question (3.00 / 0) (#7)
by TomW on Sat Feb 09, 2008 at 01:01:23 PM MST

val;

Just some answers would be nice... no need for namecalling.

I read the entire thread and did not see ANY namecalling. I and Gus gave you the answer in the first 2 posts.

I know Angus and I call him "Gus" regularly over on IRC where we chat often. If thats where you got the namecalling from.

This place has just gotten too sensitive to non issue "issues".

You are welcome.

TomW

The Truth is the Truth, even if no one believes it; and a lie is a lie even if everyone believes it

[ Parent ]

Re: AC LED Light Electronic Circuitry Question (3.00 / 0) (#9)
by valterra on Sat Feb 09, 2008 at 02:00:03 PM MST

Thanks for the answer there Tom and Gus - maybe it is the way this website gets formatted on my browser, but it looked like the Einstein quote was part of his response, which seemed out of character for this friendly board. :)

I know some about electricity and all that. I did the math as 3.6 x 5 x .02 = 0.36 Watts... well under the 1/2 watt rating. But it makes more sense that the wattage should be measured with the amount of energy the resistor is resisting.... ;) I get that now.

Since resistors are wasteful by design, I'd get the best bang for my buck by just making something with the 30 or so LEDs that would eat up 110 volts... but that is different than my LED light bulb idea. Sounds more like LED track light or LED Florescent replacement now.

[ Parent ]

Re: AC LED Light Electronic Circuitry Question (3.00 / 0) (#11)
by fungus on Sat Feb 09, 2008 at 05:36:24 PM MST

Nah thats just my signature - thought it was a cool quote :)

'Two things are infinite: the universe and human stupidity; and I'm not sure about the universe.'-Albert Einstein
Fungus - www.reenergy.co.uk
[ Parent ]

Re: AC LED Light Electronic Circuitry Question (3.00 / 0) (#12)
by elvin1949 on Mon Feb 11, 2008 at 05:34:16 AM MST

I agree, It is a cool quote.
later
Elvin

[ Parent ]

Re: AC LED Light Electronic Circuitry Question (3.00 / 0) (#6)
by ghurd on Sat Feb 09, 2008 at 12:24:10 PM MST

Not sure what LEDs you have, but I bet they are pulling more a lot than 28ma at 3.6V.
Might try 10K 1/2W, 8 pieces. 2 sets of 4 parallel, then seriesed. My ratshak still has 1/2W 10K.
G-

Ghurd.info

Re: AC LED Light Electronic Circuitry Question (3.00 / 0) (#10)
by Flux on Sat Feb 09, 2008 at 02:03:08 PM MST

If you want efficiency you will not get it by doing this sort of thing. With only 5 leds you are going to waste 90% of your power in the resistors.

Unless you adopt some form of transformer or reactive ballast you will do no good with direct connection to a high voltage. If you had 30 leds in the string things would start to look acceptable.

May be ok for a commercial manufacturer to sell this sort of thing and kid people they are saving power but it really is hopeless and you won't solve it on the cheap.

Flux

[ Parent ]

Re: AC LED Light Electronic Circuitry Question (3.00 / 0) (#13)
by ghurd on Sun Feb 17, 2008 at 11:07:09 AM MST

Idea.
Switching regulator. A newer/newish cell phone charger.
Mine puts out 5V, very efficient compared to cheap small transformers.
Should be pretty good feeding into a 100 ohm resistor and LED.
One 100 ohm resistor for each LED.

The switchers are very light weight and often are rated for 100~240VAC input.
The old transformer types are heavy with a specific input voltage.
G-
Ghurd.info

Re: AC LED Light Electronic Circuitry Question (3.00 / 0) (#15)
by oztules on Wed Mar 05, 2008 at 02:38:31 AM MST

Ghurd,
I'm assuming 60hz ac here 110v

a. Would a .8uf ac capacitor give a capacitive reactance of about 3k3ohm @60hz. ie ( C=1/2Pi x 60hz x 3300ohm = approx 8x10~7 farads)

b. And if so would this direct connect to his original circuit without resistive losses but with the current limiting required. (no 3k3 resistor)

c. If we ditch the rectifier, would not the leds do that instead.... ( a capacitor and leds only?)

Will this solve his problem regarding loss in the resistor.... no resistor at all ( or maybe a low R just to help a little at start up?)

Just pondering how to tidy up the losses, but not smart enough to know for sure.

...........oztules

[ Parent ]

Re: AC LED Light Electronic Circuitry Question (3.00 / 0) (#16)
by ghurd on Wed Mar 05, 2008 at 06:27:36 AM MST

You certainly remember more AC than I do. Use it or lose it, and I lost it.
I never trusted the cap idea for LEDs. Maybe into a bridge, then another cap, then a resistor, then the LEDs. I still wouldn't trust it myself.
The rectifier could be lost, but it would take back to back reverse connected LEDs, and they would flicker.
Many people can see LEDs flicker at 60Hz. Less than 50% duty cycle and 60Hz would be a bit obvious. It makes some people, like my wife, feel slightly ill.
G-

Ghurd.info
[ Parent ]

Re: AC LED Light Electronic Circuitry Question (3.00 / 0) (#14)
by Opera House on Wed Mar 05, 2008 at 01:18:59 AM MST

I remember the last LED question I answered. Told the poster to use a 10-33K ohm resistor. The poster interpreted that to be 10 ohm to 33,000 ohm. I got a nasty reply when he used a 47 ohm 1/4 watt and everything instantly went up in flames.

Re: AC LED Light Electronic Circuitry Question (3.00 / 0) (#17)
by TomW on Wed Mar 05, 2008 at 07:07:25 AM MST

House;

One reason I hesitate to offer much advice unless I know for a fact the other person has at least a trace of common sense.

Your example illustrates that perfectly. Anyone with any sense would know a range from 10 to 33,000 did not make sense.

Just me.

TomW

The Truth is the Truth, even if no one believes it; and a lie is a lie even if everyone believes it

[ Parent ]

Re: AC LED Light Electronic Circuitry Question (3.00 / 0) (#18)
by valterra on Wed Mar 05, 2008 at 07:42:09 AM MST

I don't think it matters WHAT you're talking about - 10 to 33,000 on ANY scale is a wide range... money, temperature, ram, hard drive space, voltage, amperage, IQ (lol) etc.

[ Parent ]

Re: AC LED Question (Grid to USB) (3.00 / 0) (#19)
by ghurd on Wed Mar 05, 2008 at 09:08:33 AM MST

Saw a grid to USB charger a couple days ago. 5VDC 500ma output.

They had that, and 12VDC to 24VDC input USB output. Same No-Name brand.
I got a 12V to USB for $2. The 12~24VDC unit is based on 34063. Maybe the grid unit is too.

Looks like a resistor swap would change it to 3.15V output for white LEDs in a fairly efficient manner.
And none of the parts are SMD. :-)
http://www.otherpower.com/images/scimages/2050/USBswitcher.jpg

Anyway, this should be easier to do efficiently with cheap off the shelf parts pretty soon.
G-

Ghurd.info

AC LED Light Electronic Circuitry Question | 19 comments (19 topical)

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