Strange electrical happening....

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Greg_Lewis
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Strange electrical happening....

Post by Greg_Lewis »

We have two upper cabinets in the kitchen that have stained glass in the doors. Years ago I put some festoon bulbs in there to light up the inside so the stained glass would look good when the doors were closed. I've decided to convert to LED strips. The festoon bulbs are 12 volts, so to test the LED strips I just pulled out a festoon bulb and temporarily connected one of the LED strips to one of the festoon socket/clip thingies. The strip lit up and I figured I was good to go.

Now here's where it gets interesting. I installed the festoon bulbs about ten years ago and put a 12-volt power source inside the cabinetry to run them. When I made the final hookup for the LED strips, I discovered that the power source was 12 volt AC and would not power the LEDs. Of course not, as LEDs require DC. But why did one of the strips light up when I connected it to the festoon wiring loom? The festoons were wired parallel, three on a loom, and I connected one lead from the LED strip to one terminal on the festoon clip, and the other lead to the other terminal. The loom I used may have had two festoon bulbs remaining in it; I don't remember but I tend to think not considering the reason for my test. But I don't see how that would matter. If LEDs only run on DC, where did the DC come from?
Greg Lewis, Prop.
Eyeball Engineering — Home of the dull toolbit.
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Bill Shields
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Re: Strange electrical happening....

Post by Bill Shields »

Some LEDs will run on 12vac as they have a built in rectifier in the power supply to supply DC from any available power source

Some will run on anything from 220 VAC down to maybe 8VDC
Too many things going on to bother listing them.
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Greg_Lewis
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Re: Strange electrical happening....

Post by Greg_Lewis »

Bill:
This is a 2835 SMD LED strip. The supplier's spec is 12V DC. There is no separate power supply or rectifier. They would not light when connected to the power supply but did light when connected to the wiring loom, which I had made 10 years ago, and which was just wire to connect the festoon bulbs to the AC power supply.

There is a small SMD component on the strip for each set of three LEDs. I would expect it to be a resistor, but could that be a rectifier? The strip is labeled along its length with + and - for attaching leads.
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Greg Lewis, Prop.
Eyeball Engineering — Home of the dull toolbit.
Our motto: "That looks about right."
Celebrating 35 years of turning perfectly good metal into bits of useless scrap.
John Hasler
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Re: Strange electrical happening....

Post by John Hasler »

LED -> Light Emitting Diode. LEDs are diodes (rectifiers) but not very good ones. They have a low reverse breakdown voltage and quite a bit of leakage current. Leaving them connected to AC for long may damage them. The SMD part is a resistor.
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Greg_Lewis
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Re: Strange electrical happening....

Post by Greg_Lewis »

Thanks, John. So in a few minutes test on AC they would work. The all do work now on DC. I tested them all on a battery and have ordered a DC power supply to run them in the installation.
Greg Lewis, Prop.
Eyeball Engineering — Home of the dull toolbit.
Our motto: "That looks about right."
Celebrating 35 years of turning perfectly good metal into bits of useless scrap.
atunguyd
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Re: Strange electrical happening....

Post by atunguyd »

As John said they are still diodes and as such with conduct on the positive half of the AC waveform. So they will flicker with AC. Essentially you have a half wave rectifier now.

The negative half is going to be hell on them though.

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