Chinese dimmer just had thermal runaway and burned up

I've been reading about dimmers here and thought I would try dimming my outdoor LED lights with one of the Chinese 12-24VDC 30A PWM dimmers found on eBay and elsewhere. The lights are running at 12VDC and consume 8.5 A. Dimmer was installed, but noticed right away that it was very warm. In an hour, almost too hot to touch. Should not have been, not at the current levels I noted. After several hours, it began to smoke and burn out. In the meantime, lights went to full brightness and probably then some. Needless to say, the only way I'll be using any more of these is with an in line fuse as well. Of course, the dimmer doesn't have one inside.

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I sure wouldn't use one even with a line fuse.


Phil Hobbs

Dr Philip C D Hobbs 
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Phil Hobbs

I'm sure it's capable of 30 amps - for a minute at most. Sounds like ty pical Chinese overestimation. I remember a guy who brought what he insisted was a 1000W amplifier in for repair (said so on the box). I showed him ho w it drew 125W max off the line. I guess maybe it could do that 500W for a thousand of a second as the rail filters discharged into a 2 ohm speaker.

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I did a little research today. I actually have/had two of these dimmers. They're all over the net. I had one that seemed to work fine (also claimed 30 A) and then I just got this other one I had all the trouble with. They look almost identical except the most recent one has an amber plastic cover that covers the screw inputs. I decided to disassemble them both. Circuit boards are different. With the one that never heats much, there is a much larger transistor than the one that burned. There are other differences too, but really don't know without going back to the images I took. So, even though they looked nearly identical, they weren't. Why would they decide to change something to the inferior when they had a working unit that was more capable beats me unless they were trying to save a few cents.

I've never trusted the Chinese stuff. I've unknowingly had their transistor substitutes in the past that never lasted as long as a Japanese equivalent. I also see 30V, 20 A switching supplies with variable voltage all over the place too, but darned if I'm ever going to buy one.

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Coming in late on this.

30A @ 24V = 720 watts. Unless the dimmer weighs as much as a standard bri ck, it is not going to pass that much current for more than a very few micr oseconds. So much for the "Rating".

Now, 8.5A at 12V = 102 watts. So, using an average of 15ma/led, that is s omething like 560 typical LEDs. Allow 450 for standard losses. So, any numb er substantially fewer than 450 LEDs in the string will represent current g oing up in heat. Think of it as 0.23 watts per LED. So, if there are only 2

00 LEDs in the string, over 50 watts is going up in heat - somewhere - as y ou experienced.

These are very rough 'back of the envelope' calculations, involving a S.W.A .G. or three. But, when considering such devices for such uses, common-sens e basic calculations can avoid very unfortunate down-line results.

Peter Wieck Melrose Park, PA

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Peter W.




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I've been using one for two years to brighten and dim about 10 amps worth of LED lights. It burned out, most likely from a power outage where the electricity kept going on and off rapidly for a few seconds last year. I always worry when that happens because it can ruin anything connected and running at the time. Anyway, just after that the

lights didn't come on that night. When I checked the fuses, both were blown. I had two fuses ahead of the dimmer, one in each line. Replaced

those and still nothing from that dimmer. So I think it was that power surge that did the damage. Second one has been going strong for 18 months now.

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it was

have been,


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That's it, no more Chinese dimmers! I just can't trust them. This week, I managed to pick up a linear variable voltage and current Astron

12 amp power supply. It won't be as efficient as switching units, but Astron is apparently a winner with amateur radio folks and reviews tout its reliability. I looked at the schematic and it seems that all of their units use the same general design, just different transformers and

components of course as size increases. I like the fact that the schematics are readily available unlike this Chinese stuff where it is next to impossible to find one! I have even been able to peek at several internally online of course, but apparently people often tweak and modify them and post images and such online. Nice to see the innards .

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Astron has been using the same basic design for many years for their linear supplies. There is almost no difference in any of them, just more pass transistors and bigger transformers. That same circit has been used in many other power supplies with minor variations. All designed around the 723 voltage regulator. It is almost like a cockroach and the circuit will not die as it is very reliable.

I have a 50 amp one that has been on 24/7 for around 30 years. Only cut off when I moved and to rearange the equiipment.

They do have two weakness in that if you push their ratings the transistor socket pins get loose and do not make good contact. If the power is cut off and right back on say as can hapen during a storm the over voltage protection will trip and you have to cut it off,count to about 10 slow and then turn it back on.

I have another supply of no name that uses the same basic circuit and it has been on over 40 years.

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Ralph Mowery

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