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Old 06-07-2008, 10:06 AM   #1
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Surge Protection Primer

Has anyone ever wondered what the little green (or other color) LED labeled 'Protected when Lit" on your surge protection device means?
You would assume that when the LED goes out, it's time to replace the device, right?

Well, I have had this suspicion for years that the "Protected when Lit" LED doesn't indicate anything except that the internal fuse is good, and that the LED is still working!

This morning, I proved my theory.
I noticed that the LED on my Radio Shack single outlet model 61-2133 which was protecting my microwave oven was out.
I opened up the unit, and found what I had anticipated. The LED is wired through a 24K resistor directly across the fused 115VAC mains!

This means that when the LED goes out, it indicates only that the LED has blown, or the fuse has blown. In my case, the appliance connected to the device was still working, so I knew that the fuse had not blown.
Checking the other components inside the device, I found everything looked good, so I will continue to use this device instead of going to Radio Shack to buy another one, as the designers had intended me to do.

Fuel for thought.



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Old 04-09-2009, 10:04 AM   #2
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FW2007: You have part of your theory proven. However, the internal fuse only protects the internal surge components, not the device plugged into the supression unit. The idea is: Most common variety surges will simply get clipped. The really big ones will cause enough current to flow through the protection MOVs to blow the fuse. Yes, the LED is on the fuse side. In such a case of large surge and fuse blown, the MOVs are most likely toasted anyway. LED is out.

Because the AC wired directly through, your device will always be powered, whether the LED is on or not. Whether the fuse is blown or not.

I recently found my LED was out. Quick disassembly revealed the problem wasn't the fuse, but the LED was bad. This is because of a poor design that drives the LED. The LED has about 3 volts across it in the forward direction, when conducting on the positive half of the AC cycle. On the negative half, the LED is back biased and doesn't light. Most LEDs don't tolerate much reverse voltage unless specially designed. I have no way to determine if the original LED had internal reverse-voltage protection diode or not. However, the LED will see 160 volts or so on the reverse
direction, which is extremely high.

My solution was to replace the LED, and put a 1N4454 diode across the LED, to conduct on the negative half of the AC cycle. Any diode will work, such as 1N400x series. The added diode then conducts on the reverse cycle. This produces only 0.7 volts of reverse voltage on the LED, which means it should last forever, intead of only 10 years.

Because newer LEDs are much more efficient, you may need to increase the value of the series resistor from 24K to something like 39K or 47K.
My replacement white LED looked like a night light, so I went up to 47K to
tone it down some.


Galvin Franks is offline   Reply With Quote
Old 04-09-2009, 10:31 AM   #3
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When you talk about the fuse, I am assuming that you are referring to the internal fuse, not the resettable circuit breaker.
These single-outlet devices do not even have a circuit breaker, so I guess that answers my question.

I never bothered to replace the LED on my unit. I just plugged it back in without it.
I'm now thinking of installing a whole-house system, but I would still use the plug-in devices locally.

I really like those efficient LED's. I used several to replace the incandescent bulb in power failure lights. I used two LED's. One for the night light, and another for the flashlight (which turns on when the power fails). The LED will run much longer on the NiCd battery than an incandescent will.

Sorry if this is off-subject, but you mentioned LED's.


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Old 04-09-2009, 10:47 AM   #4
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My whole house units are bad (black ooze coming out of them, usually indicates a mov burn out.) Light is still green
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