Electronics > Beginners

Wattage Limiting Device

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DrG:
I had to replace a floor lamp recently. It was a cheap lamp that I bought at least 10 years ago. The problem was that the base insert (looks something like a dumbbell weight) cracks with age. The gravel and clay that it is filled with comes out. It is a real mess and once it starts, there is not much you can do. I tried some glue earlier on a similar cheap lamp that suffered from the same affliction – gave me an extra year or so. Pretty pathetic that the plastic is so cheap it wears out with no use other than sitting there – good thing I don’t wear out that quickly sitting on the couch.

The lamp had this box on the cord. Every once in a while, if I plugged something else into the outlet, I would have to recycle the rocker switch to get the lamp to work. Apart from that, it was inconsequential and I never gave it much thought.



I did a little due diligence and found this short pdf[ http://www.zingear.com/ueditor/php/upload/file/20170403/1491207378338810.pdf on the unit   It is a 180 watt limiting device for lamps. I had to take a look inside.



I can certainly recognize that relay and the LM324 op amp but do not know much else. For example, is R2 (that wire) basically a fuse? Does the pot set a trip point? In any event, I put it back together again and it now has a place in my junk box(es). I read that I am not supposed to just ask “How does X work”, but I am curious to learn a little bit more about them and why I don’t see them around. Were they ever very common?

Gregg:
R2 is a shunt; or you could call it a sense resistor.  The device uses it for sensing the current to the load. 

TerminalJack505:
That's an interesting circuit.  How big is that mains capacitor on the bottom right-hand side of the board?  And what is the component in the T-92 package?  Are there any components on the other side--like a high power resistor?

amyk:
Trace out the schematic, and others here will help you understand its operation.

NiHaoMike:
That seems to be an incredibly complex way to make a circuit breaker?

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