Wattage Limiting Device - Page 1
Forum section: electronics / beginners

author: DrG
author_rank: Super Contributor
timestamp: February 05, 2020, 11:31:48 pm
content: 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.pdfon 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?

author: Gregg
author_rank: Super Contributor
timestamp: February 06, 2020, 12:57:39 am
content: R2 is a shunt; or you could call it a sense resistor. The device uses it for sensing the current to the load.

author: TerminalJack505
author_rank: Super Contributor
timestamp: February 06, 2020, 02:13:26 am
content: 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?

author: amyk
author_rank: Super Contributor
timestamp: February 06, 2020, 02:18:30 am
content: Trace out the schematic, and others here will help you understand its operation.

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 06, 2020, 02:31:00 am
content: That seems to be an incredibly complex way to make a circuit breaker?

author: DrG
author_rank: Super Contributor
timestamp: February 06, 2020, 03:50:14 am
content: I probably should have done a little better job of recording the components and even, as suggested, try to draft a schematic. The rocker switch is soldered to the board and I had to re-solder them to put it all back. I was a little reluctant to go back in there and take the camera out...then I remembered that I am the one who asked about it - and a phone camera is always handy.The T92 is a 78L12 regulator. Not surprising that it is a regulator but it is a 9V relay so I am surprised that it is not a 78L09.You can see the size of the AC cap and the value 155K I am reading as 15uF (15 x 1000000 pf), 10%, 250VThere is nothing on the other side of the circuit board. The grey block is a 224 cap (metal film?).The shunt resistor R2 is marked on the circuit board as .08 2 watt - are you sure that isn't going to act like a fuse - although at 2W that wouldn't make sense, I just don't get why the wire and not a "normal" component? Edit: but a wire of specified diameter and length might be easier to supply than a 0.08 ohm, 2W resister...so n/m.I guess that I am curious because I just don't see these on cheap floor lamps. I am wondering...Around the time of purchase, I remember that there was a hubub about halogen lamps and fire. By limiting the wattage, you could stop folks from using 200 W halogen bulbs in there - that kind of thing. I just don't know, but that was the direction I was thinking about.

author: TerminalJack505
author_rank: Super Contributor
timestamp: February 06, 2020, 04:43:57 am
content: Okay. Thanks for answering my questions. That is a 1.5uF cap. Which means that they are powering the circuit using a capacitive power supply. So, from a power supply standpoint, it likely looks like the attached.I'm not sure about the rest--so far as how the AC current is being sensed and how they are using the LM324 to 'latch' and require a power cycle when it trips.

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 06, 2020, 04:50:41 am
content: Quote from: DrG on February 06, 2020, 03:50:14 amI would have solved that by putting a thermal switch or fuse in the bulb holder.

author: DrG
author_rank: Super Contributor
timestamp: February 06, 2020, 04:52:09 am
content: Quote from: TerminalJack505 on February 06, 2020, 04:43:57 am1.5uF? oops yes, sorry about thathttp://kaizerpowerelectronics.dk/theory/capacitor-code-table/Third digit=5 so the multiplier is 100,000 = 1500000 pF = 1.5 uFNeed more time with the schematic you posted - thanksEdit: Actually understanding *some* of it, seems like the input to the 7812 would be well beyond the 35V max?

author: DrG
author_rank: Super Contributor
timestamp: February 06, 2020, 05:14:19 am
content: Quote from: NiHaoMike on February 06, 2020, 04:50:41 amI wonder about that. I read that halogen bulbs burn hotter than incandescent bulbs - I am assuming that would hold true for two bulbs of the same wattage of each type. So, simplistically, temperature more than wattage would be what you want to limit, fire hazard-wise. Hey, maybe that is why I don't see these around...then again, I don't know if there are thermal fuses in cheap floor lamps - I do know that they are very fond of putting warning labels on them.

author: TerminalJack505
author_rank: Super Contributor
timestamp: February 06, 2020, 06:08:46 am
content: Quote from: DrG on February 06, 2020, 04:52:09 amThe input to the regulator, in the simulator, is steady at 14.08V.The circuit is kind of hard to understand but the capacitor that is in series with the 120VAC will only allow so much charge to pass through. This, and the fact that the circuit will use a minimum amount of current prevents the voltage from rising any further. Basically, the circuit has to burn off as much charge as the capacitor feeds in.This would be very inefficient for circuits that have dynamic current requirements. In the case of this particular circuit, when it trips and cuts the current to the relay (which is an exceptional condition), that current has to go somewhere else. In the circuit as shown, it would go through the Zener diode.

author: DrG
author_rank: Super Contributor
timestamp: February 06, 2020, 03:48:46 pm
content: Quote from: TerminalJack505 on February 06, 2020, 06:08:46 amSome of this is actually sinking in. When I have used a 78L series regulator with mains, it has always been mains to transformer to bridge rectifier to regulator and filter caps before and after. Just didn't focus on the R1, R2, C1 (as in your sim). Ignoring certain sections is probably a very bad way to try to understand a circuit. This is why I posted it in beginner's section.Not so embarrassed though. I am a retired research scientist (unrelated field as you might guess) and I really appreciate folks, like yourself, taking the time to explain some things. It does have an effect. Thanks again.The red denotes where the rocker switch is attached, connecting/disconnecting L out of the circuit.

author: TerminalJack505
author_rank: Super Contributor
timestamp: February 06, 2020, 05:20:10 pm
content: The lack of a transformer means this particular circuit isn't isolated from mains voltage. So be careful if you play around with it while it is powered up!

author: DrG
author_rank: Super Contributor
timestamp: February 06, 2020, 08:10:03 pm
content: Quote from: TerminalJack505 on February 06, 2020, 05:20:10 pmNoted!

author: DrG
author_rank: Super Contributor
timestamp: February 07, 2020, 08:27:37 pm
content: I decided to go in another direction.

author: TerminalJack505
author_rank: Super Contributor
timestamp: February 07, 2020, 08:34:23 pm
content: If you were just going to throw it in a box and forget then that is probably the best use you can make of it.You have some spare parts for other projects now. The case looks like it could come in handy.