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T³: Heated Blanket Teardown

Sarah takes apart a non-functional heated blanket to have a look around

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While at a local thrift store recently, I came across a non-functioning heating blanket. The place was willing to give it to me for very cheap since they just wanted it off their hands. I know a thing or two about electronics and was curious to see what was in the guts of this blanket, so I figured that I would take it apart on camera for anyone else that might also be curious.

When I was taking it apart, I found a fuse which I decided to take out and test. It was both broken and a common enough rating that I decided to try and replace it, to see if that would fix the heating element. A lot of the time fuses blow because another electronic piece has stopped functioning, so I knew it might take more than that, but I decided to give it the ol' college try.

Once I opened it up, it was pretty simple, and consisted of what you would expect.

Instides of the heated blanket

There was a switch for power, a potentiometer for the heat level, an LED indicator and lots of capacitors and resistors of various sizes.

Back side of the heated blanket PCB

On the back there were more resistors in chip form, lots of rectifier diodes, what I assume is another AC to DC converter, regular diodes, a LM339DG comparator chip for dealing with input from the dial, and an MC14541B programmable timer chip for the 10-hour switch-off safety feature.

Photo of the origional fuse and the replacement fuse in the heated blanket

There was a 2A 250V fuse on the board, which I removed and replaced with a fuse with the same rating (although a slightly larger size, since I couldn't find the same size in my toolbox). I didn't think this would fix the blanket, and when I plugged it in after replacing the fuse, the light (which I found out was not an LED but a little bulb) didn't light up, so I figured it wasn't working.

However – as we were filming me saying it wasn't working, I felt the blanket heating up! The little bulb just wasn't functional, and may have been what burnt the fuse out. YAY! I am a happy little hacker.

Hope you enjoyed my teardown, let me know what you think and if you have done anything similar. Until next time! ^_^

Comments 19 comments

  • What's that music?

  • oh holy molly... There is much more to fuses than the voltage and current ratings.

    And if you feel the need to play with the AC mains side of stuff, please consider using a variac to gradually apply power while watching input current. I do this whether testing a 5VA or 1000 kVA transformer.

  • Tube light? ARGH! :)

    There was once a world without LEDs :) The Venerable NE2 still lives! :)


  • One other thing: at about 4:12 into the video, you are talking "giga ohm resistors" -- it seems to me you've probably mis-read the code, as that would be higher than the trace-to-trace resistance of the printed circuit board.

    Also, I suspect that the thing over in the corner, at about 2:15 into the video, that you're guessing is a "wave rectifier", is likely something along the lines of an SCR that the unit uses to control the power to the actual blanket. After all, the control unit is basically just a glorified light-dimmer.

  • Yes - I did something similar once. And it was a career changer! After 4 years of floundering in college changing majors and just not 'finding myself' in general, I stopped at a Goodwill store and saw an FM receiver - tube type from the 50's or so - and it was marked down to 2 or 3 dollars as it was non-working. I bought it, took it home and looked it over. Power on made the rectifier tube begin to overheat so there looked to be a possible shorted component somewhere. Further inspection showed a B+ wire passing through a sheet metal bulkhead had it's insulation cut through. Replacing that fixed it. I felt so great at fixing it that I changed majors to electronics and had a successful 40 year career from that point on.

    So finding old stuff at a thrift store can have hidden unexpected benefits.

  • I had a heating pad once that I was using as a warmer for a sick pet. The automatic 2-hour shutoff wasn't a feature I wanted, so I opened the controller, and found well-marked pads for choosing 2 or 10 hour shutoff and to disable it entirely. I documented it on my website, and I've actually gotten a couple of emails thanking me for doing so, as other people doing similar things could use the information.

  • Yes, everyone is correct. The bulb is in fact a neon lamp. The ballast resistor is located next to JP3 and in the photo is clearly damaged (burnt). Probably the rating on this resistor is too low. It looks to be a 1/2 watt. Try replacing it with a 1 watt. The lamp should now light up.

    • I'm not convinced that burned resistor is for the neon lamp. Following the traces for the neon bulb makes me think that the surface mount R12 (47k) is the ballast resistor.

      It's difficult to tell in the photo where the burnt resistor passes through the board, but I think it is attached to the SE1 wire. (The return for the H1 loop of the blanket?) Maybe the blanket only half works with that resistor blown. What blew that resistor? Maybe the H1 loop of the blanket is suspect? (Suspect electric blankets are fire-hazards, not something I would want to sleep under.)

    • In my experience (dating back a half century), 1/2 watt ballast resistor is plenty at 120V (or even 240V) for a small neon lamp. (Back when neon lamps were common, 1/2 watt resistors were by far the least expensive. A 1/4 watt cost maybe 4x what a 1/2 watt back in those days, so you'd only use the 1/4 watt if you needed the small physical size.) (You might need a 1 watt if it's powered by 1kV or higher -- I have seen that, though usually the neon lamp is there to "bleed" the high voltage down to something safer when power is removed.) I'd be suspicious that either the lamp actually failed (I have seen one or two short), and/or the wrong ballast resistor got put into place. (I've got a Dr. appointment to get to so don't have time to look up the value and, hey, last time I needed that number was about 40 years ago, and I've slept since then!)

      • OK, now that I'm back from the eye doctor (still a bit blurry), looking at the DigiKey site mentioned elsewhere (and their datasheet), I'd say that something around 150K to 200K ohms is a good guess. (I've seen manufacturing errors where a 150 ohm resistor is substituted for a 150K -- which lets the "magic smoke" out of things.)

        Anyway, 120V across a 150K resistor is going to dissipate 0.096 watts, so assuming there's a zero voltage drop on the lamp (not likely!) a 1/4 watt resistor is overkill.

      • Yeah, a half watt is plenty at a reasonable current, but many manufacturers run the neon bulb at way too much current to get more brightness. In those circumstances, the resistor usually burns before the neon bulb dies.

  • Sarah,

    I think your hair is lovely (and I have a bit of envy -- my hair is far too curly to have it that long). However, for safety's sake, I really think you should do something to "tie it back" when you're soldering or working with a soldering iron.


    • Thank you, and you are very correct, I should have it back, and I should also have been wearing safety goggles (which I wasn't). I tie my hair back and wear glasses when I solder at my desk =P Though there have been many times I have burnt my hair a little from a soldering iron.

  • Always nice to find a low cost fix and keep something out of the landfill. If you want to try replacing the little bulb, it looks to be a neon lamp. You can get a replacement for under a dollar - http://www.digikey.com/short/3z2vc1

    • That's pretty cool, thanks! I had just never seen one, do you know what kind of life span they have?

      • Definitely a neon bulb. I'm not sure what life span, but as Member #134773 says they generally only work above 70-80V (depending on the specific model). What is interesting about them is with DC only one of the two electrodes inside will glow. Reverse the DC polarity and the other electrode will glow. I forget which one, but it might be the negative/ground side as electrons would be flying off that end and colliding with the neon atoms causing them to glow. (I could have my reasoning backwards though.)

        If you have a variable DC supply that can supply up to 100VDC you can test this for yourself. Connect the legs of a loose (known working) neon lamp to your supply. Set the output to zero and then turn the supply on. Slowly raise the voltage until you see a little glow on one of the electrodes. Turn off the supply, and switch the lamp around and then turn it back on. The other electrode should glow. Be careful though. With too much voltage you could push too much current through the bulb. This will heat up the gas inside, and if the gas gets too hot it may cause the glass envelope to fail showering the surrounding area with glass fragments. So wear your safety glasses and/or do the testing inside a clear plastic bottle that would contain any glass shrapnel. This happened to a classmate of mine when we were doing a lab on inductive kick. She hooked her circuit up wrong and got a huge kick. Burned up the momentary switch and exploded her neon bulb with a bright flash. We teased her that her shadow was etched on the wall behind her. ;-)

        With AC, each positive and negative peak cause the two electrodes to glow alternately. Persistence of vision causes us simple humans to think both are lit at the same time. (Remember, the DC voltage that causes the lamp to glow would be the AC Peak voltage. That's why the AC and DC ratings for these types of lamps are different on DigiKey.)

        I found this page with a Google search for "neon lamp failure modes". Apparently the normal failure of these lamps is an increasing firing voltage, which translates to a higher resistance. Not something that would trip the fuse, so don't be too confident that you found the source of the fuse failing.

      • I agree that it's a neon lamp. Trace the circuit, though: as with an LED, there should be a "limiting resistor" (also known as a "ballast resistor") in series with it. If the lamp is the cause of the problem, the resistor is probably blown too. BTW, the neon lamp requires in the neighborhood of 70 V to "fire", so likely is in parallel with the heating element. Neons will work on DC, but since they can also work directly on AC, they are usually connected to the AC line (though a ballast resistor). I would be inclined to open the control box back up, and after tracking down the ballast resistor and checking it with the controller not plugged into the wall, use a multimeter in AC Volts, probably 200 V range, to see if there's voltage across the neon lamp, or across the ballast resistor (indicating current is getting through) with the thing plugged in and turned on.

        As for life span, IIRC, they quoted "50,000 hours", though in reality, the only thing that really kills a neon lamp is "abuse". When they go bad due to usage, they often do so slowly, starting to "flicker" in the last few thousand hours.

        Oh, yes: Back in the 60s, we had an electric blanket that had quit working, but my mother, being very thrifty, kept it and we used it as a non-electric blanket.

  • I hacked a heating blanket: my wife and I wanted to have a timer so that we could set it, go to bed. and an hour (or whenever) later, the heating level would go down or go off (separate preferences for his side and her side). I wrote about it at http://www.galacticstudios.org/heating-pad/

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