Electronics > Projects, Designs, and Technical Stuff
Designing a Burn Down Transformer
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james_s:
There are lots of expensive but simple things out there. The market is small so there's little incentive for anyone to jump in and drive the price down and most of the people using them lack the skills to build one themselves.
sinB:
Thank you all for sharing!


--- Quote from: Marco on July 22, 2019, 08:45:37 pm ---So what does the hi-pot device from the heating wire company cost?
These are quite dangerous devices by the way, that's a lot of power at high voltage ...

--- End quote ---
I could not find the exact one from the heating wire company, but cheap chinese ones is sold for ~500EUR on EBAY.
I may look more into that later, cause.. what's the fun in that.
Dangerous devices indeed. I do not have any plans of playing with this without necessary safety measures.


--- Quote from: MagicSmoker on July 22, 2019, 10:12:35 pm ---Use the neon sign transformer to encourage a partial insulation breakdown to finish the job, so to speak, and the isolation transformer to energize the heating cable at a reduced power level. Old-school NSTs have magnetic shunts in the transformer that turn them into current-limited power sources so are almost ideal for hi-pot testing as-is, while supplying an isolation transformer at less than its rated primary voltage will also limit the maximum current it can deliver, though additional resistance or reactance might be warranted for extra safety.

--- End quote ---
That's a good idea, my first thought was a current limited microwave transformer, but an old school neon transformer sounds better.
Though my guts are telling me that HV DC is better as it is generating a more stable arc while "burning", thus making it more effective.
What's your thoughts on that?

Also, I'm not sure about how much current is needed to generate a sufficient "weld" for lower voltages to pass through, but I guess that will be one of the things I will test on some scrap heating cables when that time comes.

I completely agree with the additional resistance, I'll have to pick up some high voltage resistors.


--- Quote from: digsys on July 22, 2019, 11:48:35 pm ---The only way we could find the break, and luckily it was a ring loop, was to inject a good quality RF signal into the cable and trace out the circuit - then looked for obvious "dips" , "inconsistencies" - unfortunately there were none - then drilled "breaks" in the cable run - re-checked with the RF, and when that didn't help, injected 24VAC and tested a load at the break. A slow process, but worked out ok.

--- End quote ---
Sounds like a lot of fooling around, but interesting anyway, will look into it.


--- Quote from: IanB on July 23, 2019, 12:15:12 am ---First of all: this is an underfloor heating element designed for direct connection to the mains. I do not see the point of an isolation transformer here?

--- End quote ---
One practical reason:
Since the heating cable often is buried on top of a reinforcing mesh which often is earthed (in scandinavia atleast), the current does not have a long way to go before tripping the RCD under these conditions.


--- Quote from: IanB on July 23, 2019, 12:15:12 am ---If the buried heating element has a break and has gone open circuit, then a constant current source with a high compliance voltage can arc across the gap and produce a hot spot where the gap is. A thermal camera may then be used to locate the hot spot under the floor and let you know where to drill.

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That sounds right, though I think a "permanent weld" (like the method in the guide I posted) is better when possible. That way I can supply lower voltages while locating the exact point without having to be using safety equipment. To me it sounds better, although it's a theoretical approach.
IanB:

--- Quote from: sinB on July 23, 2019, 08:48:09 pm ---
--- Quote from: IanB on July 23, 2019, 12:15:12 am ---If the buried heating element has a break and has gone open circuit, then a constant current source with a high compliance voltage can arc across the gap and produce a hot spot where the gap is. A thermal camera may then be used to locate the hot spot under the floor and let you know where to drill.

--- End quote ---
That sounds right, though I think a "permanent weld" (like the method in the guide I posted) is better when possible. That way I can supply lower voltages while locating the exact point without having to be using safety equipment. To me it sounds better, although it's a theoretical approach.

--- End quote ---

If a "permanent weld" means to somehow repair a break in the heating element by melting it back together, then I have my doubts about this being effective or reliable. If you heat up the break in the wire to the point where the ends melt, then surface tension will tend to cause each end to ball up and recede from the gap, making the gap bigger.

IMHO the only reliable way to effect a repair is to dig up the floor and splice the broken ends back together with a proper crimped join.
james_s:
Yeah I certainly wouldn't trust a non-invasive weld like that. That said, maybe it works, if the wire is buried into the floor then breaks would likely be hairline fractures from thermal cycles. I have "fixed" incandescent lamps before by applying power and tilting the thing around until the filament touches and welds. It was a favorite trick of mine when I was a kid.
sinB:

--- Quote from: IanB on July 23, 2019, 11:50:13 pm ---If a "permanent weld" means to somehow repair a break in the heating element by melting it back together, then I have my doubts about this being effective or reliable. If you heat up the break in the wire to the point where the ends melt, then surface tension will tend to cause each end to ball up and recede from the gap, making the gap bigger.

IMHO the only reliable way to effect a repair is to dig up the floor and splice the broken ends back together with a proper crimped join.

--- End quote ---
Those were not so carefully chosen words from my part.

What I meant was to spark the gap with high voltage to make enough "permanent" conductivity for lower voltage to pass through before finding the exact point of the damage with the thermal camera while the heating cable is energized with lower voltage. And then of course chisel down at that point and repairing the heating cable.

I may be misunderstood, but the guide I linked to suggested so.

I guess it also depends on factors such as depth. If the cable is not too deep the high voltage arc should leave a thermal footprint alone. Then if the cable is deeper it would be best to have enough conductivity to supply the cable with lower voltage over a longer period of time and then see where the cable gets cold (and of course a brighter spot as a result of higher resistance at the end).
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