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Old 15th September 2013, 12:09 PM   #1
mdm102 is offline mdm102  Russian Federation
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Default Resettable fuses in power rails thoughts

Hi,

What if to use these PTCs instead of traditional fuses in amp power rails, but use these thermally coupled to ensure they're share the same temp and will open together at once. To be sure they specs are close enough to act simultaneously, cutting both rails at once, it could be possible to match every pair using powerful lab power supply to measure current pass ability of each one before they'll open and make a pairs based on these results.
There are not much high voltage high current ones available on the market, but there are some that could be used in up to 100W class AB amps I suggest. There are numbers of PTCs offering quite low initial resistance, but I don't know how far it will rise in case of significant current will flow.
I suggest also matched ones possibly could be used as pairs in each of power rail to double max current or to cut resistance.
What you think?
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Old 15th September 2013, 12:17 PM   #2
Mooly is offline Mooly  United Kingdom
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Not a good idea imo. They have too high an internal resistance (in other words they will modulate the rails at high current) and they are not fast enough for the type of protection needed in amp.
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Old 15th September 2013, 01:05 PM   #3
DUG is offline DUG  Canada
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Not a good idea imo. They have too high an internal resistance (in other words they will modulate the rails at high current) and they are not fast enough for the type of protection needed in amp.
That is what I would also answer.

They are good if:

Automatic reset is required.
You can afford the V drop.
You don't need fast protection.
They are rated for the voltage that they see when open. (I not only let the smoke out once but some internal bits as well)

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Old 15th September 2013, 01:15 PM   #4
FoMoCo is offline FoMoCo  United States
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If you have a fault, or even a bass note of low enough frequency, the two rails will not have equal current and will not open simultaneously. It depends on your amplifier topology as if this is important or not, but the hypothesis of both acting simultaneously is false.

To be on the safe side, I'll be the third no.
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Old 15th September 2013, 01:34 PM   #5
DUG is offline DUG  Canada
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If you want fast "simultaneous" dual rail protection then use dual active current sensing and opto-isolated drive to a normally ON FET in each rail.

You will need a little bit of filtering to prevent false triggering for current surges under normal operation.

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Old 15th September 2013, 02:01 PM   #6
mdm102 is offline mdm102  Russian Federation
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Thanks for the input guys, that was just a theory that have to be checked. I've found this paper http://old.iihr.uiowa.edu/~hml/peopl...ter/pptc01.pdf there is mentioned that there are some of PPTC with a fast reaction time of a few milliseconds, but I haven't found these yet, would be cool to check them in practice if these are exist.
I found such type fuses are been used in amps. I remember some time ago I had a commercial integrated amp Atoll IN50, which had a resettable fuse in it, after pushing it hard for some time it cuts the output and then reset after a several seconds of cooling if turn the volume down. Don't remember where it was located, at the speaker output or at trafo's primary, but if I can remember it was rated 3A, so mostly like to be in the OP as it has 160VA 2x30 or 2x35v trafo which is far lower consumption for that.
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Old 15th September 2013, 02:06 PM   #7
Mooly is offline Mooly  United Kingdom
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Probably in the speaker feed to make it "punter proof"

A common use for these polyswitches was in satellite receivers in the feed to the LNB on the dish. If a short occured in the wiring then the supply was protected and when the fault was cleared so the supply came back up. Thats really their intended type of application.
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Old 15th September 2013, 04:56 PM   #8
DUG is offline DUG  Canada
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...there is mentioned that there are some of PPTC with a fast reaction time of a few milliseconds...
The more overcurrent they are heated by the faster they open...all fuses are like that.
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Old 15th September 2013, 05:55 PM   #9
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Notice that PTC fuses has a max voltage which isn't too high.
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Old 15th September 2013, 06:01 PM   #10
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can we think of some relay circuit that will disconnect power rails when high current flows. and switch on back after certain time?
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