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craigb(at)onthenet.com.au
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europabill(at)bellsouth.n
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 Posted: Sat Jan 10, 2009 5:18 am Post subject: polyswitches |
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Craig,
For what it's worth, that's what I'm planning on using in my Europa. I've
designed and etched a printed circuit board for the prototype. There's been
a lot of round & round on the use of poly fuses in lieu of circuit breakers,
but I still think this is the best option. Just turn off the power to that
particular fuse for a few second and it's reset. Don't ever have to
replace, and it's simple.
Just my 2 cents worth.
Bill
A010 Monowheel Classic
Building since '95. When will it ever end.....
---
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frans(at)paardnatuurlijk.
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| Posted: Sat Jan 10, 2009 6:52 am Post subject: polyswitches |
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craig bastin wrote:
It appears to me that these are PTC-resistors. Downside of these things
are that they will never really cut the power, but remain in such a
state that the heat development is just enough to increase the
resistance enough to remain in some sort of balance. Another problem is
their relatively high resistance. You will loose some voltage over these
things, depending on the amount of current you are asking. Near their
cut-off current, the voltage drop is sufficient to convert some of the
energy into heat, it is the heat which will increase resistance even
further, and that is why there is some limit in the amount of current
you can ask from it.
With other words: These things have no clear on-off states, but
everything in between instead. Depending on your purpose, this is
allowable, or it is not.
It might be an option to power non-trivial small devices through them
(like a GPS) where the current remains significant below the cut-off
current, but I would not use them for, let's say, the fuel pumps, the
flaps, or the radio. In general, all equipment with changing current
demands and higher current demands are unsuitable.
Oh, and you can't instantly reset them. They need to cool down first
after they "tripped". Not bad for your GPS (unless you are in the middel
of an GPS-IFR-approach), but not so smart for the fuel-pumps, flaps, etc.
Actually, I would stay away from them.
Just my 2 cents.
--
Frans Veldman
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asarangan(at)gmail.com
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| Posted: Sat Jan 10, 2009 9:29 pm Post subject: polyswitches |
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There are pros and cons.
Nothing beats the reliability and speed of fuses, but they are inconvenient to replace. Circuit breakers are more convenient although less reliable, but is a happy medium. Polyswitches are useful in unreachable areas, but are slower to trip and sensitive to ambient temperature.
It depends on what you are trying to protect. If you are trying to protect your electronics from surges, then polyswitches are probably the least favorable. Your electronics would probably be fried before the switch trips. But in mechanical drives and motors they could be ok.
On Sat, Jan 10, 2009 at 9:50 AM, Frans Veldman <frans(at)paardnatuurlijk.nl (frans(at)paardnatuurlijk.nl)> wrote:
[quote]--> Europa-List message posted by: Frans Veldman <frans(at)paardnatuurlijk.nl (frans(at)paardnatuurlijk.nl)>
craig bastin wrote:
> --> Europa-List message posted by: "craig bastin" <craigb(at)onthenet.com.au (craigb(at)onthenet.com.au)>
>
> for those of you up to the electrics stage, i found these
>
> [url=http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID 592]http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID 592[/url]
>
> they look like they might be a good replacement for circuit breakers
It appears to me that these are PTC-resistors. Downside of these things
are that they will never really cut the power, but remain in such a
state that the heat development is just enough to increase the
resistance enough to remain in some sort of balance. Another problem is
their relatively high resistance. You will loose some voltage over these
things, depending on the amount of current you are asking. Near their
cut-off current, the voltage drop is sufficient to convert some of the
energy into heat, it is the heat which will increase resistance even
further, and that is why there is some limit in the amount of current
you can ask from it.
With other words: These things have no clear on-off states, but
everything in between instead. Depending on your purpose, this is
allowable, or it is not.
It might be an option to power non-trivial small devices through them
(like a GPS) where the current remains significant below the cut-off
current, but I would not use them for, let's say, the fuel pumps, the
flaps, or the radio. In general, all equipment with changing current
demands and higher current demands are unsuitable.
Oh, and you can't instantly reset them. They need to cool down first
after they "tripped". Not bad for your GPS (unless you are in the middel
of an GPS-IFR-approach), but not so smart for the fuel-pumps, flaps, etc.
Actually, I would stay away from them.
Just my 2 cents.
--
Frans Veldman
[b]
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tonyk(at)kaon.co.nz
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| Posted: Sun Jan 11, 2009 12:03 pm Post subject: polyswitches |
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| Quote: | >> It depends on what you are trying to protect. If you are trying to protect your electronics from surges, then polyswitches are probably the least favorable. Your electronics would probably be fried before the switch trips. But in mechanical drives and motors they could be ok.
|
[quote]But then again a circuit breaker won’t protect against surges either as both are current limiting devices, not voltage limiting devices. I know of a number of aircraft flying with polyfuse protection instead of conventional breakers and have heard no adverse comments. I also use these on my model railway and can attest to their effectiveness in shutting the load down well before burning out the wiring and the great thing is that they do automatically reset once the power is removed from the circuit or the fault goes away. In the last rendition of my wiring on the Europa I dumped the majority of conventional breakers and went back to polyfuses . You do need to ensure that there is some means of isolating power from the circuit but this would normally be provided for by the on/off switch. Remember that the primary purpose of a fuse or breaker is to protect the wiring from excessive current - they aren’t designed to protect the equipment on the end of the wire. Tony[b]
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