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SNIP
have an awfully high holding current; that is, the
current to maintain the device temp to hold it in the tripped
state.
Per the manufacture's data sheet, they define "hold current: maximum
current device will sustain for 4 hours without tripping in 20C still air.
This is not the current to hold the device in the
tripped state. The current to hold the device in the tripped state can be
calculated from the manufactures value Pd (Power dissipated in tripped
state). A 30R 5 amp has a Pd of 3 W, which would yield about 1/4 amp
flow to keep the device in the tripped state.
As far as I can see, PTCs do not have an interrupt
rating like a circuit breaker would. Since there are no mechanical
contacts that could fuse together before the device trips, I am not sure the
interrupt rating in the classic sense is appropriate. The max current the
device can handle is 40A (R30 series), after 40A the device blows all its smoke
out and stops conducting electricity (that is until the device is repacked with
smoke, available from Lucas.) To ensure the devices would work in my
application, I bought one and set up a test across a fully charged 12v
battery. 20+ direct shorts across the battery yielded positive tripping
and no warming of the 20 ga test leads.
I feel comfortable that PTCs, properly applied in
my application will work well, but please keep in mind this is just one man's
opinion and anyone else's mileage may vary.
Joe
----- Original Message -----
Sent: Sunday, February 18, 2007 10:26
PM
Subject: [FlyRotary] Re: CBs and
fuses
> SNIP
> >
> > I am curious what PTCs you are
using. I used PTCs
> > on some circuits. I used
> > a
Buss ATC form breaker, removed the bimetallic and
> > soldered a PTC
to the
> > frame. Then plugged into a ATC fuseblock.
This
> > worked well, bit I am
> > curious if you found a
packaged PTC.
> >
> > Thanks,
> > Joe
> >
Concerning the PTC devices, I was interested in them
and a friend also building a Cozy wants to use the PTC devices but I am
reluctant to say to use them. If correct, many of the ones Mouser lists
that allow high current (1 - 18 amp) seem to have an awfully high holding
current; that is, the current to maintain the device temp to hold it in the
tripped state. Be careful of the device you choose, it's holding current
could be 50% of the trip value (Littel 30R series for instance). This
would not be good for protecting the wiring.
Also, you need to know the max interrupt
current. A battery can dump a lot of current through a fuse before it
goes and it must be able to interrupt that current. A typical 5 amp fuse
can flow over 75 amp in a battery supply situation before it blows.
Some fuses might actually weld together and never interrupt the current when
used in a battery powered situation. I had to do some UL testing on
some fuses many years ago and it would surprise you how much current
a small fuse will pass before blowing. Will your wire take the
current that a direct short can deliver, of course different for each
situation, something to think about even when using breakers.
This is why they need to rated for battery supply use.
Wendell
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