Solid State Fuse

[engineer]

It's indeed true that at the first look, few people think about
conditions.

It'll have warmed up to 0 C roughly at apogee. as temperature increases
with altitude at some point, but in the course of getting there,
travelling a mile per second will heat up the nose a bit. Vacuum like
conditions are a 10 minute thing, and I think dissipation is the only
real concern.. to provide enough thermal mass where necessary.

Thanks Pete though, for giving it some thought.

Geoff

 

[engineer]

Two, some, not many, simple, uncomplicated...

it's not hard to understand the goals.

 

[Mark]

I don't think that can be relied on. Once the polyswitch is in the
high resistance (tripped) state, then it requires a lot *less* current
to keep it there (I^2 R). So it could be that when the short circuit
is removed, the circuit keeps drawing enough current to keep it
tripped.

John
Plot the VI curve for the load and for the poly, if they cross at one
point, then the system is monostable will RESET without removing the
power. If they cross at 2 or more points then the system is bi-stabe
and can hang up as you describe. But for the load lines to cross at 2
points, one has to exhibit a negative resistance region. Does the
poly exhibit a negative resistance region? I don't think that it
does... like an incandescent light bulb, the resistance changes but
it never goes negative.

Mark



I think if both the load and the poly are positive resistances then

 

[engineer]

Good suggestion. A quick search led to a NASA webpage, and they talk
about it a little there.

The idea of splicing isn't important, a ground in the circuit is quite
alright.

 

[engineer]

How about a FET, a oneshot for the 3 second delay and a comparator to
trigger it.

should be under $10

you gave nto specification on speed of the fuse. Emulating a slow blow would
not be a problem.

If used in a vacuum, you'll have to provide a conductive path for the heat
generated in the current sense resistor and the Mosfet Rds. Even if it is
low if there is no convection a few 10s of mW can cause significant
temperature rise unless you provide a thermally conductive path to a
heatsink.

The objective, on a good day, would be to remove the offending short
before our small batteries are drained or wire insulation melts.

Heat dissipation is a primary concern, RF corona effects are another.
Shorts are unlikely, but no one could predict what events can take
place. Short and long blasts of gases at 1000 F are a real hazard..
might not happen, but as likely could.

 

[John Devereux]

John
Plot the VI curve for the load and for the poly, if they cross at one
point, then the system is monostable will RESET without removing the
power. If they cross at 2 or more points then the system is bi-stabe
and can hang up as you describe. But for the load lines to cross at 2
points, one has to exhibit a negative resistance region. Does the
poly exhibit a negative resistance region? I don't think that it
does... like an incandescent light bulb, the resistance changes but
it never goes negative.

I think if both the load and the poly are positive resistances then

I will assume you were about to say "the system will not hang up" or
words to that effect!

Anyway, I *have* had systems where this happens. It had a negative
voltage convertor, and other switch mode supplies in it. I guess these
must have presented the negative resistance load which you describe.

Thinking about it, amost *all* circuits with switch mode supplies
probably have negative resistance, since they draw less current the
more voltage you feed them with.

 

[engineer]

www.sugarshot.org

 

[engineer]

www.sugarshot.org

 

[[email protected]]

Sweet mother of Jesus, he's serious. I wouldn't let you within 300
yards of a lit match, never mind a rocket...

 

[engineer]

There will be threadlocker. Thanks though.

 

[engineer]

I love how cheeky everyone is.. mind you it's better than being a jerk
outright.

Mr. Pete. I think about this stuff all the time. I'm in conversations
with 10 and up to 50 project members daily.

And many a circuit jock pops in, sure as anything that they are the
first one to ever think of something.

Project wide, it's more my concern than anyone else's, which is the way
I would have it.

 

[engineer]

If you understand what's involved with the match and treat it with due
respect, it's safe to use.

Au contraire, when all is said and done, safety will be designed and
tested in... this is no 'experiment', it's a very deliberate effort.

 

[[email protected]]

"if you understand what's involved with the match"

That's just it; you haven't demonstrated that you understand much
relating to electronics.

"it's a very deliberate effort"

I've seen your deliberate efforts. Just warn people in a 400 mile
radius OK?

 

[John Larkin]

John
Plot the VI curve for the load and for the poly, if they cross at one
point, then the system is monostable will RESET without removing the
power. If they cross at 2 or more points then the system is bi-stabe
and can hang up as you describe. But for the load lines to cross at 2
points, one has to exhibit a negative resistance region. Does the
poly exhibit a negative resistance region? I don't think that it
does... like an incandescent light bulb, the resistance changes but
it never goes negative.

For an incandescent, more voltage always makes more current, at least
until it burns out. For the polyfuse, more voltage makes less current
in the active region, approaching a
constant-power/constant-temperature mode, so there's incremental
negative resistance, like a tunnel diode but a lot slower. As you
point out, it must have a negative resistance region to be bistable.

So, is it possible to make an oscillator from a polyfuse?

John

 

[Ken Taylor]

It's indeed true that at the first look, few people think about
conditions.

It'll have warmed up to 0 C roughly at apogee. as temperature increases
with altitude at some point, but in the course of getting there,
travelling a mile per second will heat up the nose a bit. Vacuum like
conditions are a 10 minute thing, and I think dissipation is the only
real concern.. to provide enough thermal mass where necessary.

Thanks Pete though, for giving it some thought.

Geoff

Have you spoken with the likes of the Surrey micro-satellite group?
Don't have a link on hand sorry (will look later though) but they're
probably handy being UK also. Or folk like Microsat
(http://www.microsatsystems.com)

Ken

 

[Winfield Hill]

engineer wrote...

Two, some, not many, simple, uncomplicated...
it's not hard to understand the goals.

My circuit is simple and it has 10 essential parts,
plus a few convenience parts, like an LED indicator.

 

[Terry Given]

I will assume you were about to say "the system will not hang up" or
words to that effect!

Anyway, I *have* had systems where this happens. It had a negative
voltage convertor, and other switch mode supplies in it. I guess these
must have presented the negative resistance load which you describe.

Thinking about it, amost *all* circuits with switch mode supplies
probably have negative resistance, since they draw less current the
more voltage you feed them with.

yep, they do. thats why an input filter can make a stable smps
oscillate. Likewise driving a smps with a smps can cause problems - Fred
Lee's crowd at VPEC have written dozens, if not hundreds, of papers on
this particular subject.

Cheers
Terry

 

[engineer]

The rocket part of it is being handled by people who are experts in
that aspect, and
everything happens with a view to safety first, and then success.

There is a review process to get permission to launch something, and we
will be in the middle
of a desert probably. I think some kind of horizontal excursion is
out of the question, as the rocket will be equipped to recognize
attitude with substantial reserve resources, and made to cancel the
flight in any such event.. and at any time... despite my confidence in
my immediate group, I remain convinced that the more critical control
funcitions are going to work precisely because I'm the fall guy for the
works, no matter who designs which parts.

Serious rocketeers always think first about safety of others.

Also, rocketry is a subject unto itself. A casual observer would have
little idea of physical operation and what goes into development.

The basic process is a simple execution of events based on sensing.
What I'm adding to it is
failsafe operation, and some calling of the tune for the band.

These guys are very clever. Some RF, some software, analog, digital,
math, physics, and then most are very strong in technical electronics.


It stands to be a significant achievment in both global networking and
amateur rocketry, and could influence future directions in satellite
deployment methods.

Ok, 400 miles, will do.

 

[Phil Allison]

"Frank Bemelman"
"Phil Allison"

and transformers plus tweeters in hi-fi speaker boxes.

That's a nice application, with tweeters. Didn't know that. What
are typical values used for tweeters?

** For the average 20 / 25 mm dome tweeter, a device rated to trip at just
under 1 amp is typical. This allows the long term rms current flow to be
close to the max dissipation of the voice coil - ie about 10 watts (
allowing that the copper wire heats and R goes high by 50% )

An important point is to fit the device in series with any LC filter
components that feed the tweeter so when it trips and goes high R, the LC
filter is still loaded by the low impedance of tweeter and NOT high one of
the device.

This is to avoid the LC filter turning into a series tuned circuit,
presenting a very low impedance to the drive amp at and near resonance and
simultaneously boosting the voltage level delivered to the device and
exceeding it ratings.




........ Phil

 

[Genome]

yep, they do. thats why an input filter can make a stable smps oscillate.
Likewise driving a smps with a smps can cause problems - Fred Lee's crowd
at VPEC have written dozens, if not hundreds, of papers on this particular
subject.

Cheers
Terry

I think you might find that Middlebrook had something to do with it.......
Mind you I have no idea about the historical perspective.

http://ardem.com/

Ooooh

Input Filter Considerations in Design and Application of Switching
Regulators
IEEE Industry Applications Society Annual Meeting, 1976 Record, 366-382
(IEEE Publication 76CH1122-1-IA).

BTW, who the **** is Fred Lee? Perchance.... does he take Middlebrooks
explanation of the problem and generate shitloads of IEEE papers about how
to solve it with an AVR implementing an appropriate PID controller in
suftware?

DNA

DNA