Solid State Fuse

[engineer]

Since there are so many technologically informed people here, I'll try
an interesting question.

What kind of device could be contrived to make a self-resetting (after
say, 3 seconds) circuit breaker? It has to be 'solid state' - no
moving parts, handle a few amps of current at 10 volts or so, and work
1000% of the time.

A simple, unstoppable power switch that functions only to interrupt
current for a few seconds upon overload.

What comes to mind is a mosfet with some kind of optoisolator /
comparator setup.. involving a button cell to power the opto.. but this
is the naive view.

I'm thinking of a device which can be spliced into a DC carrying
wire... a self-resetting circuit breaker. It should cost less than $10
to implement in parts,the big requirement is that it could never have
any failure modes except when hit by lightning, so to speak. It has to
stay cool at 1 to 10 amps, in a vacuum.

No moving parts.

Geoff

 

[Phil Allison]

"engineer"

Since there are so many technologically informed people here, I'll try
an interesting question.

What kind of device could be contrived to make a self-resetting (after
say, 3 seconds) circuit breaker? It has to be 'solid state' - no
moving parts, handle a few amps of current at 10 volts or so, and work
1000% of the time.

A simple, unstoppable power switch that functions only to interrupt
current for a few seconds upon overload.

What comes to mind is a mosfet with some kind of optoisolator /
comparator setup.. involving a button cell to power the opto.. but this
is the naive view.

I'm thinking of a device which can be spliced into a DC carrying
wire... a self-resetting circuit breaker. It should cost less than $10
to implement in parts,the big requirement is that it could never have
any failure modes except when hit by lightning, so to speak. It has to
stay cool at 1 to 10 amps, in a vacuum.

No moving parts.

** You have just described a PolySwich.

http://www.circuitprotection.com/polyswitch.asp


As widely used for protecting re-chargeable battery packs, small motors and
transformers plus tweeters in hi-fi speaker boxes.




......... Phil

 

[Phil Hobbs]

It should cost less than $10
to implement in parts,the big requirement is that it could never have
any failure modes except when hit by lightning, so to speak. It has to
stay cool at 1 to 10 amps, in a vacuum.

Umm...why a vacuum? Vacuum is a pretty fine insulator. How are you
planning to get the heat out, then? Or are you just shooting the
breeze? (Not that that isn't allowed, you understand. About 70% of
s.e.d. posts are of that sort.)

Cheers,

Phil Hobbs

 

[Mook Johnson]

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.

 

[engineer]

Going to space for 1 minute, effectively.. but will be low atmosphere
for 8 minutes or so.. I'm not going, but our project is.
It's not a theoretical problem.

The problem with a polyswitch is it resets after the fault is cleared
and power is removed. I need something that will reset automatically,
the idea is a nut will rattle loose and short something, or similar
event, and the circuit needs to get power restored
so I'd imagine a device that would cycle it in an out often (but not
RF often).

Imagine a can with some circuits in it, a battery, and some pocket
change, a few washers, nuts, and. Between say three circuits, a
minimum of two of them have to function to execute an event upon
certain condition or time greater than Tx. As the can is rattled,
wasker, nuts, screws, change, and a dash of fire and brimstone interact
with the circuits occasionally only at the connectors to the boards.
The electronics are put inside, the switch turned on, and the can is
shaken vigorously for a minute, during that minute, the 'mains' have to
be protected from total failure, and certain events have to take place
irrespective of individual or temporary failures.
We haven't worked short-proof into the scheme yet, and can't risk
melting the mains while assuring that anything that can use power gets
it. It's probably the most obsessive project I'll ever be involved
in, it has to work on absolute terms, even though we aren't sending
people up. If we don't have a clean power protection solution we
have to go with hard wire and crossing fingers, and I don't want to go
that way, though it merits consideration.

The specific heat of the chips is often adequate, but at any
appreciable power (>100mW) potting or heat sinks will be used.
There is another cooling idea.. we might bring along a gas cannister.
The trick is running the transmitter in the vacuum, when it is needed
most.. or highly desired.. at some 20 watts.. but that will be dealt
with.


Thanks,
Geoff

 

[engineer]

Going to space for 1 minute, effectively.. but will be low atmosphere
for 8 minutes or so.. I'm not going, but our project is.
It's not a theoretical problem.

The problem with a polyswitch is it resets after the fault is cleared
and power is removed. I need something that will reset automatically,
the idea is a nut will rattle loose and short something, or similar
event, and the circuit needs to get power restored
so I'd imagine a device that would cycle it in an out often (but not
RF often).

Imagine a can with some circuits in it, a battery, and some pocket
change, a few washers, nuts, and. Between say three circuits, a
minimum of two of them have to function to execute an event upon
certain condition or time greater than Tx. As the can is rattled,
wasker, nuts, screws, change, and a dash of fire and brimstone interact
with the circuits occasionally only at the connectors to the boards.
The electronics are put inside, the switch turned on, and the can is
shaken vigorously for a minute, during that minute, the 'mains' have to
be protected from total failure, and certain events have to take place
irrespective of individual or temporary failures.
We haven't worked short-proof into the scheme yet, and can't risk
melting the mains while assuring that anything that can use power gets
it. It's probably the most obsessive project I'll ever be involved
in, it has to work on absolute terms, even though we aren't sending
people up. If we don't have a clean power protection solution we
have to go with hard wire and crossing fingers, and I don't want to go
that way, though it merits consideration.

The specific heat of the chips is often adequate, but at any
appreciable power (>100mW) potting or heat sinks will be used.
There is another cooling idea.. we might bring along a gas cannister.
The trick is running the transmitter in the vacuum, when it is needed
most.. or highly desired.. at some 20 watts.. but that will be dealt
with.


Thanks,
Geoff

 

[Winfield Hill]

engineer wrote...

What kind of device could be contrived to make a self-resetting
(after say, 3 seconds) circuit breaker? It has to be 'solid state'
- no moving parts, handle a few amps of current at 10 volts or so,
and work 1000% of the time.

It's not a difficult problem: I designed an accurate fast electronic
fuse that quickly opens the circuit after the current threshold is
exceeded for a settable number of A-s. (Note, passive devices such
as polyswitches are inaccurate, and cannot be programmed as to current
threshold and fault time.) It can either stay off until the power is
cycled, or follow another electronic rule, such as a timeout or a
software-programmable reset. It lights an indicator when it's blown.

I devised the fuse to protect wideband RF-amplifier modules from
blowing out when they felt they were being mistreated. In our case
the offending condition most likely continued after a fuse-off event,
so a power-cycle reset was deemed appropriate. Not one wideband amp
has died since it was installed, knock-on-wood, knock-on-wood.

 

[engineer]

Thanks for the concision and good points. At an altitude of 62 miles,
there isn't much air, but the high-side time of the flight cycle should
be under 10 minutes.. 1 up 8 down.. but there is some guesswork still.

We have team of technical whizzes, but I thought I'd air the question
out here and get some good ideas from other viewpoints.

 

[Pooh Bear]

the idea is a nut will rattle loose and short something

I suggest you use some thread locking compound !

Graham

 

[Phil Allison]

"Pooh Bear"

I suggest you use some thread locking compound !

** Specially on all his own loose screws.





........ Phil

 

[engineer]

I didnt think it had to be a difficult problem, but I thought there was
a chance there were devices out there that fit the bill, and someone
might have some useful suggestions. I'd like to keep the parts count
down to two, and have yet to think of an implementation that would
operate the gate, but haven't put myself to task either. One
consideration is the mosfet needs to be set up in a way that it
defaults to ON in most conceivable failures within the circuit that
drives it.. A current sense resistor voltage, amplified, on a
comparator that drives an optoisolator that pulls that gate low..
otherwise held high by pullup resistor, with some RC nudging into the
region of desired times.

 

[PeteS]

I didnt think it had to be a difficult problem, but I thought there was
a chance there were devices out there that fit the bill, and someone
might have some useful suggestions. I'd like to keep the parts count
down to two, and have yet to think of an implementation that would
operate the gate, but haven't put myself to task either. One
consideration is the mosfet needs to be set up in a way that it
defaults to ON in most conceivable failures within the circuit that
drives it.. A current sense resistor voltage, amplified, on a
comparator that drives an optoisolator that pulls that gate low..
otherwise held high by pullup resistor, with some RC nudging into the
region of desired times.

One problem not addressed is ambient temperature. if you are sending
something into nearspace, then you'll need to make sure there is
sufficient current in your sense and control circuitry to keep it warm
enough to operate (by the same token it's difficult to dissipate heat
in a near vacuum - a physical heat sink would probably be required)

Just a wrinkle you may wish to consider.

Cheers

PeteS

 

[Rene Tschaggelar]

Since there are so many technologically informed people here, I'll try
an interesting question.

What kind of device could be contrived to make a self-resetting (after
say, 3 seconds) circuit breaker? It has to be 'solid state' - no
moving parts, handle a few amps of current at 10 volts or so, and work
1000% of the time.

A simple, unstoppable power switch that functions only to interrupt
current for a few seconds upon overload.

What comes to mind is a mosfet with some kind of optoisolator /
comparator setup.. involving a button cell to power the opto.. but this
is the naive view.

I'm thinking of a device which can be spliced into a DC carrying
wire... a self-resetting circuit breaker. It should cost less than $10
to implement in parts,the big requirement is that it could never have
any failure modes except when hit by lightning, so to speak. It has to
stay cool at 1 to 10 amps, in a vacuum.

No moving parts.

Have a look at the LT1153, a PMOS control circuit that
gives it the PMOS fuse like characteristics.
Requires a proper VCC, no cable splicing.

Rene

 

[Frank Bemelman]

Going to space for 1 minute, effectively.. but will be low atmosphere
for 8 minutes or so.. I'm not going, but our project is.
It's not a theoretical problem.

The problem with a polyswitch is it resets after the fault is cleared
and power is removed. I need something that will reset automatically,
the idea is a nut will rattle loose and short something, or similar
event, and the circuit needs to get power restored

The polyswitch doesn't know (or care) if the circuit is opened
before or after it. So it already has auto-reset, no need to
cycle the power after the short is removed.

[snip]

 

[Frank Bemelman]

"engineer"


** You have just described a PolySwich.

http://www.circuitprotection.com/polyswitch.asp


As widely used for protecting re-chargeable battery packs, small motors 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?

 

[Frank Bemelman]

"Pooh Bear"


** Specially on all his own loose screws.

Hahaha, that's funny

 

[John Devereux]

The polyswitch doesn't know (or care) if the circuit is opened
before or after it. So it already has auto-reset, no need to
cycle the power after the short is removed.

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.

 

[GregS]

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

I think the ones I used were about .5 to 1 ohm, and took a hike
when they heated up. You have to design the system including the resistance to
get all the proper levels and protection. Tweeters come in all varieties, so
different values are needed.

greg

 

[Winfield Hill]

engineer wrote...

I didnt think it had to be a difficult problem, but I thought
there was a chance there were devices out there that fit the
bill, and someone might have some useful suggestions. I'd like
to keep the parts count down to two,

Oops, you just made the problem difficult, unless you're willing
to design and procure a custom IC.

and have yet to think of an implementation that would operate the
gate, but haven't put myself to task either. One consideration
is the mosfet needs to be set up in a way that it defaults to ON
in most conceivable failures within the circuit that drives it..

Perhaps using a depletion-mode FET would be appropriate. This
defaults to on, even with no applied power. It does require
an opposite polarity voltage from the source to turn it off.

A current sense resistor voltage, amplified, on a comparator
that drives an optoisolator that pulls that gate low...
otherwise held high by pullup resistor, with some RC nudging
into the region of desired times.

That's more than two parts.

 

[engineer]

Very useful. Have you had your domicile torched lately?