Solid State Fuse

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

Reply to
engineer
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"engineer"

** You have just described a PolySwich.

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

........ Phil

Reply to
Phil Allison

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

Reply to
Phil Hobbs

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.

Reply to
Mook Johnson

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

Reply to
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

Reply to
engineer

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. :-)

--
 Thanks,
    - Win
Reply to
Winfield Hill

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.

Reply to
engineer

I suggest you use some thread locking compound !

Graham

Reply to
Pooh Bear

** Specially on all his own loose screws.

....... Phil

Reply to
Phil Allison

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.

Reply to
engineer

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

Reply to
PeteS

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

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Reply to
Rene Tschaggelar

"engineer" schreef in bericht news: snipped-for-privacy@e56g2000cwe.googlegroups.com...

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]
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Thanks, Frank.
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Reply to
Frank Bemelman

"Phil Allison" schreef in bericht news: snipped-for-privacy@individual.net...

and

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

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Thanks, Frank.
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Reply to
Frank Bemelman

"Phil Allison" schreef in bericht news: snipped-for-privacy@individual.net...

Hahaha, that's funny ;)

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Thanks, Frank.
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Reply to
Frank Bemelman

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 Devereux
Reply to
John Devereux

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

Reply to
GregS

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

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.

That's more than two parts. :-)

--
 Thanks,
    - Win
Reply to
Winfield Hill

Very useful. Have you had your domicile torched lately?

Reply to
engineer

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