Microamp Fuses

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Jason C Wells
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Microamp Fuses

Post by Jason C Wells »

I looked at the big online retailers for the smallest fuse I could find. They were in the milliamp range. Can anyone point me to a device used to protect circuits in the 10, 50, 100 microamp range?

Thanks,
Jason
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Richard Hull
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Re: Microamp Fuses

Post by Richard Hull »

Wow! Micro amps! It is tough to have a true fusible link in that range. You will probably have to go to something totally electronic which would be setable. (variable trip point) If it is in a HV circuit, it would probably have to float or be isolated in some heroic fashion. Fairly easy to design overall, even if in an HV situation.

Curent spikes in this range would be common and a bear in constant reseting unless you also adjusted the fault delay time (slo-blow)

Inquiring minds want to know............What is the app for such a "fuse".

Richard Hull
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Fusion is the energy of the future....and it always will be
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Jason C Wells
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Re: Microamp Fuses

Post by Jason C Wells »

The app is to keep from blowing up microammeters.
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Re: Microamp Fuses

Post by Jeroen Vriesman »

Isn't it easier to protect the meters with a zener-like circuit?

What is the max voltage over the meter? A circuit with an opamp, zener and mosfet to limit the voltage over the meter should do the trick.
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Bob Reite
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Re: Microamp Fuses

Post by Bob Reite »

two silicon diodes across the meter terminals (connected opposite polarity) will limit the voltage across the meter to 0.6 volts which should be enough to keep it from getting damaged, even though the meter still might "peg" if the duration of the fault is long enough. You'll need to check with a controlled current source that the diodes do not affect the full scale accuracy of the meter you are using. I've done it with 50 microamp movements and it works fine as a typical 50 microamp meter has a resistance of 3500 ohms which gives 0.175 volts at full scale.
The more reactive the materials, the more spectacular the failures.
The testing isn't over until the prototype is destroyed.
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Jason C Wells
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Re: Microamp Fuses

Post by Jason C Wells »

And if the diodes fail?
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Re: Microamp Fuses

Post by John Futter »

Use the anti parallel diodes as suggested
if you are worried over the diodes going open circuit put a zener across the meter movement as well ie 2.7 volt they fail short circuit
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Rich Feldman
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Re: Microamp Fuses

Post by Rich Feldman »

Jason,

When selecting any overcurrent protection device, you need to start with:

- maximum fault current which must be protected against. This applies to fuse-like devices as well as shunt protectors, such as the diodes under discussion.

- maximum voltage when the circuit has opened. Applies only to fuses etc. that are supposed to switch to a very high resistance state.

For example, look up common glass cylinder fuses. e.g. Cooper Bussman AGC 5-amp and 1/20-amp.
Both are rated for 250 VAC. In a high voltage circuit, sustained current through a blown fuse could set fire to the fuse holder.
Both can interrupt 10,000 amps in a 125 volt circuit. But in 250-volt circuits, the fuses max fault currents (interrupting ratings) are respectively only 200A and 35A.

If this is for a kilovoltmeter application, there is a consideration other than protecting the meter movement.
In case the meter coil fails open, or becomes accidentally disconnected (e.g. by broken wire),
the associated connections in back of control panel could see the full kilovoltage.
Usual practice is to prevent that with a voltage-limiting shunt across the meter. PN diodes, neon lamp, gas tube surge arrestor...
All models are wrong; some models are useful. -- George Box
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Re: Microamp Fuses

Post by Corby Dawson »

If you use a power shottky diodes across the coil they will limit at plus minus .3VDC also you can get ones rated at 1 amp easily so I doubt they will fail!

Corby
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