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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 27th Jul 2018, 11:06 am   #1
60 oldjohn
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Default What is this modern part ?

It is in the supply lead of a 6 v battery, and measures ~4mm dia. A pin head is included for scale.


TFL, John.
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Old 27th Jul 2018, 11:24 am   #2
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Default Re: What is this modern part ?

Most likely a
GEMOV/ AKA Metal Oxide Varistor/ AKS voltage dependant resistor.
GEMOV = General Electric Metal Oxide Varistor.

I would "guess" you stole that from a computer power supply.
Photos are too blurry to read the part number

Joe
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Old 27th Jul 2018, 11:26 am   #3
Boater Sam
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Default Re: What is this modern part ?

Fuse.
Can't quite read the printing.
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Old 27th Jul 2018, 11:30 am   #4
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Default Re: What is this modern part ?

I agree, it's unlikely to be a varistor in that position, though it does look like one. Measure the resistance.
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Old 27th Jul 2018, 11:32 am   #5
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Default Re: What is this modern part ?

PPTC, re-settable fuse.

Lawrence.
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Old 27th Jul 2018, 11:40 am   #6
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Default Re: What is this modern part ?

Colloquially (originally trade-name) "polyfuse"- you often find them in applications like this where there is potential for a high fault current.
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Old 27th Jul 2018, 11:58 am   #7
60 oldjohn
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Default Re: What is this modern part ?

Part measures 3.2 ohms either way. Zoom in on this photo.


John.
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Old 27th Jul 2018, 12:17 pm   #8
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Default Re: What is this modern part ?

Could be a Polyswitch self-resetting fuse originally introduced by Raychem, now part of Tyco Electronics (IIRC).
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Old 27th Jul 2018, 2:38 pm   #9
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Default Re: What is this modern part ?

Agree that it is probably a polyswitch or similar.
Widely used to protect battery circuits from faults.
They normally have a low resistance, but overcurrent causes heating which increases the resistance and produces more heat, until the device latches into a high temperature and NEAR open circuit state.
The drawback is that a small current flows indefinitely, and if not noticed will over discharge the battery in a day or two.
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Old 27th Jul 2018, 6:48 pm   #10
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Default Re: What is this modern part ?

I have seen the larger Polyswitch, I now believe this is one. Thanks everyone, this thread may be closed.


John.
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Old 27th Jul 2018, 7:07 pm   #11
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Question Re: What is this modern part ?

Quote:
Originally Posted by broadgage View Post
The drawback is that a small current flows indefinitely, and if not noticed will over discharge the battery in a day or two.
Since I know nothing about these things, I have found the above to be interesting and educational. But I'm a bit confused by the above quote. If this device has the general properties of a fuse, it will, therefore, be in series with a load. And if that load is 'switched off', then no current will flow from the battery. Therefore, I fail to understand the comment "a small current flows indefinitely . . . (etc)". Where is that current going?

Al.
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Old 27th Jul 2018, 7:14 pm   #12
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Default Re: What is this modern part ?

I think the suggestion is that a heavy current may flow under fault conditions. Without the protection of the Polyfuse, it may be sufficient to be a fire risk etc., with the protection, it's limited to a safe level.
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Old 27th Jul 2018, 7:49 pm   #13
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Default Re: What is this modern part ?

Broadgage was highlighting the problem of an apparently unpowered device being neglected for some time once the polyswitch (apologies for my earlier nomenclature slip!) has gone high resistance and thus trickling the battery down. (I think). A blown fuse would at least completely break the circuit. (With the proviso that I have very occasionally found that a failed fuse still registers a high, rather than infinite, resistance- this being highly unusual and anomalous, though, rather than a normal feature of operation as with the polyswitch.)

I know from experience that it's all too easy to overlook the fact that something still has power applied if it appears to be dormant without any obvious power lights etc.!
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Old 27th Jul 2018, 8:39 pm   #14
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Default Re: What is this modern part ?

A fuse that has gone bang and plated the inside of the tube with a metal film can still exhibit a measurable resistance.
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Old 27th Jul 2018, 9:16 pm   #15
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Default Re: What is this modern part ?

Quote:
Originally Posted by Skywave View Post
Since I know nothing about these things, I have found the above to be interesting and educational. But I'm a bit confused by the above quote. If this device has the general properties of a fuse, it will, therefore, be in series with a load. And if that load is 'switched off', then no current will flow from the battery. Therefore, I fail to understand the comment "a small current flows indefinitely . . . (etc)". Where is that current going?
These devices can often be used in place of fuses, but are not directly comparable.
A tripped polyswitch has a high resistance, but is not a true open circuit like a blown fuse.
Therefore if the overload or short circuit persists, rather than being momentary, the polyswitch will remain in a high resistance state, and a small current will flow from the battery, via the polyswitch and the fault. This will deep discharge the battery and probably kill it.

This current is not fixed, but varies INVERSELY according to the supply voltage. So a tripped polyswitch in a 6 volt circuit will pass about twice the current that it would in a 12 volt circuit.
All very odd and counter intuitive.
The current that a tripped polyswitch passes also depends on the ambient temperature and airflow.

The original polyswitches also had a feature whereby they changed colour slightly when tripped. Mustard yellow at room temperature, and a slightly deeper and more orange colour when tripped.

They come in various voltages, but unlike fuses, a high voltage polyswitch wont work correctly at a very much reduced voltage.
At a very rough guide, the circuit voltage should be at least 10% of the maximum rated voltage of the polyswitch.
A 250 volt fuse will work fine in a 6 volt circuit.
A 250 volt polyswitch wont work well in a 6 volt circuit, the internal resistance might drop a volt or more, which is of no consequence on a 240 volt circuit, but is a serious loss with only 6 volts.

They are interesting devices and I suggest buying a few small ones to experiment with. They are very cheap online.
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Old 27th Jul 2018, 11:11 pm   #16
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Thumbs up Re: What is this modern part ?

Ah! I understand now: thank you.
As I said, I know nothing about these devices - or, rather, didn't - until now. I think I will take up your idea and buy a few with which to experiment. I can now see that in some applications they could be very useful indeed.

Al.
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Old 28th Jul 2018, 12:07 am   #17
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Default Re: What is this modern part ?

They are very good for protecting small transformers because they pick up heat from the core and windings as well as the normal mode of operation.
They make the equipment user wait for the transformer to cool down before they reset.
This makes the user do something about the fault instead of adding a bit of foil from a fag packet to a conventional fuse.
They dissipate about a watt when tripped for a big one.
http://uk.farnell.com/w/search/prl/r.../3?st=polyfuse
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