Battery charger gismo

[duke37]

My brother has given me a very unenthusiastic battery charger to look at. I have evicted the spiders and brushed out the chaff.
There is no name or model, it is labelled 6 Amp and has a 6V/12V switch. It does not have an ammeter but has 6 leds.

The main charging circuit has a bridge rectifier, two 5Wr1J fat resistors in parallel. a gismo and a 10A fuse.

The auxilliary circiut contans an LM358 dual op-amp, several S9014 npn transistors and the leds.

Action!
With a bulb as 1.5A load the first led lights and the bulb lights up for about 15 seconds, then the current gradually drops and rests at about 250mA. The only current control is the gismo which gets HOT.

Gismo
I assume this is a thermistor. It is about 19mm square and about 2mm thick. labelled
L30
700
150T
I do not find this when googling. I would like to try a replacement.

Help appreciated.

 

[KrisBlueNZ]

Is it a PTC resettable fuse? Something like http://www.digikey.com/product-detail/en/RHE1500/RHE1500-ND/467783 ? Can you post a photo?

 

[duke37]

Looks just like the digikey picture. I would not describe it as a fuse since it takes considerable time but perhaps this is how they work.
I do not know what parameters to look for, perhaps 6A 20V.

 

[KrisBlueNZ]

They can take a while to open, depending on the ratio between the actual current and the trip current. If the actual current is only slightly higher than the trip current, they can take minutes to open. If it's several times the trip current, they open in a few seconds.

They're specified with a maximum holding current and a minimum trip current, and a withstand voltage. Grab some app notes from Raychem and other manufacturers. They're pretty well documented.

 

[duke37]

Thanks Kris, I don't know how you do it all.

I have found on cpc.co.uk
Bourns multifuse radial
MF-R600-0-99 6A 30V Stock FF01367
Holding 6A Tripping 12A

They have one in stock and I can buy one. I also want some other bits from them so will get an order off today.

Never met one of these before, you learn something every day. unfortunately I forget something every day!

 

[KrisBlueNZ]

Cool Trevor Yeah, polyswitches aka polyfuses etc have been around for over a decade now. They're good for reducing service call-outs - once the fault is fixed, everything returns to normal.

I know what you mean about forgetting something every day! I used these things extensively in my job, so I'm not likely to forget about them. As for everything else I've forgotten... even Google couldn't keep track of that much information!