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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 15th May 2021, 10:17 am   #1
livewireless123
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Default Thermistor, any info?

I found some of these in a bits-box while having a sort out, look like thermistors in a glass tube with a tiny bead inside.
At normal room temp' they measure around just over 200K ohm. when heated I can push them down to about10K by holding a soldering iron underneath.
wonder if anyone knows anything more about them and what their applications could be? Mike.
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Old 15th May 2021, 10:29 am   #2
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Default Re: Thermistor, any info?

look like RA53 to me!!

Joe
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Old 15th May 2021, 10:37 am   #3
CambridgeWorks
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Default Re: Thermistor, any info?

Not RA53, they are typically around 5k at 20C ambient.
Rob
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Old 15th May 2021, 10:53 am   #4
ms660
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Default Re: Thermistor, any info?

An STC RA25 would fit the bill at 200k.....first link in here:

https://www.vintage-radio.net/forum/...ad.php?t=47316

Lawrence.
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Old 15th May 2021, 2:01 pm   #5
Restoration73
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Default Re: Thermistor, any info?

The thermistor bead is in a vacuum so I don't think it is designed to sense external heat.
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Old 15th May 2021, 2:44 pm   #6
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Default Re: Thermistor, any info?

They were mostly used as amplitude regulators in valve signal generators.
They are very hard to find if you want one.
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Old 15th May 2021, 2:53 pm   #7
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Default Re: Thermistor, any info?

The spade connectors may suggest some sort of accessory for something?
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Old 15th May 2021, 7:29 pm   #8
Leon Crampin
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Default Re: Thermistor, any info?

I think this is an "anti tinkle" telephone thermistor. The spade connectors on leads are the give away and I seem to remember it should have a yellow rubber sleeve marked 1A-1.

The idea was that with extensions wired to the network and the thermistor in series with the bell, dialling pulses from the extension would not energise the bell - so no tinkle on dial. An incoming call however gives a solid blast of AC which heats the thermistor, lowers its resistance and allows the bell to ring.

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Old 15th May 2021, 8:57 pm   #9
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Default Re: Thermistor, any info?

This is my memories of why & where a thermistor 1a-1 was used. In plans where there were two or more teles in circuit ( pre LJU days) where the teles were wired in PLAN 1A fashion, the bell circuit (from memory) was broken by the switch hook contacts.
But I've just refreshed my memory of n806 and (as far as I can see) the thermistor is only fitted in share service installation models.
i seem to remember this was to try and eliminate something we norther engineers say regularly in areas of poor mains earths of bell tinkle due to stray AC from the mains supply returning to a decent earth at the exchange via the tele bell circuit.
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Old 17th May 2021, 2:50 pm   #10
livewireless123
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Default Re: Thermistor, any info?

Thanks for all the informative replies on this one, I have not had much to do with 'phones apart from fitting new bt plug leads to a batch of old dial 'phones.
I wonder if they could be used to stabilise the amplitude in a valve sig genny.
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Old 17th May 2021, 3:03 pm   #11
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Default Re: Thermistor, any info?

Post #9
The thermistor 1A rang a bell with me. No pun intended.
Whilst sorting my thermistor box a short while back I found some. Written on bag 150K +, probably at 22C or so.
Rob
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Old 19th May 2021, 1:57 am   #12
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Default Re: Thermistor, any info?

John Linsley Hood designed a low-distortion transistorised oscillator published in Wireless World September 1977 which uses an RA53 in the loop amplifier. I built one and it did work a treat with four frequencies from 30Hz to 10kHz IIRC - switching frequencies takes a long time to stabilise, seconds anyway! Same principle as the the lamps in the HP 200 series audio oscillator.
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