Thermistor for oscillator

[MrThermister]

Hi, I am looking for working examples of the RA53 NTC thermistor -
made by STC/GEC in the UK. It was a favourite means of stabilizing
Wein bridge oscillators. The RA53 was encapsulated in a glass tube
about 25mm long by 2mm in diameter, with two leads and a resistance of
about 5kOhms at RT. It is now sadly obsolete. Am based in the UK,
but willing to deal worldwide. Please respond to
[email protected] or to this group. Thanks and regards,
MrThermister.

 

[Baron]

MrThermister inscribed thus:

Hi, I am looking for working examples of the RA53 NTC thermistor -
made by STC/GEC in the UK. It was a favourite means of stabilizing
Wein bridge oscillators. The RA53 was encapsulated in a glass tube
about 25mm long by 2mm in diameter, with two leads and a resistance
of
about 5kOhms at RT. It is now sadly obsolete. Am based in the UK,
but willing to deal worldwide. Please respond to
[email protected] or to this group. Thanks and regards,
MrThermister.#

A thermistor with similar characteristics is commonly found in
computer power supplies that have variable speed fans. The old HP
Vectra (486 Vintage) used one to good effect. As do some Compaq
models.

 

[Eeyore]

Hi, I am looking for working examples of the RA53 NTC thermistor -
made by STC/GEC in the UK.

Good Luck !

Is it for a repair ?

Graham

 

[Eeyore]

MrThermister inscribed thus:


A thermistor with similar characteristics is commonly found in
computer power supplies that have variable speed fans.

No it isn't.

The RA53 was a low mass (bead) thermistor in a glass tube. The thermal time
constant and indeed the heating power which is critical for oscillator
stabilisation would be wildly different for one thing. I'm also doubtful that
the resistance is going to be the same too.

Graham

 

[Sjouke Burry]

Hi, I am looking for working examples of the RA53 NTC thermistor -
made by STC/GEC in the UK. It was a favourite means of stabilizing
Wein bridge oscillators. The RA53 was encapsulated in a glass tube
about 25mm long by 2mm in diameter, with two leads and a resistance of
about 5kOhms at RT. It is now sadly obsolete. Am based in the UK,
but willing to deal worldwide. Please respond to
[email protected] or to this group. Thanks and regards,
MrThermister.

Use a very small lamp instead,like the ones used in internal lighting
of switches.
The positive temp co. of these can stabilize a wien bridge nicely.
You can even see the resistance change with the small current applied
by an ohm-meter.

 

[legg]

Hi, I am looking for working examples of the RA53 NTC thermistor -
made by STC/GEC in the UK. It was a favourite means of stabilizing
Wein bridge oscillators. The RA53 was encapsulated in a glass tube
about 25mm long by 2mm in diameter, with two leads and a resistance of
about 5kOhms at RT. It is now sadly obsolete. Am based in the UK,
but willing to deal worldwide. Please respond to
[email protected] or to this group. Thanks and regards,
MrThermister.

You might consider adapting the glass-encapsulated ptc's in DO7 or
DO35 body sizes. Thermally isolating the parts might give usable
performance, particularly with DO7, where the thermistor is not
physically coupled to the glass - hence the relatively long thermal
time constant in measurement applications - 55seconds.

http://www.thermistor.com/ptctherms.cfm

http://www.thermometrics.com/htmldocs/numindex.htm#PTCTherm

Removing the leads and replacing them with lighter gauge wire would
increase this further.

Murata makes PTC thermistors in SMD 0402 packages, which is pretty
small, however they don't begin to see 4K7 impedance till their bodies
hit 65degC or more, which is hardly likely to fit into a low-power
scenario.

http://www.murata.com/catalog/r01e.pdf

I recall that the RA53 had to be protected from stray light, if
repeatable performance was to be achieved at 20degC.

RL

 

[legg]

Hi, I am looking for working examples of the RA53 NTC thermistor -
made by STC/GEC in the UK. It was a favourite means of stabilizing
Wein bridge oscillators. The RA53 was encapsulated in a glass tube
about 25mm long by 2mm in diameter, with two leads and a resistance of
about 5kOhms at RT. It is now sadly obsolete. Am based in the UK,
but willing to deal worldwide. Please respond to
[email protected] or to this group. Thanks and regards,
MrThermister.

I'm sort of curious why the ptc ended up being used - I assume it was
because of the greater beta. An NTC of sufficient sensitivity could
ideally be used to stabilize amplitude if it's position were swapped
with the reference R.

RL

 

[Baron]

Eeyore inscribed thus:

No it isn't.

The RA53 was a low mass (bead) thermistor in a glass tube. The
thermal time constant and indeed the heating power which is critical
for oscillator stabilisation would be wildly different for one
thing. I'm also doubtful that the resistance is going to be the same
too.

Graham

Hi Graham,
I agree that the RA53 is a low mass device! However I did say
similar! I had a look at a couple today, that I salvaged. At room
temperature they both measure 4.65K. In terms of size they are about
the same as the glass headed map pins (2.5mm) diameter and 1.5mm
thick.

FWIW I also measured a salvaged thermister from a computer mainboard
(the one under the cpu) which is physically similar in bead size
(about 0.5mm, I couldn't measure the diameter) to the RA53... That
measured a whopping 15K at room temp.

 

[Eeyore]

Eeyore inscribed thus:

I agree that the RA53 is a low mass device! However I did say
similar! I had a look at a couple today, that I salvaged. At room
temperature they both measure 4.65K. In terms of size they are about
the same as the glass headed map pins (2.5mm) diameter and 1.5mm
thick.

I see. That's different to what I thought you meant in that case. I haven't seen
one of those.

FWIW I also measured a salvaged thermister from a computer mainboard
(the one under the cpu) which is physically similar in bead size
(about 0.5mm, I couldn't measure the diameter) to the RA53... That
measured a whopping 15K at room temp.

The ones I've seen of that type have a much larger bead than the RA53 and
critically the leads are much thicker and will conduct heat away, which means
its self-heating won't be any use for stabilising an oscillator.

Graham

 

[Baron]

Eeyore inscribed thus:

I see. That's different to what I thought you meant in that case. I
haven't seen one of those.



The ones I've seen of that type have a much larger bead than the
RA53 and critically the leads are much thicker and will conduct heat
away, which means its self-heating won't be any use for stabilising
an oscillator.

Graham

If the ones from under the cpu could/can be used, the one I measured
today was like a narrow strip of very thin flexible pcb, about an
inch long. The bead looked like a black dot at one end connected to
flat conductors that came out at the other. When a cpu is in place
this would be pressed up against the bottom simply by the springiness
of the lead/s. Crude but effective !