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Thermistor for oscillator

Discussion in 'Electronic Components' started by MrThermister, Jul 19, 2007.

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  1. MrThermister

    MrThermister Guest

    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
    or to this group. Thanks and regards,
  2. Baron

    Baron Guest

    MrThermister inscribed thus:
    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
  3. Eeyore

    Eeyore Guest

    Good Luck !

    Is it for a repair ?

  4. Eeyore

    Eeyore Guest

    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.

  5. Sjouke Burry

    Sjouke Burry Guest

    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.
  6. legg

    legg Guest

    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.

    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

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

  7. legg

    legg Guest

    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.

  8. Baron

    Baron Guest

    Eeyore inscribed thus:
    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

    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.
  9. Eeyore

    Eeyore Guest

    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.

  10. Baron

    Baron Guest

    Eeyore inscribed thus:
    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 !
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