[time-nuts] Fwd: HP10811 Oscillator Thermal Fuse

Jeremy Nichols jn6wfo at gmail.com
Thu May 11 20:03:05 EDT 2017

I had a vaguely similar situation with a pressure switch for the pump in my
home's water supply. The electrical contacts in the pressure switch have to
handle about 10 Amps at 250 Volts (our line voltage is on the high side).
Over the years, the contacts burn and become "ohmic" contacts, gradually
increasing in resistance. Early one morning the contacts finally got so
resistive they couldn't pass enough current to start the pump. Since the
water pressure never came up, the switch never shut off. It got hotter and
hotter and finally burst into flame. I found out about it at 4:00 AM when a
smoke alarm woke me. An extinguisher was sufficient to dowse the flames
although the local fire fighters were called as a precaution, me being
still half asleep. When I replaced the pressure switch, I added a thermal
fuse as protection should that problem ever recur.


On Thu, May 11, 2017 at 4:04 PM Donald E. Pauly <trojancowboy at gmail.com>

> https://www.febo.com/pipermail/time-nuts/2017-May/date.html
> This is the third report of an open thermistor which would have
> resulted in a fire in an HP10811 oven.  There are a dozen other
> problems that could cause such a fire.  See
> http://gonascent.com/papers/hp/hp10811/thermal.jpg .  The leads are
> long enough that they can be soldered quickly without a heat sink if
> the socket is to be replaced for better reliability.  A hemostat can
> also be used to heatsink the leads one at a time.  We have the 108° C
> fuse which is prone to nuisance blows.  HP has superceded it with a
> 125° C version.  Panasonic seems to make a direct replacement.  It is
> stocked by Digikey for under a dollar.  I can find no reports of a
> nuisance blow of the 125° version of the thermal fuse in a HP10811.
> All nuisance blows seem to be in the 108° version.
> In the 1970s, many consumer electronic products with 60 cps power
> transformers had a thermal fuse inside them.  I bought a Sony real to
> real stereo tape deck which was not working with that problem.  It was
> a nuisance blow since no problems existed in the tape deck.  I
> installed a fine gauge piece of solder since the fuse was close to
> 180° C.  I used it for over two years with no problems.
> Later model switching power supplies have windings of several volts
> per turn.  Shorts in these transformers will blow the fuse or destroy
> the switching transistors.  A 60 cps transformer operates at a small
> fraction of a volt per turn. A few shorted turns in a will not draw an
> excessive primary current but merely causes a hot spot.  The hot spot
> will grow as additional turns short.  Line current will not be greatly
> excessive even as the transformer heats up.  It can catch fire well
> before the fuse blows.
> I saw a living room that caught fire because of a 60 cps transformer
> in a stereo receiver.  The line breaker never tripped.  But for the
> grace of G-d, the house would have burnt down.  A thermal fuse in the
> transformer would have prevented several thousand dollars of damage.
> That would have paid for over a thousand thermal fuses.
> πθ°μΩω±√·Γλ
> -------- Forwarded message ----------
> From: David G. McGaw <david.g.mcgaw at dartmouth.edu>
> Date: Wed, May 10, 2017 at 8:20 PM
> Subject: Re: [time-nuts] Fwd: HP10811 Oscillator Thermal Fuse
> To: Discussion of precise time and frequency measurement <
> time-nuts at febo.com>
> I too have had a fuse open up due to a failed thermistor in a HP10811.
> David N1HAC
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