HP10811 Oscillator Thermal Fuse
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 HP has superceded it with a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
WB0KVV
-------- Forwarded message ----------
From: David G. McGaw david.g.mcgaw@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@febo.com
I too have had a fuse open up due to a failed thermistor in a HP10811.
David N1HAC
Hi
A bit in jest:… that compares to 3,957 incidences of open fuses that had nothing to do with
a thermal runaway. Of those, the majority 3,721 resulted in the 10811 being tossed
in the garbage as “another junker”……(yes, those are estimates, but I’d bet they are close
based on the number of 10811’s made and how flakey those fuses are).
Bob
On May 11, 2017, at 7:04 PM, Donald E. Pauly trojancowboy@gmail.com wrote:
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
WB0KVV
-------- Forwarded message ----------
From: David G. McGaw david.g.mcgaw@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@febo.com
I too have had a fuse open up due to a failed thermistor in a HP10811.
David N1HAC
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
There's an excellent article out there on the web called "Low Voltage, the incompetent ignition source" I highly recommend a read.
I dealt with a case like this a couple years ago. Failed fet in an H bridge caused a fault which the brick SMPS picked up as a short, and went into "hiccup" mode on.
The energy delivered in "hiccup" mode was about 1W average, and that was enough, after several hours, to cause ignition and sustained flame on the PCB.
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.
Jeremy
On Thu, May 11, 2017 at 4:04 PM Donald E. Pauly trojancowboy@gmail.com
wrote:
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
WB0KVV
-------- Forwarded message ----------
From: David G. McGaw david.g.mcgaw@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@febo.com>
I too have had a fuse open up due to a failed thermistor in a HP10811.
David N1HAC
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To unsubscribe, go to
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