[time-nuts] Fwd: HP10811 Oscillator Thermal Fuse

Donald E. Pauly trojancowboy at gmail.com
Tue May 9 21:48:34 EDT 2017


I disagree about permanently jumpering the thermal fuse.  The
oscillator is rated down to -55° C and the oven can maintain 82° C at
that temperature.  This is a 137° C rise.  It is likely capable of
more heat than that.  At 25° C, it would therefore be able to reach at
least 25+137=162° C.  This is near solder melting temperature and
would quickly cook the components.  It is rated to work at 71° which
would give a temperature of 208° C and do serious damage.  That
thermal fuse is very important.  There are a dozen failures which
could result in a thermal runaway.

This web page discusses the fact that a thermal fuse should be 30° C
above normal operating temperature to prevent nuisance blows.
HP originally used a 26° C margin and later changed it to a 33°
margin.  This explains the frequent failures.  This poster used the
wrong type of fuse but the correct type is commercially available.


---------- Forwarded message ----------
From: Bob kb8tq <kb8tq at n1k.org>
Date: Mon, May 8, 2017 at 6:07 PM
Subject: Re: [time-nuts] HP10811 Oscillator Thermal Fuse
To: Discussion of precise time and frequency measurement <time-nuts at febo.com>
Cc: "Donald E. Pauly" <trojancowboy at gmail.com>


You will get a lot of diversity of opinion on the topic of the thermal
fuse on the 10811. My feeling is that they are a nuisance and
contribute very little to the design. I’d just short it out and move
on. In the era of failure prone heater transistors or faulty
thermistors, the fuse may have made sense. That era ended before the
10811 went into production.


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