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From: firstname.lastname@example.org (Kooner)
Subject: Re: question on measuring temperarures at high voltages
Date: 23 Oct 2002 10:10:00 -0700
NNTP-Posting-Date: 23 Oct 2002 17:10:00 GMT
"Ban" wrote in message news:...
> "Kooner" schrieb im Newsbeitrag
> > I tried to measure the temperature of a LT1084 voltage regulator that
> > is floating at 10kV. The 10kV is the output of a switching power
> > supply. The regulator is in a TO-247 package. I placed a thermocouple
> > on the body, which is supposed to be isolated from the leads and case
> > (which are at high voltage). The thermocouple was plugged into a hand
> > held temperature meter powered by a 9V battery.
> > My question is why does the meter give an invalid reading? The
> > temperature just bounces around. I'm guessing it's one of two things:
> > 1. The thermocouple acts as an antenna picking up EM fields that
> > induce a voltage.
> Yes, this will probably be the reason. When switching off the circuit to be
> measured do you get a stable reading?
Yes I get a stable reading when I shut the power off.
> > 2. The body is not totally isolated from the high voltage leads.
> > Therefore the 10kV forces a current along the body, to the
> > thermocouple, to the hand held meter to ground. But the hand held
> > meter is not referenced to ground??
> When the meter is placed on an isolating surface(rubber mat, dry wood,
> plastic) it will still work, as long as there is no connection to mains. If
> there is it will never work again.
> I wouldn't hold that meter in the hand! consider that a 10kV potential can
> bridge a substantial distance of air and zap...
> do not fiddle around with voltages that high without a decent training.
> There are temperature sensitive self adhesive plastic dots available, which
> change the color when the set temperature is exceeded.
> There are also infrared sensors for contactless temperature measurements.
> Since your application doesn't require the accuracy of a thermocouple, be
> smart and use one of those.
Well, I do have a little training working with high voltage. I'm a
junior EE studying/measuring someone else’s design of a 10kV
power supply used to power a traveling wave tube (TWT). Our end
product is a microwave amplifier.
The LT1084 is used to power the filament or "heater" in a TWT. I can
power up the LT1084/filament circuit without turning on the high
voltage, and safely measure the power dissipation at 4.5W at worst
case and a case temperature at 115C. The LT1084 is also mounted to a
small crappy heat sink and there's no room for a better one.
The problem is I need to measure the temperature of the LT1084 in the
end product - at high voltage. Thermal dots have a 15C window which
in not accurate enough. Infrared sensors can be better if they have an
emissivity setting on them. But I couldn't use it anyway because the
LT1084 is part of a high voltage module that gets incased in a red
block of potting material to prevent arcing and corona. The block of
potting material has other power dissipating components in it also has
a thermal conductivity of 1 W/(mK) which is a lot better that air. I
can estimate the LT1084 temperature, but with these other variables it
really would be guess. My guess is it gets to hot, but I can't prove
it without a valid measurement.
Wasn't there a famous scientist who said, "We know what me measure."
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