More on CHT probe replacement

As long as the higher wattage resistor fits physically and is the
appropriate resistance (82 ohms in this case), it will work just
fine... It will be just loafing at a small fraction of a watt and so
should never 'burn out from thermal stress.... Physically larger is
usually more mechanically robust, as Dave surmises...

Resistors start at 1/8 watt at most electronic suppliers and go up into
the hundreds of watts... The larger units will not be available in all
resistances, especially in the megohm range... For what you need it
in the airplane, a wirewound "flameproof" of 2 watts is fine...

denny

Jim ...

The three main resistor characteristics are value (ohms), tolerance (%), and
power handling capability (watts). There are other second-order effects,
but let's not worry about them right now.

You've got the value nailed.

You've got the tolerance pretty well in hand. Tolerance is pretty much
linear with dollars. That is, a 5% resistor might cost a dime. A 0.5%
resistor may cost a dollar. A 0.05% resistor might be $10, and so on. For
your "hammer, file, kick in the edges and weld shut" project 5% is well
within any reasonable design consideration.

Wattage is a little like a wooden deck. A fly lands on it, no big deal.
You drive a motorcycle on it and it may groan, but will probably not break.
Drive a HummV on it and you've got matchsticks. Ohm told us that the
power dissipated in a resistor is equal to the current through the resistor
(squared) times the resistor value (P = I^2 * R).

Let's say, just for grins, that the current through the resistor is 10 mA
(ten milliamperes) or so. This 82 ohm resistor will then be dissipating
about 8 milliwatts. (0.008 watts). A 3 watt resistor won't even start to
get warm.

The general rule of thumb is that you select a resistor with double the
expected wattage next-size-up. That is, if your resistor is actually
dissipating 1 watt, double is 2 watts, next size up is 3 watts. However, to
be dissipating 1 watt, the current through the resistor is going to be about
100 mA, which is WAY THE HELL more than I really think you are dissipating.
That's why I asked you to check the current through the resistor (or the
voltage across it, same thing) so we could see what the resistor was REALLY
dissipating. You'd probably want to make this measurement fairly close to
the top end of the scale where the current is most probably the greatest.

Some posters have postulated that a larger wattage resistor will be more
resistant to vibration and the like. I respectfully disagree. I'd much
rather something about the size and mass of a toothpick than size and mass
of a large ball bearing bouncing around. If we can get a handle on exactly
how much power this sucker is dissipating, we may be able to do a Rat Shack
fix for a buck or so.

Keep me posted.

Jim



"Jim Burns" wrote in message
...
Thanks Jim, I appreciate the link.

Not having a background in electronics, what ultimate difference would a
resistor of higher wattage rating have on the circuit, the gauge, or the
probe? If the circuit is only drawing mA's, 1) are there "smaller" mA
rated
82ohm resistors available or 2) with a mA rated resistor would the
required
ohm rating change? Forgive my ignorance regarding resistors, I basically
understand what they do, but do not know the implications of the
sizes/capacities. I'm just looking for a way to get out of several hours
of
tedious PITA work with my upper torso stuffed up underneath our panel.

Damn, you're a pretty good teacher! Thanks. Everything you wrote was
basically what I thought, but wasn't sure of. Seems every electrical item
has a sizing procedure, and that's what I didn't know. I can size motors,
fuses, wiring, and larger items, but I had no idea how to figure the
required size of a resistor.

Thanks again for the explanation and your help. I'll hook it up when I get
the parts and let you know the draw.
Jim

"RST Engineering" wrote in message
...
Jim ...

The three main resistor characteristics are value (ohms), tolerance (%),
and
power handling capability (watts). There are other second-order effects,
but let's not worry about them right now.

You've got the value nailed.

You've got the tolerance pretty well in hand. Tolerance is pretty much
linear with dollars. That is, a 5% resistor might cost a dime. A 0.5%
resistor may cost a dollar. A 0.05% resistor might be $10, and so on.
For
your "hammer, file, kick in the edges and weld shut" project 5% is well
within any reasonable design consideration.

Wattage is a little like a wooden deck. A fly lands on it, no big deal.
You drive a motorcycle on it and it may groan, but will probably not
break.
Drive a HummV on it and you've got matchsticks. Ohm told us that the
power dissipated in a resistor is equal to the current through the
resistor
(squared) times the resistor value (P = I^2 * R).

Let's say, just for grins, that the current through the resistor is 10 mA
(ten milliamperes) or so. This 82 ohm resistor will then be dissipating
about 8 milliwatts. (0.008 watts). A 3 watt resistor won't even start to
get warm.

The general rule of thumb is that you select a resistor with double the
expected wattage next-size-up. That is, if your resistor is actually
dissipating 1 watt, double is 2 watts, next size up is 3 watts. However,
to
be dissipating 1 watt, the current through the resistor is going to be
about
100 mA, which is WAY THE HELL more than I really think you are
dissipating.
That's why I asked you to check the current through the resistor (or the
voltage across it, same thing) so we could see what the resistor was
REALLY
dissipating. You'd probably want to make this measurement fairly close to
the top end of the scale where the current is most probably the greatest.

Some posters have postulated that a larger wattage resistor will be more
resistant to vibration and the like. I respectfully disagree. I'd much
rather something about the size and mass of a toothpick than size and mass
of a large ball bearing bouncing around. If we can get a handle on
exactly
how much power this sucker is dissipating, we may be able to do a Rat
Shack
fix for a buck or so.

Keep me posted.

Jim



"Jim Burns" wrote in message
...
Thanks Jim, I appreciate the link.

Not having a background in electronics, what ultimate difference would a
resistor of higher wattage rating have on the circuit, the gauge, or the
probe? If the circuit is only drawing mA's, 1) are there "smaller" mA
rated
82ohm resistors available or 2) with a mA rated resistor would the
required
ohm rating change? Forgive my ignorance regarding resistors, I
basically
understand what they do, but do not know the implications of the
sizes/capacities. I'm just looking for a way to get out of several
hours
of
tedious PITA work with my upper torso stuffed up underneath our panel.