More on CHT probe replacement

BTW, that 82 ohm power resistor (??) you need is available at www.mouser.com
5 watt cement coated fireproof resistors (280 CR5 series) are 39 cents in
onesies. 3 watt flame retardant resistors (283 series) are also 39 cents in
onesies.

Mouser has no minimum order and a decent shipping policy.

Somebody needs to measure the current that these things are sucking. In
order to dissipate 1 watt in an 82 ohm resistor, you need a bit more than
100 mA (0.1 amps) to flow. My gut says that this is not happening.

Jim



"Jim Burns" wrote in message
...
But I think the width of the needle of our gauge will compensate for that.
It's an old style gauge with only a dot at 200F and the next dot is at
400F.
The best we can do is try to keep the needle below the 400F dot a tad. I
guess we could also calibrate it with a pan of oil, a hot plate, and a
candy
thermometer.

Thanks Jim, I appreciate the link.
I also found some 82ohm 2W 5% flameproof resistors on ebay.
Some more goggling found that this Rochester probe w/82ohm resistor "work
around" has been used by owners of Pipers, Bonanzas and Commanders, all with
the AC Spark Plug CHT gauge. The 82ohms is the one constant in everything
I've read, the W varied between 2 and 3Watts and the tolerance was either 2
or 5%.

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.

Instrument Tech of Dallas TX seems to have quite a bit of experience with
the combo and emailed me the American Bonanza Society newsletter that
contains the article. The original question was published June 1990, and
the explanation and work around was published some time shortly after
however I do not have that date.

Jim


"RST Engineering" wrote in message
...
BTW, that 82 ohm power resistor (??) you need is available at
www.mouser.com
5 watt cement coated fireproof resistors (280 CR5 series) are 39 cents in
onesies. 3 watt flame retardant resistors (283 series) are also 39 cents
in
onesies.

Mouser has no minimum order and a decent shipping policy.

Somebody needs to measure the current that these things are sucking. In
order to dissipate 1 watt in an 82 ohm resistor, you need a bit more than
100 mA (0.1 amps) to flow. My gut says that this is not happening.

Jim



"Jim Burns" wrote in message
...
But I think the width of the needle of our gauge will compensate for
that.
It's an old style gauge with only a dot at 200F and the next dot is at
400F.
The best we can do is try to keep the needle below the 400F dot a tad.
I
guess we could also calibrate it with a pan of oil, a hot plate, and a
candy
thermometer.

Jim Burns wrote:

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 don't have a background in electronics, either, but I'd think 3 watts is way
overkill from an electrical point of view, *but* a higher power rated resistor
is also more robust from a mechanical point of view, and may stand up better to
the vibration environment. Just an idea from an ignorant observer.

Dave

I don't have a background in electronics, either, but I'd think 3 watts is
way
overkill from an electrical point of view, *but* a higher power rated
resistor
is also more robust from a mechanical point of view, and may stand up
better to
the vibration environment. Just an idea from an ignorant observer.

Dave

You summed up my thinking pretty close. I'm thinking if 3W is overkill,
what are the downsides of it? Shorter probe life? shorter gauge life?
shorter resistor life? If it doesn't detrimentally effect the operation of
either the probe or the gauge and the life of the components isn't
compromised, then the only downside I see is about $0.10. But like I said,
I don't have much knowledge in this area but I'm willing to learn so I defer
to the pros.

Thanks
Jim

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.