Re: More on CHT probe replacement

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" <jim@xxxxxxxxxxxxxxxxxx> wrote in message
news:11hesjlpb5i0b91@xxxxxxxxxxxxxxxxxxxxx
> 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" <jburns3@xxxxxxxxxxxxxxxxxx> wrote in message
> news:c7udnZ2dnZ1iLrrwnZ2dnR_Dit6dnZ2dRVn-z52dnZ0@xxxxxxxxxxx
> > 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.
>
>


.

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