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#11
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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 |
#12
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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. |
#13
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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. |
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