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Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
power and a functional voltage limit are different... you can hit the voltage limit long before or after the watt limit would indicate by calculation. right ?
Registered Member #2939
Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615
Voltage rating has almost nothing to do with power, and everything to do with construction. A long skinny resistor will handle more volts than a short fat one rated at the same power. Even choice of construction materials will affect voltage handling. Short answer: for voltage handling physical size is a better indicator than power rating .
Registered Member #2939
Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615
Thats just ohm's law, and not really relevant to the voltage rating of a resistor except at DC. Pulse handling is a different beast. Its entirely possible to have a 1Meg resistor that will handle a 15kV pulse, but only have a power rating of a watt (which would imply a limit of 1000V ).
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714
Plasma, the book page you cited might be confusing to noobs.
Right after the introduction of voltage rating, it mentions V = sqrt (P * R). Could be more clear immediately that putting P-max into that formula does not give you V-max, which is an independent limit in the data sheet. Fortunately, it then mentions critical resistance value. At foot of the page, it's beginning to say that for resistors whose R value is above critical, the V-max rule gets you before the P-max rule. ( Both rules apply to steady state operation, and have exceptions for pulses and small duty cycles. )
To beat more on the dead horse, and certainly not to say anything MrMilStar doesn't know:
Consider a series of 1/4 watt 200 volt resistors. The first part does not mean all R values can dissipate 1/4 W, it means stay below 1/4 W to avoid overheating. The second part does not mean all R values can withstand 200 V, it means stay below 200 V to avoid overvolting.
I agree with all that 2Spoons has said about material and construction details, which underlie the P and V ratings. Some materials are more tolerant than others of high temperature. Some resistive materials and dielectrics are more tolerant than others of high electric field strength, even at high temperature.
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Seeking spec on HV resistors
