Do I have to rate the wattage of the resistor on the amount of current that the resistor is actually using or the total flow of current through that circuit?
Thanks
2007-06-21 17:23:25 · 4 answers · asked by FastEddie 5 in Science & Mathematics ➔ Engineering
Total flow of current through the resistor.
2007-06-21 17:30:47 · answer #1 · answered by John S 6 · 0⤊ 0⤋
That's a tricky question - you should rate the resistor for at least 150% of the calculated current through the actual resistor times the source voltage across the resistor circuit.
Less than this rating often lets the smoke out of the resister during actual operation in a real circuit over time.
2007-06-22 00:36:40 · answer #2 · answered by LeAnne 7 · 2⤊ 0⤋
It's the current through the resistor itself that matters. It causes the resistor to dissipate power equal to the square of the current times the value of the resistor. The power is in watts if you work in amps and ohms.
Depending on your application, you may want to down-rate the manufacturer's power rating for the resistor, for two reasons:
(1) The resistor value can change substantially when it gets hot.
(2) Resistors can fail if kept hot for hundreds of hours.
For example, if you have a current of 20mA through a 1 kohm resistor the power dissipation will be 0.0004 Ã 1000 = 0.4W, so in theory a 0.5W resistor should be fine. However, if you actually try this you will find that the resistor gets quite hot, so if your application is critical it might be better to use a 1W resistor instead.
2007-06-22 05:27:18 · answer #3 · answered by rrabbit 4 · 1⤊ 0⤋
The rating is based on the current through the resistor.
For sense resistors, you want them to remain as cool as possible, so you don't want them to heat up much, at all.
Whatever the maximum expected current (to sense) is, multiply it by at least 5, and size the sense resistor to that current squared multiplied by the resistor value.
.
2007-06-22 10:44:02 · answer #4 · answered by tlbs101 7 · 1⤊ 0⤋