Maximum rating of resistor

I have a fusible resistor

In some situation resistor will damaged.

How can I calculate the maximum current a resistor can handle with a safe limit?

is this resistor can handle inrush current from a ceiling fan motor ?

• P=R*I^2 – JRE Jun 16 '19 at 15:09
• Therefore I=sqrt(P/R) – JRE Jun 16 '19 at 15:09
• Basic power formula. P=I^2 * R. Remember that the power rating depends on the ambient temperature. – Unknown123 Jun 16 '19 at 15:11

The maximum steady-state current depends on the resistor rating and (at least above some minimum) the ambient temperature.

Surge ratings should be in the data sheet. Fusible resistors are designed to fail “open” in a fairly predictable manner, without undue flame, smoke and other drama.

I would think that a fusible wire wound resistor would be more appropriate than a film type for an application with relatively long duration surges.

• This. "You read the data sheet." Any other answer is a guess, and I wouldn't guess when safety is at stake. – Elliot Alderson Jun 16 '19 at 16:15

Can “this resistor can handle inrush current from a ceiling fan motor?”

maybe NOT. When you choose a smaller speed, with a smaller capacitor, the motor appears much lower resistive since lower back EMF AC Voltage is lower, the motor stall/starting current can up to 10 x max rated current or 100x power drawn thru the same resistor. This power drops rapidly only if the fan slowly rises in speed quickly.

So a better solution is no resistor but a suitable similar capacitor in series to raise the reactive impedance. This must be chosen carefully to match the fan load characteristics otherwise, the cap will fail from its dissipation factor ( % of applied power)

There are many levels or meanings of "safe":

1. Safe from self-damage or less than rated power max. temperature 250'C (?)
2. Safe for the circuit it protects: Current limit reduces with rising R but "may" result in more power dissipated in the resistor from a steady current.
3. Safe for long component life, failure rate doubles every 10'C rise above room temp
4. Inflammable or flame-proof: (UL-1412) only by design of part and usage.
5. Safe to touch from heat or less than user pain threshold, 85'C(?)

A "fusing resistor" is one that fails in open condition from excess heat well above max rated power and resulting temperature.

A wirewound (WW) resistor can be flame-proof yet fuse at 25x its rated power in 60 seconds.

This is just an example of a WW Resistor spec.

Notice that as the (de-) Rated Load decreases with rising ambient temperature.
This means self-heating of the part rises to this max temp ( 275'C here at max rated Power)

If you know the lifespan is cut in half for each 10'C rise, eventually you get to a temperature where it fails in sixty seconds. That is the fusing temperature.

For safe operation and good design guidelines, we generally choose to use less than 60% of it's rated power so that the temperature rise is < 60% of it's maximum temp.