## Resistor power ratings in surge conditions?

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Hi,

I have read many resistor datasheets but can't find a curve or anything that would relate to resistors being used in surge conditions, only steady-state DC or AC conditions.

So how can I know if a 10R 100mW resistor can be used with five times the rated current and thus 25 times the rated power for 160 microseconds, or what power rating would be enough? I would hate if it started acting like a fuse and blow up.

I have two devices that connect together with a cable, and the master device gives also +5V power to the slave. There is some 10uF capacitance in the slave box, but the current consumption is very low, like 10mA max, so I have put a 10R resistor to limit the inrush current to 0.5A when the cables are connected on the fly.

Datasheets say that in steady-state DC or AC conditions a 100mW power rating is more than enough as it is calculated from resistance and current. 10R@100mW=100mA.

The resistors seem to be tested at surge conditions of 2 or 2.5 times the nominal rating, but the thing is that in my case the surge current starts at 500mA.

And since 5V with 0.5A means 2.5W of initial power dissipation, and because of the exponential decay, the power dissipation level of 100mW is reached after 160us.

If you know the surge profile You may be able to estimate the energy dissipated during Your surge condition and compare it to specified surge condition.

If they are at similar levels then perhaps there is some reasonable hope it might survive Your conditions.

We are after all talking thermal failure which is predicated on energy being dissipated.

Jepael,
I don't think you will have a problem as a surge of 160usec is not going to cause the 10R to heat up fast enough to burn it up. REmember, a surge is generally a voltage transient in which case a transorb on the inputs will keep the voltage where you want it, and a current surge on start up is short lived as well. 2.5 watts of dissipation for 160usec is in my book nothing to worry about, but to help you sleep better use a 1/4 watt resistor in place of the 1/8 watt you currently have.

Oh, the .5 amp would only happenm if there were a dead short on the load side of the resistor...yes electrolytics appear as a short for a few microseconds on initial charge up but again, they change resistance values as they charge. YOur .5 amps would have to be a constant short for more than say 15 seconds before you start to fry the resistor.

Jim

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It might work since the resistor is low and the conductivity of the film is high. Might want to make a test setup with even higher peak current and test.

I have found that high value resistors used in voltage dividers decrease in resistance if exposed to many ESD events.

It all starts with a mental vision.

Surge ratings are published for surge rated resistors. And are fairly complex too, depending on the duration of the pulse. If your pulse is not rectangular you have to compute an rectangular equivalent.

Like this appnote on Koa Speer SG73 series.