# Power resistor dealing with quick high current surge

I am currently working on a circuit to deal with the high inrush current of high value capacitors.

I'm calling this a 'capacitor adder' as the rest of the circuit (output load) is already powered while the caps are slowly charged over a current limiting resistor. The resistor is then shunted by a relay when the capacitors are (almost) fully charged.

My question is concerning the wattage of the power resistor as the simulator shows power consumption of over 11 watts but in a short burst of about 1.1 seconds until it lowers to 1 watt. Does this mean only a resistor rated over 11 watts is suitable? I'm thinking because of the short time frame this will not give the resistor time to warm up and so maybe a 5w wire-wound resistor may be suitable.

Any other improvement I could make?

Thank you ESE community!

• What is the peak current through the resistor? You need to worry about that as well. Commented Jan 11, 2020 at 18:12
• looks like just under 480mA peak current through the resistor. Commented Jan 11, 2020 at 18:15
• You have to consider that users switch the power supply on and off - even in very short intervals. Think about the worst case. At some time, this will always occur. Commented Jan 11, 2020 at 18:26

Any other improvement I could make?

There are resistors specifically designed to handle surges. In fact all will but, few have the information in their data sheet that allows you to make the right choice.

For instance, the Bourns CRS0805 range of surface mount resistors have a nominal power dissipation value of 0.25 watts but, the data sheet tells you that if the surge is no more than 1 second, the power can be a little over 1 watt. For 10 ms the power dissipated can be about 10 watts: -

So, if you choose the right resistor you’ll get the info you need in the data.

I’m not saying this range is suitable for your needs of course; just look for resistors with surge handling capabilities and inspect the data sheet for the power vs duration graph.

• and the single pulses used in these charts are rectangular, and to convert from an exponential decay pulse, you use width of τ/2 where τ is time constant of voltage decay (not power decay) (??) Commented Dec 29, 2020 at 21:12
• Here is an instructional flowchart! vishay.com/docs/48516/… Commented Dec 29, 2020 at 21:48

Does this mean only a resistor rated over 11 watts is suitable?

Resistor power rating typically indicates maximum steady-state power at which resistor reaches maximum temperature at some ambient temperature. If you want it to be cooler power must be lower and if power is intermittent, it can be higher.

This document describes how to calculate power and energy handling of a resistor and how to calculate pulse power and energy. It splits power handling capability in to two categories based on pulse duration. If pulse is shorter than cross-over point time duration, it is assumed that energy is dissipated mostly in resistance element. If pulse is longer, it is assumed that power is dissipated in entire component. Either way, peak power can be greater than the power rating.

Preferred approach is to refer to datasheet of specific component, because overload characteristics are highly dependent on materials, geometry, maybe mounting and other factors.

Any other improvement I could make?

Make sure your design is good for worst use case. Also that one failure (shorter or open component) would not cause fire or other safety hazard.

• An improvement to this answer would be a short summary of the solution in case the link dies at a future time.
– BobT
Commented Jan 11, 2020 at 21:01
• Answers that are just links to another site are discouraged. If Vishay reorganizes their web pages or changes a single letter in the URL then your answer becomes worthless. Commented Jan 12, 2020 at 1:47