# Pulsed power of a Resistor

Discussion in 'General Electronics Chat' started by Jebasingh, Mar 28, 2011.

1. ### Jebasingh Thread Starter New Member

Mar 1, 2011
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Hi all

Can anyone give me the formula or the method to calculate the pulse power dissipation by a resistor?

For example have a resistor value of 100Ω, 0805 package, and the pulse voltage of 500, 1000Volts and pulse width of 1ms.

Regards
Jebasingh

2. ### Wendy Moderator

Mar 24, 2008
21,413
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With a square wave or PWM it is the percentage of the on time. If a square wave is on 50% that is the RMS.

RMS is what you are actually referring to. A sine wave is 0.707 RMS, for example.

3. ### eblc1388 AAC Fanatic!

Nov 28, 2008
1,543
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Form where you have got the information that you can apply voltage of over a hundred volt to a 0805 resistor?

Their small physical size means that they are not generally designed for that much voltage. Better check the specification first before you even consider apply the voltage.

4. ### Wendy Moderator

Mar 24, 2008
21,413
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I had missed the voltage. You might run into arcing if it gets too extreme, as the gap sizes are pretty small.

5. ### SgtWookie Expert

Jul 17, 2007
22,202
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An 0805 size resistor might have a working voltage of 100v-150v, and max overload of 200v-300v - but beyond that, good luck. Even 2512 size SMD's are typically limited to 400v or less.

Another item you may run into trouble with is thermal shock; a brief, intense burst of heat during the event, and then a cooling period, will tend to tear the resistor loose from it's end caps.

6. ### tom66 Senior Member

May 9, 2009
2,613
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The space between pads on an 0805 is 0.04" or about 1mm. It takes about 10kV to 30kV/cm to arc through air. Your voltage of 1000V is *dangerously* close to the limit of about 1-3kV, that's ignoring the fact that the resistor element is likely to arc way before that.

Assuming the resistor didn't arc, it would dissipate an instantaneous power of V^2/R = 10kW. Over 1ms, that resistor would convert 10 joules of energy into heat. It would likely be toast.

Last edited: Mar 28, 2011
7. ### Jebasingh Thread Starter New Member

Mar 1, 2011
16
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Main idea is to check the withstanding capability of the resistor to ESD
generally ESD voltages will be in the range of 500, 1000V etc....

8. ### Wendy Moderator

Mar 24, 2008
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But not in the ms range. ESD falls more in the ns to µS range on the discharge, unless it is a massive event. It also can run much higher voltages.

One of the ways to combat ESD damage is to slow the discharge rate. It is the intense current that goes with the discharge as much as the voltage that does damage. This is why many ground straps use 10MΩ resistors, to bleed off the ESD in a controlled manner.

While resistor can be damaged by ESD, it won't be power. The thermal shock for an intense event you describe will be mechanical in nature, as one of the guys said earlier.

9. ### tom66 Senior Member

May 9, 2009
2,613
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Not to mention, ESD is often high impedence.

10. ### Wendy Moderator

Mar 24, 2008
21,413
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That is actually why the time durations are so short. There are exceptions, like lightning.