> From:         Eisermann, Phil [Ridg/CO][SMTP:peiserma@RIDGID.COM]
> Sent:         Tuesday, February 10, 2004 2:46 PM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      [EE]:  resistor pulse capability

> Hi list,

>         does anyone know how to calculate how much of
> a current pulse resistors can tolerate? I'm looking
> at tech note TN-95 from KOA SPEER as a starting point.

>         If I ignore the fet resistance and assume a perfect
> capacitor for worst-case approximation, then I basically
> have a 14.4V battery charging a 33uF cap through a 3R9
> resistor. Following the tech note shows an equivalent 50W
> pulse for 200usec. So where do I go from here? Is that
> correct, and is that too much for an 0805?

> Phil Eisermann

      The few data sheets I have here in front of me show an 0805
      resistor as having a continuous power rating of 1/10 watt.
The critical parameter is the temperature of the resistance
material in the part. Integrating an exponentially falling power
function which starts at 50W and falls with a time constant
of 200usec would give the total energy dumped in the
resistor during the pulse. The total energy for a simple
exponential works out to 50W x time constant = 50 * 3.9 * 33e-6
= 6.4 mJ. At 1/10W, or 100mJ/sec, this much energy would
be deposited in 0.064 sec or 64 msec. Your circuit is
dumping the same amount of energy in 0.2 msec, which is
320 times faster. The amount which this will raise the
temperature over and above the static value for a DC current
rated at 1/10W depends on the thermal time constant of the
package, which is the unknown. None of the data sheets I
have address this parameter.

John Power

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