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.

        The lengthy description is: I'm helping diagnose
a problem on a product that we buy from another vendor.
The product is sold in the US and Europe. It is powered
by a 14.4V battery. It is a 3R9 0805 that is failing.
Currently we have around 30 out of 1500 that failed.
All failures occured in January, and all in the US. No
failures of this nature in Europe. The vendor thinks it
may be related to the inrush current from plugging in the
battery but I have my doubts. It seems strange that they
only failed in the US, and only in January. My first
suspicion would be condensation or ESD.

        circuit is a 14.4V battery powering an LM2594M
through a FDN5618P p-channel MOSFET and a 3R9 0805
resistor. The input to the regulator has a 33uF electrolytic
in parallel with a 100nF ceramic. The battery remains
disconnected from the circuit until the user presses a
button. It is controlled by a 16F73 that keeps the circuit
powered up after the button is pressed. Current draw from
the circuit in normal operation is between 40mA and 90mA.

        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?

        Thanks for any and all ideas!

Phil Eisermann
Electronics Engineer
The Ridge Tool Company
(440)329-4680


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