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Old 22nd Oct 2020, 3:02 pm   #72
ms660
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Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
Default Re: Motorola 5A7A dead.

The current flowing through R4 will be a lot higher than the DC load current (a good rule of thumb is twice the load current) and that the resistors power dissipation rating should be chosen to suit, eg: if the load current is 0.06 Amps (60mA) then multiply that value by 2 eg: 0.12 Amps, and use that to calculate the power dissipation in the resistor (Isquared*R) eg: 0.12squared*R eg: if say R = 280 Ohms then Pdiss = 0.12squared*280 which would equal approx. 4 Watts dissipation so choose a power dissipation rating that's at least twice that value for a happier cooler life for the resistor and anything that's in close proximity to it.

Lawrence.
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