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Topic: Transistor power dissipation question (Read 577 times) previous topic - next topic


I'm looking for a transistor that can switch a 3.3v 350 mA load.

Is this one rated .5 amps with 625 mW power dissipation tough enough?

Or should I use something like this one with a 1.5 amp rating and up to 1 watt power dissipation.

Thanks for you advice.


I'd go with the overkill. ;) 

In theory, it looks like either one should work.  But, power & current ratings depend on ambient temperature and in the real world the temperature around the transistor is often higher than "allowed" in the spec, and the device has to be derated.   Sometimes, the power rating is only valid when a heatsink is used.    You really need to give yourself some safety margin.

...In case you don't know, power is calculated as Voltage times Current.  In a switching application, you have the C-E saturation voltage, which should be around 0.3V, and your current of around 350 mA (which is determined by the supply voltage and your load resistance/impedance).  That puts power dissipation at around 1/10th of a Watt.  When the transistor is "off", the full-voltage (3.3V) is across the transistor, but almost zero current flows, for zero power.   (Things get a little more "dangerous" if the transistor is used in "analog mode" where there is voltage and current at the same time.)

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