Question on BJT transistor sizing (W)

Thread Starter

mbohuntr

Joined Apr 6, 2009
432
If I am using a piezo siren as a load, and it consumes 120mA, and I use it as a common emitter with a 1K base resistor, How do I determine the wattage for the transistor? I\(_{}b\)(12mA) + I\(_{}c\)(120mA) X V (12V) = Watts? = 1.3 watts? It doesn't seem like it should be that easy...
 

#12

Joined Nov 30, 2010
18,217
I see. The piezo is in the collector side. That removes the 12V load from the transistor and the power rating goes way down...to about .0084W + Vce sat times .12A

A schematic might have gotten through to me better.
I'm not quite fully awake right now.
 
Last edited:
Hard stuff. Anyone who disagrees is either a fool or a liar.

Wattage for the transistor would be the current that it is switching multiplied by the voltage drop across the junction.
 

#12

Joined Nov 30, 2010
18,217
No. Post #5 has it right. 12 ma times .7 (Vbe) plus .12A times the saturation voltage from collector to emitter, which is usually less than .7 volts.

You haven't told which transistor you are using so I can't look up the Vce (saturated) of the transistor.
 

#12

Joined Nov 30, 2010
18,217
Vce Sat at 120 ma seems to be .15 volts.
Vbe at Ic = 100ma = .9V

Power = (.012 x .9Vbe) + (.12 x .15Vce sat)
P = .0108 + .018
P = .0288 watts
 

Attachments

Last edited:

Thread Starter

mbohuntr

Joined Apr 6, 2009
432
Thank-you very much! Both you and CHunter! That is why I like this site so much. Good people, great help! It seems like I'm within the safe zone for this circuit. Thanks again! :D
 
Top