# Question on BJT transistor sizing (W)

#### 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
Let me take a stab at it...

(12ma times Vbe) + (120ma times Vce) = P
(.012 x .7) + (.12 x 12) = P
.0084 + 1.44 = 1.4484W

#### â¬Hunter

Joined Jul 6, 2013
33
Looks about right to me.

#12

#### â¬Hunter

Joined Jul 6, 2013
33
On a more serious note it looks terrible.

#12

#### #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.

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#### â¬Hunter

Joined Jul 6, 2013
33
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.

#### mbohuntr

Joined Apr 6, 2009
432
So. . 7* 132ma?

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#### #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.

#### mbohuntr

Joined Apr 6, 2009
432
Sorry, forgot that... Right now I have a TO 92 3904 in the circuit.

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#### #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

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#### 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!

#### #12

Joined Nov 30, 2010
18,217
That's what the "Thanks" button is for. Bottom right corner of the posting box. Hey, it's the only pay I get here.