Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

[SOLVED] Transistor power calculation

Status
Not open for further replies.

darryl_co

Member level 1
Joined
Feb 23, 2011
Messages
34
Helped
6
Reputation
12
Reaction score
5
Trophy points
1,288
Activity points
1,521
How do I calculate the power flowing through the transistor. Should I take Vcc*Ic or Vce(sat)*Ic. Vce(sat) as in mentioned in the Datasheet of a transistor. If instead of a relay it was a resistor and input was a square wave signal, does the calculation remain the same. Sample values would be Vcc 35 VDC and resistor value 2K2
 

Attachments

  • Transistor01.gif
    Transistor01.gif
    1 KB · Views: 3

srizbf

Advanced Member level 5
Joined
Apr 3, 2010
Messages
1,995
Helped
417
Reputation
840
Reaction score
328
Trophy points
1,363
Activity points
10,985
power dissipated in a tranasistor is Vce*Ic.
if the transistor is used for switching then , Vce(sat)*Ic.

If the resistance of the relay is equal to the resistor value connected , then it is same.
 

SunnySkyguy

Advanced Member level 5
Joined
Sep 26, 2007
Messages
6,744
Helped
1,675
Reputation
3,348
Reaction score
1,644
Trophy points
1,413
Location
Richmond Hill, ON, Canada
Activity points
50,737
Transistors are rated for Vce(sat) at Ic,Ib where it is typically Ic/Ib=10. Better devices mayy be rated for 20 up to 50

The results for coil or Rc would be same only if coil resistance was same as a fixed resistor.

Pwr= Ic*Vce and will be saturated if Ic/Ib matches rating on the device.

When you look closely at VI curves, when any transistor or diode is saturated you can extrapolate the VI curve as it becomes a fairly constant resistance. sometimes referred to as Rce the incremental slope. V/I
 

darryl_co

Member level 1
Joined
Feb 23, 2011
Messages
34
Helped
6
Reputation
12
Reaction score
5
Trophy points
1,288
Activity points
1,521
Can I use the 2N5551 in this circuit or I need to change with a higher power device
 

Attachments

  • Tx Power.jpg
    Tx Power.jpg
    167.8 KB · Views: 7

KlausST

Super Moderator
Staff member
Joined
Apr 17, 2014
Messages
19,725
Helped
4,344
Reputation
8,697
Reaction score
4,304
Trophy points
1,393
Activity points
130,563
Hi,

IC in the 2N5551 is about 16mA.

******

Power doesn't flow in a transistor. It is dissipated. It heats up the transistor.

Only in device like a motor, a speaker or a LEDs it is (partly) transformed into other physical power (mechanical, noise, light)

For all other devices (that don't transform power), the dissipated = heating power is the current multiplied with the voltage across the device: P = I x U. This is true for DC....With AC it is more complicated..

Klaus
 

darryl_co

Member level 1
Joined
Feb 23, 2011
Messages
34
Helped
6
Reputation
12
Reaction score
5
Trophy points
1,288
Activity points
1,521
Hi,

IC in the 2N5551 is about 16mA.

******

Power doesn't flow in a transistor. It is dissipated. It heats up the transistor.

Only in device like a motor, a speaker or a LEDs it is (partly) transformed into other physical power (mechanical, noise, light)

For all other devices (that don't transform power), the dissipated = heating power is the current multiplied with the voltage across the device: P = I x U. This is true for DC....With AC it is more complicated..

Klaus
So should I calculate wattage consumed by the transistor as Vcc *16mA or voltage across transistor *16mA
 

KlausST

Super Moderator
Staff member
Joined
Apr 17, 2014
Messages
19,725
Helped
4,344
Reputation
8,697
Reaction score
4,304
Trophy points
1,393
Activity points
130,563
Hi,

Always use voltage_across_device to calculate dissipated power.

*****
For switching a relay you should consider using a single FET instead of two bjts. More easy.

In either way: don't forget the freewheeling diode across the relay coil.

Klaus
 
Status
Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top