[SOLVED] Transistor power calculation

[darryl_co]

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

 

[srizbf]

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.

 

[D.A.(Tony)Stewart]

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]

Can I use the 2N5551 in this circuit or I need to change with a higher power device

 

[KlausST]

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]

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]

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