# Hi There! Also in need of URGENT HELP!!!

Discussion in 'Homework Help' started by Rhysmachine101, Jun 19, 2011.

1. ### Rhysmachine101 Thread Starter New Member

Jun 19, 2011
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0
Hi everyone! I'm Rhys, pleased to meet you all!

Hoping to get some really quick help with a voltage regulator problem.
I need help calculating the power dissipated in a bipolar transistor that is set up as a voltage regulator.

I've been trying to find info for two days now- my textbook, which has been really good up till now, is useless.

Here is the circuit:
Vin= 25V
Vz=15V

I am not too sure what formual to use; I have unfortunately seen a few I think the correct one is P=Ic(Vce+(Vbe/Beta))?
Is Ve= Vo?
Therefore is Io=Ie = 14.3V/10k?
I am not sure whether Ie is still roughly = Ic or not?
I am not sure how to find Ib, is it = to Vin/R1?
Any help would be appreciated!

Thanks,

Rhys.

Last edited: Jun 19, 2011
2. ### Jony130 AAC Fanatic!

Feb 17, 2009
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The power dissipated in the BJT is equal
Ptot ≈ Vce * Ic
So you need to know voltage across BJT and the load current.

3. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
Do you understand how the regulator works in principle?

What is the purpose of the zener diode for instance?

What is the difference between the output voltage and the zener diode voltage?

What parameters (circuit values) do you need to know to find the power dissipation in the regulator transistor T1?

4. ### Rhysmachine101 Thread Starter New Member

Jun 19, 2011
5
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Thanks for the quick reply, I am not sure how to calculate Vce, though? Is the load current 14.3/10k in this instance? I am fairly sure it is?
Thanks again!

5. ### Rhysmachine101 Thread Starter New Member

Jun 19, 2011
5
0
I think so... The zener diode forms a constant voltage reference, right?

The output voltage is Vz- Vbe(0.7) isn't it?

6. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,990
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The load current is equal to
Vce = Voltage on the collector - Voltage on emitter = Vc - Ve

7. ### Rhysmachine101 Thread Starter New Member

Jun 19, 2011
5
0
Thanks again, Jony!

So I load = 14.3/10k = 1.43mA?

Vc= Vin? Vo= Ve?

So Vce= 25-14.3= 10.7V?

Ptot ≈ Vce * Ic ?

Ptot≈ 10.7x 0.00143=0.0153W? Is this correct? 15.3mW?

Feb 17, 2009
3,990
1,115