# serial regulator

Discussion in 'General Electronics Discussion' started by pwnstars, Feb 28, 2016.

1. ### pwnstars

15
1
Oct 6, 2015
Hi guys... I would appreciate if you could tell me if these calculations are correct or not?

D2 is 16V
HFE in the transistor is to be calculated as 50
S1 is Connected

Ur5 = Ucc-Vdz-Ube
Ur5 = 24V-16V-0.6 = 7.4V

Ir5= Ur5/r5
Ir5 = 7.4/1000 = 7.4mA

Ir5 = Ib = 7.4 mA

Ic= Ib* HFE
Ic = 7.4mA*50 = 370mA

Ie = Ic + Ib = 377.4mA

Ue = Udz - Ube =
Ue = 16 - 0,6 = 15,4V

thanks

2. ### Alec_t

2,737
728
Jul 7, 2015
This bit is: "Ue = 16 - 0,6 = 15,4V".
Starting with that, work out the currents in R6 and RL. The total of those two gives Ie.

5,164
1,079
Dec 18, 2013
NO I don't think its quite right. What is Ur5 please.

Edit: Brain fart moment... sorry.

Last edited: Feb 28, 2016
4. ### pwnstars

15
1
Oct 6, 2015
Arent Ur5 correct?

R6 and RL would be 160 ohm
Ue = 15.4V/160 = 96.3mA

5. ### pwnstars

15
1
Oct 6, 2015
Sorry meant 130 ohm total.

let me try all over again.

Ur5 = Ucc - Uzd
Ur5 = 24V- 16V = 8V

Ir5 = Ur5 / r5
Ir5 = 8V / 1000 = 8mA

Ib = Ir5 = 8mA
Ub = uzd = 16V

Ue = Ub - 0.6V
Ue = 16- 0.6 = 15.4V

Ir6 = Ue / r6
Ir6 = 15.4 /1000 = 15.4 mA

IRL =Ue /RL
IRL = 15.4mA/150 = 102.6 mA

Ue = Ir6 + IRL
Ue = 15.4mA + 102.6mA = 118mA

6. ### Alec_t

2,737
728
Jul 7, 2015
Better, but you're not quite there yet .
No. You're forgetting that some of IR5 flows through D2.

7. ### pwnstars

15
1
Oct 6, 2015
So the current through base is calculated as emitter current devided by the Hfe of 50?
118 mA / 50 = 2.36 mA
Which makes current through zener diode:
Ur5 - Ub
8 mA - 2.36 mA

Last edited: Feb 29, 2016

2,737
728
Jul 7, 2015
Got it!

9. ### Colin Mitchell

1,418
314
Aug 31, 2014

Last edited: Mar 3, 2016
10. ### Alec_t

2,737
728
Jul 7, 2015
Not so, Colin.
Spice confirms them (as do members on another forum where the same topic is raised).

Eh? Most of the current goes through the 150Ω resistor, not through the zener.
So the rest of the world is wrong . Hmmm.

11. ### Colin Mitchell

1,418
314
Aug 31, 2014

Last edited: Mar 3, 2016
12. ### Alec_t

2,737
728
Jul 7, 2015
I certainly can, but this thread is all about encouraging the OP to do the calculations.
How do you get that figure? R1 is the main current-limiter for the 16V zener. The only way 200mA could flow would be if there were reverse-voltage breakdown of the transistor collector-base junction.

13. ### Colin Mitchell

1,418
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Aug 31, 2014
What current flows through the zener when the switch is open?

14. ### Alec_t

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Jul 7, 2015
That's a calculation I can do in my own head . It's a bit more than when the switch is closed, but not the ~200mA you allege . Are you asking for an answer for yourself, Colin, or asking the OP to do the math?

15. ### BobK

7,671
1,681
Jan 5, 2010
So can I. I get approximately 9mA.

Here is my math:

Voltage at the top of the Zener: 15V.
Voltage at the top of the resistor, 24V

Voltage across R5 24 - 15 = 9V

Current through R5 = 9 / 1000 = 9mA

Current into base is negligible, 15mA / beta.

Colin, why don't you show us your calculation that comes out to 200mA, so we can point out where you are wrong.

Bob

16. ### Colin Mitchell

1,418
314
Aug 31, 2014
You are right. I read the circuit incorrectly.

The only comment I can make is the fact that the transistor will deliver a maximum of about 300mA due to the gain of the transistor.

Last edited: Mar 3, 2016
17. ### Colin Mitchell

1,418
314
Aug 31, 2014
The figures should be:
base current = 24 - 16 = 8v current = 8mA.
All this means is this: the maximum emitter current can be as high as 50 x 8 = 400mA.
but the actual emitter current is determined by the load.