# serial regulator

Discussion in 'Homework Help' started by pwnstars, Feb 29, 2016.

1. ### pwnstars Thread Starter Member

Feb 29, 2016
44
1
Hi guys, im a new electronic student, but im stucked in the basics
So would appreciate some help correcting my calculations in this curcuit.

D2 is 16V
S1 is connected

here is my calculations

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

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

Ir5 = IB
8mA = 8mA

Ue = Uzd - Ube
Ue = 16V - 0.6V = 15.4V

Ir6 = Ue/R6 =
Ir6 = 15.4V/1000 = 15.4mA

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

Ie = Ir6+ IRL
Ie = 15,4mA + 102,6mA = 118mA

Ic = Ie-Ib
Ic = 118mA - 8mA

Is there some truth in these calculations?

Best, Peter

2. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
You need to annotate your diagram with the definitions of the variables you use. When you say Ur5, that is ambiguous because you are not only requiring the reader to assume that it refers to the voltage across R5, but you are requiring them to guess what the polarity of that voltage drop is. Similarly, when you talk about Ir5 you are requiring the reader to guess what direction you are defining that current to be in. Engineering is not about guessing -- make your definitions clear.

This would appear (again, you are forcing people to guess) to say that all of the current that flows through R5 must then flow into the base of Q4. Does this seem reasonable?

Apr 5, 2008
15,802
2,387
Hello,

When you say that Ir5 is Ib. How does the zener diode regulate?
A zener diode will need a minimal amount of current to be able to regulate.

Bertus

4. ### pwnstars Thread Starter Member

Feb 29, 2016
44
1
Thanks guys.
I have subtraced the .5mA zenior current from the calcs and added my values to the picture..

5. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
What is your reasoning for concluding that the zener current is 0.5 mA?

What would the zener current be if the wire going to the base of the transistor were cut?

Apr 5, 2008
15,802
2,387
Hello,

Have a look at the datasheet of the zener for the minimal current.

Bertus

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7. ### pwnstars Thread Starter Member

Feb 29, 2016
44
1
I meant 5 mA, sorry. and i've looked it up in the datasheet.

If i cut the base of the transistor then the zener would pass all the current delivered to it.
And with that knowledge i have no explaination for why the current wouldn't pass more than 5mA.. (most likely because it's wrong)

I cant seem to find the solution at this point

8. ### pwnstars Thread Starter Member

Feb 29, 2016
44
1
Is it right to say that the current through R5 is:
(Ucc - Udz)/R5
(24V-16V)/1000ohm = 8mA ?

Apr 5, 2008
15,802
2,387
Hello,

Yes, the current in R5 is 8 mA.
This current is split into the zener current and the base current of the transistor.
The currents will be different in the case the switch is open and closed.
You will have to make calculations for both cases.

What is the gain of the transistor?
Have a look in the datasheet.
For the calculations I would use the minimum gain.

Bertus

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10. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
You are on the right track and making good observations. We can explore this a bit more, but your next posts provide a better opportunity.

11. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
Yes, this is correct. And you correctly tracked your units -- bravo!!

Like a normal diode, a zener diode will hold a constant voltage across it (if possible) while allowing whatever current through it that is needed to hold that voltage.

So if the base of the transistor is cut off, then all of the current will go down through the zener. With the transistor in the circuit, some of it will go into the base and the rest will go through the zener. The amount that will go into the base will be dictated by the circuit to the right of the transistor combined with the current gain (beta) of the transistor.

12. ### pwnstars Thread Starter Member

Feb 29, 2016
44
1
According to the data sheet the minimum gain is 40.

I assume the next stage is to calculate the currents after the emitter. Is that right?

Uemitter = Uzener - Ubasis/emitter diode
Uemitter = 16V - .6 = 15.4V

current through the resistances would be calculated as the emitter voltage devided by the resistance values.

15.4V/1000 = 15.4mA
15.4/150 = 102.6mA
total current ( current-emitter) = 118mA

am i on the right track? :b

13. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
The typical gain is probably in the 100 to 300 range, but using 40 will make most of your calculations more conservative, which is usually (not always) good.

Watch those units. 16 V is a voltage, while 0.6 is just a number. You can't subtract a number from a voltage. It should read

16 V - 0.6 V = 15.4 V

What those units! 15.4 V / 1000 is 15.4 mV. What you meant was

15.4 V / 1000 Ω = 15.4 mA
15.4 V / 150 Ω = 102.6 mA

This is only IF the switch is closed -- something that you haven't indicated one way or the other.

Yes, you are.

So what would the base current in the transistor be, assuming a beta of 40?

14. ### pwnstars Thread Starter Member

Feb 29, 2016
44
1
Sorry, I'll make sure to add units

Assuming the gain is 40.
Base current is: emitter devided by 40
118 mA / 40 = 2.95 mA

And as far as i know the collecter current is approximately the same as emitter.

Doesn't the base current determine if the transistor is closed or open?

Apr 5, 2008
15,802
2,387
Hello,

When there is 2.95 mA going to the base of the transistor, how much current is going to the zener diode?

Also calculate the currents for the open switch.

Bertus

16. ### pwnstars Thread Starter Member

Feb 29, 2016
44
1
The remaining current from the resistance (R5) will go through the zener diode.
8 mA - 2.95 mA = 5.05 mA

Im not sure how to calculate the open switch.

Apr 5, 2008
15,802
2,387
Hello,

You already have calculated the current through the 1k resistor.
This is the only one connected to the regulator when the switch is open.

Bertus

18. ### pwnstars Thread Starter Member

Feb 29, 2016
44
1

I dont really have a clue how to go from here.

Am i supposed to change resistance values?
Everything is constants to me.

Apr 5, 2008
15,802
2,387
Hello,

Think of it like the switch and the RL do not exist, like this:

This would be the situation when the switch is open.
Now calculate all currents.

Bertus

20. ### pwnstars Thread Starter Member

Feb 29, 2016
44
1
Ohh.. of course

Current at emitter is calculated by voltage at emitter devided by R6
15.4V / 1000 Ω = 15.4 mA

Base current is emitter current devided by the Hfe of 40

15.4 mA / 40 = 0.385 mA

Current through zener is current in r5 subtract with current at the base
8 mA - 0.385 mA = 7.615 mA

Current at collecter is current at emitter subtract current at base
15.4 mA - 0.385mA = 15.015 mA