# Homework Help: Transistor as a switch

1. Jun 21, 2014

### -SJ-

Hello,
I've spend a lot of time at following example of my homework but still unable to find a solution. In fact I have no idea how to solve that, any help would be very appreciate.

1. The problem statement, all variables and given/known data
Transistor KFY34 has to switch a load of nominal supply voltage 24V and resistance Rs=60Ω.
a)Draw a schematic of electrical circuits.
b)Determinate Ib while the load is switched on.
c)Determinate currents and voltages in the circuit I B = f(t), IC= f(t), UBE = f(t) a U CE = f(t) with the load switched on and off and draw them to a graph.
d)Decide if additional cooling is necessary.
KFY34: NPN, h21E = 35 až 125, UCB0 = 70V, UCER = 50V, ICmax = 500 mA, UCES ≈ 1,5V, PC = 800mW

2. Relevant equations
Ib=Ic/β
Rb=(Uin-Ube)/Ib

3. The attempt at a solution

2. Jun 21, 2014

### Staff: Mentor

The voltages and currents you have drawn are the maximum allowed specifications, not the values that the transistor experiences in this circuit. How could you get 50V or 70V across the transistor, with only a 24V power supply? :biggrin;

Calculate the voltages in the circuit assuming two conditions: 1) the transistor is cut off (Off state), 2) the transistor is in saturation (On state).

3. Jun 21, 2014

### -SJ-

Thank you for your answer, 70V with a 24 power supply is a great mistake of mine, of course :)
1)if the transistor is cut off, there would be a full voltage at it, wouldn't it?
2)in saturation mode there's should be voltage drop of 0,6-0,7 due to diode-juction.

4. Jun 21, 2014

### Staff: Mentor

Correct. Vce would be 24V in cutoff. What would Ic be?

Somewhere around there, perhaps Vce would be a bit lower than 0.6V. So what does that mean for Ic? And then how can you work back to finding the Ib needed to support that Ic?

5. Jun 22, 2014

### -SJ-

If 24V be cutoff, there will be none Ic.
And there would be relative hight Ic while transistor switched on determinated by Rd. But not sure how to calculate it because I don't know beta neither Rd.

6. Jun 22, 2014

### Staff: Mentor

Compared to 60 Ohm, you can hopefully neglect the internal resistance of the transistor. You can probably assume it conducts perfectly (just with the internal voltage drop).

7. Jun 22, 2014

### -SJ-

So, Ic=400mA. Hm, but what about Ib if assumed we don't know beta?

8. Jun 22, 2014

### Staff: Mentor

In practice, you'll never know β. You design for the worst case, plus allow an extra margin for safety. If you don't observe this cautious approach, then you risk losing the transistor.

So, what worst-case β will you design for?

9. Jun 22, 2014

### -SJ-

Ic can't get over 500mA, so beta should be appropriate to that. But we don't know Ib, do we?

10. Jun 22, 2014

### Staff: Mentor

You are trying to determine what value you will set IB ..... such that the transistor is guaranteed to operate in saturation, regardless of who builds it. So you base your calculations on the worst-case you can anticipate. You will need to refer to the device's specs for this.

11. Jun 22, 2014

### -SJ-

I've read some pages about transistors but nowhere found how to calculate it. In fact, I have no idea how to set Ib and any guidance would be really appreciate.

12. Jun 22, 2014

### CWatters

You cited the relevant equation in the OP.

Edit: Well ok you need to rearrange it a bit. Ic=Ib*β

However β isn't a constant. It varies because the manufacturing process isn't perfect. If you need to achieve a certain Ic you had better assume β is at the lower end of the specified range when you calculate how much Ib you need to feed it. Otherwise not all transistors of that type would work.

Last edited: Jun 22, 2014
13. Jun 22, 2014

### -SJ-

But which value of beta I should assume to determinate Ib? I've tried to look up it's range in a data sheet but it is not stated there. How can I find it out?

14. Jun 22, 2014

### Staff: Mentor

You are not finding Beta information in the datasheet? Can you post a link to the datasheet?

15. Jun 22, 2014

### CWatters

Google suggests this transistor dates from the 1970's and found what I think is a Czech data sheet.

http://www.datasheetarchive.com/dl/Scans-048/DSAGER000349.pdf [Broken]

Looks like the static Current Gain is "Proudove zesileni" and is the parameter h21E.

Note that the gain is specified at various values of IE and temperature.

Last edited by a moderator: May 6, 2017
16. Jun 22, 2014

### Staff: Mentor

Refer to part (d) in your first post.

17. Jun 23, 2014

### -SJ-

Hello,
I am sorry for my very late response due to my work responsibilities. Thanks for help with the current gain, I've find some data sheet but was looking for the greek symbol beta and didn't realized that h21e may be the same.
It looks like the current gain is 35-125 for this particular transistor manufactured by Tesla (indeed wide range seems to me), so the base current would be Ib=4mA while h21e=125 and Icmax=500mA. But is there a guarantee the transistor will be in saturated region with so low current? If the internal resistance is negligible are we able to determine the voltages across the transistor how we are asked?

18. Jun 23, 2014

### Staff: Mentor

4mA would be the best case. You have to plan for the worst case where you need more base current.

19. Jun 23, 2014

### Staff: Mentor

.... the base current would be Ib= ... mA while hFE=35 and Icmax=500mA.

To ensure the transistor will be saturated for all β in the range, you have to take the conservative approach.

20. Jun 24, 2014

### -SJ-

OK, for h21e=35 is the base current Ib=14mA. But what happens if we set this value and the h21e will be 125?

Last edited: Jun 24, 2014