# Homework Help: Voltage regulator

1. May 12, 2015

### M P

1. The problem statement, all variables and given/known data
https://physicsforums-bernhardtmediall.netdna-ssl.com/data/attachments/66/66455-bb02bb0d3c839e914352eb0379b453f2.jpg [Broken]

2. Relevant equations

3. The attempt at a solution
Transistors TR2 and TR1 amplify the current being regulated through the regulator. The current gain of transistors is not important since the regulator will increase/decrease the current through pass transistors to maintain output voltage.

Anyone can help if I am on the right track?

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2. May 12, 2015

### Staff: Mentor

At first glance, I don't think that's what the extra transistors are doing. Look at the equivalent circuit for what's inside of a typical 3-terminal linear regulator like an LM7805. What is different about that regulator's internal main circuitry compared to the extra transistors outside the 3-terminal regulator in this problem? It's a subtle but important difference, which is part of the answer to this problem, I believe...

Last edited by a moderator: May 7, 2017
3. May 12, 2015

### CWatters

TR1 is to increase the current capability but TR2 has a slightly different purpose. I had to think about it for quite awhile. Hint: What does the regulator have (or usually has) that the transistor TR1 doesn't have?

Edit: Berkeman posted his reply while I was still thinking about it.

4. May 12, 2015

### M P

All I could see is input output ground

5. May 12, 2015

### Staff: Mentor

I mean the equivalent circuit for what is inside the regulator...

6. May 12, 2015

### Staff: Mentor

BTW, as I think about it more, I think CWatters is probably correct. I initially thought that the extra transistors may be to support low dropout regulation when the 3-terminal regulator does not have sufficient input-output voltage difference across it. But with the way this is put together, it's not a low-dropout add-on. Go with CWatter's hints on this one...

7. May 12, 2015

### M P

let me bring this to another level

Starting current
Thermal protection
Ref V
Series Pas element
SOA protection
Current Gen etc....

8. May 12, 2015

### M P

Small current flows through?

9. May 12, 2015

### M P

All I thought is those TR 1 and 2 are transistors pass switched on by a drops from R1 and R2.

10. May 12, 2015

### Staff: Mentor

One way to start to approach a problem like this is to just come up with some voltage numbers and resistance values, and visualize what is going to happen. Then based on that first cut, refine your voltage numbers and resistance values to get closer to what the real behavior would be.

So assume the 3-T regulator is an LM7805, so the output voltage is 5V, right? Then make Vin = 8V and have a 1V drop across R1 to make 7V into the input of the 3-T regulator. Assume you get about 0.5V across R2, and work out what happens in the circuit. Can you post your thoughts about that starting point? That's what I did to start to understand CWatter's hints versus what I initially thought the circuit did...

11. May 12, 2015

### M P

Thanks berkeman tha you not giving upon me
So we have 6.5V and that need to be rectified by TR2?

12. May 12, 2015

### Staff: Mentor

No, rectifying is not what TR2 does. Can you post a copy of the circuit with the voltages that you work out for the first-cut input voltages I mentioned?

13. May 12, 2015

### M P

That is what I think you were saying right?

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14. May 12, 2015

### M P

So the volt drop should turn those TR1 and TR2 ??

15. May 12, 2015

### Staff: Mentor

Closer. Remember I said 8V in and 0.5V drop across R2. So the emitter of TR1 should be at 7.5V, not 6.5V.

So now to get closer to operation, we need less voltage drop across R2, so say it is only 0.3V. Will that turn on TR1? And if so, do you see what CWatters was hinting at, about providing extra current to the output?

And to start to see what TR2 does, increase the input voltage to 10V, and adjust R2 so that TR1 is on with 7V at the input to the 3-T still. What does changing the value of R2 do? What happens when you get about 0.7V across it?

16. May 13, 2015

### CWatters

Expand on SOA. That includes the feature I was thinking of in post 3.

17. May 13, 2015

### M P

"SOA is defined as the voltage and current conditions over which the device can be expected to operate without self-damage"
So TR2 protects the circuit by the sound of it?

18. May 13, 2015

### M P

I did not understand the first bit and you are saying we need to rise input to 10V. Would not that give 9.5V at R2?
From clues of CWatters I think TR1 is a transistor pass and TR2 protects the circuit from self damage.

19. May 13, 2015

### CWatters

I note the load isn't specified. What sort of load might cause damage to the regulator?

20. May 13, 2015

### Staff: Mentor

That's pretty accurate. Can you describe how TR2 does this protection task? It's a very common circuit in power supply design...