# Calculating resistor values for transistor

1. Jan 10, 2017

### nothing909

<< Mentor Note -- Two threads merged into one >>

As a project I have to design a switching circuit for a switched reluctance motor. I've picked all the components I need and I've tried to design the circuit.

Open collector - 74LS06N
Optocoupler - TLP 251
IGBT - stgp6nc60hd

Does the circuit look okay and will it work or are there any additions to be made?
Also, is the current limiting resistor I calculated correct for that optocoupler?
Please don't suggest any improvements, I just want to know if that design will work.

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2. Jan 10, 2017

### Staff: Mentor

Looks close. Which optocoupler? What does the datasheet recommend for the LED current? What is the CTR (current transfer ratio) of the opto, and what is the gm of the output transistor? What is the gain of the final motor drive transistor, and is it sized right for the motor current?

3. Jan 10, 2017

### nothing909

I'm kinda new to all this and I don't have a clue what you're talking about.

What do you mean which optocoupler? It's a TLP251. Why do I need to find the LED current, CRT and gm?

4. Jan 10, 2017

### nothing909

1. The problem statement, all variables and given/known data
How do I calculate the values of current limiting resistors I need at the base and collector of transistors. What information do I use off the datasheet of devices and what formulas do I use to calculate the values?

5. Jan 10, 2017

### Staff: Mentor

Your question is far too general to be addressed without writing an entire book on the subject! There are many different circuit configurations that use different approaches, and how the circuit is meant to behave has a bearing on it, too.

I suggest that you prepare a specific example (circuit layout, chosen transistor with its data sheet, any necessary operating conditions, etc.) and show an attempt. We can't help you if you don't give us something to work with.

6. Jan 10, 2017

### nothing909

Can you take a look at this circuit build. I have to design a switching circuit for a switching reluctance motor. Can you tell me if the circuit design as it is will work or if there is any changes that need to be made to it?

The devices used:

Open collector - 74LS06N
Optocoupler - TLP 251
IGBT - stgp6nc60hd

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7. Jan 10, 2017

### Staff: Mentor

The LED current is set by the LED volteae drop, the 5V and that resiator. The CTR and gains are to check that your circuit has enough current drive for the motor.

Last edited: Jan 10, 2017
8. Jan 10, 2017

### nothing909

10mA for the LED I think.

I don't know where to find the CTR

I think this is the gm, i don't really know what I'm looking at though :

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9. Jan 10, 2017

### Staff: Mentor

It appears to me that your circuit diagram is a "concept level" diagram, rather than a working circuit design. The lack of current limiting resistors for the base circuits of the transistors is an indication of this. The "open collector" designation on the input transistor leads me to think that this might represent a generic open collector output from some IC chip, possibly a microcontroller for example.

The optocoupler in your circuit diagram doesn't look like a TLP 251. Check the data sheet. Is your design "married" to this particular part or are you free to choose another?

To determine the values for any current limiting resistors you'll need to pick out the various minimum, maximum, and typical currents for the selected components. That's where the datasheets come in. With the chosen supply voltages, required load currents, and typical transistor characteristics (such as base-emitter potential drop) you should be able to make a first guess at component values. Stick them into the circuit and do a more rigorous analysis.

If you're still at the very beginning of designing your circuit you should be doing some research. Look for manufacturer's application notes and implementation examples for their component families related to your application. There are lots of optocouplers and optoisolators out there, for example, and you might find an agreeable starting place from one of their app notes.

10. Jan 10, 2017

### Staff: Mentor

The gm of the final drive transistor that sinks the motor current? What are you expecting the motor current to be?

11. Jan 10, 2017

### nothing909

3.21A per phase

12. Jan 10, 2017

### nothing909

I only have the TLP251 available. Is it possible to use it in this design?

I don't really know how to calculate values I need for resistors for the base of transistors. I don't know what I'm looking at when I look at the datasheets.

13. Jan 10, 2017

### Staff: Mentor

Can you say more about this project, and what class it is for? Are you in high school, or undergraduate university? It's a basic project, but if you don't know what to do with the CTR and transistor gains, it's hard to understand why you were assigned this project. It's a fun project to learn from, but it sounds like you have some constraints in what you can ask for and what advice you can receive...

14. Jan 10, 2017

### nothing909

It's a college assigned project, I'm not doing it myself

15. Jan 10, 2017

### Staff: Mentor

So please tell us how you calculate the motor drive current provided by the final transistor based on the input LED current limiting resistor and the parameters of the opto and other transistors in the circuit. You've gotten a lot of hints and guidance so far...

16. Jan 10, 2017

### nothing909

I honestly don't have a clue, can you help me a little more, sorry

17. Jan 10, 2017

### Staff: Mentor

Well, it's hard to understand how you could have been assigned this project without having learned about the subject matter in the classroom and homework assignments. What class is this for? How many other project members are there?

18. Jan 10, 2017

### nothing909

It's not for any specific class. The project I do throughout the year and it's graded at the end. there's like 14 other members

19. Jan 10, 2017

### nothing909

Just by looking at that circuit shown and the devices I picked, what is actually wrong with it. What do I need to change or add in?

20. Jan 10, 2017

### Staff: Mentor

Is this your freshman year? What electronics classes have you had so far? What textbooks or other resources have you been learning from? Maybe we can point you to better information sources to help your learning...