# Transistor questions in control a stepper motor

1. Jul 26, 2016

### Sherldonnnn

Hello,

I am Sherldonnnn. I'm very happy I can ask my questions here to get your help. If you can help me solving my questions that will be very nice.

Now I am trying to control a stepper motor with a simple transistor-based switch. I still with to consult with the experts here on some basic questions:

The transistor I bought from kynix semiconductor has labeling as "S8050 D 331". I used a multimeter to measure its hfe value and it shows 186 (varying between 186 and 188).

However, when i browsing kynix electronics .I found a document from http://www.kynix.com/Detail/650928/S8050.html which appears to me somewhat confusing:

1. There are hfe1, hfe2, hfe3for this S8050 transistor and for each there are associated MIN, TYP and MAX value. for hfe1and hfe3there areonlya MIN value, but for hfe2the values MIN, TYP and MAX areallexist. Why is the difference?

2. What does hfe1, hfe2and hfe3mean? If they all exist, do I still have a unique value? Which one of them correspond to the186value which I got from the multimeter?

3. Why for each of them there are a range of values (MIN, TYP, MAX)? If the hfe has, for example like hfe2, MIN value as low as100and MAX as high as400, then how can I know exactly which hfe value the transistor will operate on, and consequently, how should I design the circuit (resistance values, saturation mode, etc.)?

I could hardly see any clue in this data sheet. Help will be greatly appreciated.
Thanks a lot.
Regards,
Sherldonnnn

2. Jul 27, 2016

### Baluncore

The value of Hfe is hard to control during manufacture so devices are sorted into different grades or ranks after manufacture. The value of Hfe is also current dependent, which is why they identify three different ranges.

The min=120, typ=110, max=400 seems irrational since the typical is less than the minimum. Either there is a typo in the data sheet or the typical and the maximum refers to all classes of the device.

Low Hfe increases the base current required to drive the output load. High Hfe slows down the transistor in high speed applications because it multiplies the miller capacitance between base and collector.

You should always design for the worst case values. That is why the minimum and maximum values are given.
An analogue circuit is designed to minimise gain variation due to Hfe. Hfe spread and variation is then not critical.
A digital circuit will need some way of limiting the base current to the maximum required for the lowest Hfe.

Can you post the circuit diagram that shows how you will use this transistor.

3. Jul 28, 2016

### Tom.G

I agree, looks like an error in the data sheet; I would choose for 110 for both min and typ, and probably design for 100 or less.

You will notice that the three lines of hFE data are for different Collector currents, Ic. This is another case of a confusing data sheet. The hFE of a transistor varies with Collector current and these three lines show the hFE to expect at different currents. In the US at least, data sheets do not number the entries, just leave them all as hFE. The 'binning' for gain is presented as a field in the part number, such as "S8050-1", "S8050-2"; or more often as "S8050-A", "S8050-B". If such binning is used, they typically appear as separate columns on the data sheet.

4. Aug 3, 2016

### Sherldonnnn

Yes,I agree with you .It's really difficult. I am trying to make a diagram. Hope you don't feel offend. I can't do it very fast.Thanks for your suggestion!

5. Aug 3, 2016

### Sherldonnnn

Yes, maybe the datasheet was wrong. Thanks . I will check it out and carefully read the other documents.