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Electronics => Beginners => Topic started by: MarkS on January 29, 2012, 04:50:18 am

Title: Can someone explain this transistor datasheet to me?
Post by: MarkS on January 29, 2012, 04:50:18 am
http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00001168.pdf (http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00001168.pdf)

I am looking at this for a linear voltage regulator circuit. Looking at the voltages and current ratings, everything is negative. I need to calculate the value of a resistor based on the transistor's Vbe rating, which is -1.2V(?!). This means I would need a resistor with a negative Ohm rating. Either that isn't correct or I need to source the resistor from the free energy market. :o

I'm obviously reading this wrong...
Title: Re: Can someone explain this transistor datasheet to me?
Post by: metalphreak on January 29, 2012, 04:57:11 am
if Vbe (base to emitter) is -1.2v then Veb (emitter to base) would be +1.2v

Negative voltage with a normal resistance gives you a negative current (ie flows in the opposite direction). The postive/negative stuff is just convention to define direction.
Title: Re: Can someone explain this transistor datasheet to me?
Post by: vtl on January 29, 2012, 04:57:47 am
Its a PNP transistor so its backwards compared to an NPN
Title: Re: Can someone explain this transistor datasheet to me?
Post by: amspire on January 29, 2012, 05:00:23 am
When you calculate the resistor needed to give a certain negative voltage drop at a specified negative current, you will get a positive resistor value. Anytime you get a negative resistance value, you have got the a sign around the wrong way somewhere.
Title: Re: Can someone explain this transistor datasheet to me?
Post by: MarkS on January 29, 2012, 05:04:34 am
OK, so how do I calculate the resistor value for figure 23 on page 25 of this voltage regulator (L78S05C): http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00000449.pdf (http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00000449.pdf)

Rsc = Vbe / Isc, where Isc = 500 ma. Unless I am mistaken, that is 2.4 Ohms.

The transistor listed in the datasheet is no longer produced (or I cannot find it), so I am having to find an equivalent part. Maybe I made a mistake on the transistor?
Title: Re: Can someone explain this transistor datasheet to me?
Post by: amspire on January 29, 2012, 05:25:52 am
Quote from: MarkS on January 29, 2012, 05:04:34 am
OK, so how do I calculate the resistor value for figure 23 on page 25 of this voltage regulator (L78S05C): http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00000449.pdf (http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00000449.pdf)

Rsc = Vbe / Isc, where Isc = 500 ma. Unless I am mistaken, that is 2.4 Ohms.

The transistor listed in the datasheet is no longer produced (or I cannot find it), so I am having to find an equivalent part. Maybe I made a mistake on the transistor?

That is right, but you wouldn't use a darlington transistor for Q2 - just a conventional PNP transistor. You can use any PNP transistor as long as it can handle the absolute maximim current of the L78SXX family of regulators.

A non-darlington transistor will have a Vbe of about 0.7V, so that means a 1.2 ohm resistor at 500mA.

Richard

Edit: The current limit in Fig 23 only limits the current in the bypass transistor, not the regulator. So it is a bit odd setting the current limit to 0.5A in the bypass transistor when the regulator can still pass 2A.

Looks like the datasheet does not set a maximum current, just a typical short circuit current of 3A. So I would pick a PNP transistor for Q2 with that will saturate to less then 0.5V at 6A with a base current of less then 0.3A.
Title: Re: Can someone explain this transistor datasheet to me?
Post by: Lesolee on January 29, 2012, 05:44:38 pm
The key thing to understand about that data sheet is that they are trying to sell that product, not to help you out. Whoever drew the circuit diagram doesn't even know the basics of how to draw a circuit. It also looks like a potential oscillator to me.

I strongly recommend you do it another way.  I just checked out the RS components web site and there are loads of 5A regulators for fixed and variable outputs. Buy one of them (or bigger) and you will have less trouble. That transistor you picked has a current gain of 4 at 4A which is really not very useful!

I am of course assuming you NEED a linear regulator. Otherwise a simple switcher module will be far more efficient (less hot).
Title: Re: Can someone explain this transistor datasheet to me?
Post by: MarkS on January 29, 2012, 08:04:58 pm
Well, I made a mistake! :-\ I was looking at the various circuits and saw "short circuit protection" and thought "reverse polarity protection". :o I know better. Brain fart!

Anyway, I do appreciate the help. I did learn some things I didn't know.