Transistor hfe

[sureshot]

Thank you AK ! I have taken all that onboard. I know I'm not going to pin down an exact value for R7, that fell into place 2 pages or more ago, as there are so many variables. My grammar understanding in context of a subject, any subject far out weights my maths capacity. I use this to gauge what I'm being advised. Experts stand out !

This is very helpful in determining what path to follow. I've studied many pages across a lot of sites, and many data sheets, also looking where people have gone with similar projects before. I will build up this circuit using a single transistor first, just to confirm I've got it right. A model working is better than a huge project flop.

As it comes together I will leave some header pins at appropriate points for testing, jumper in place when the circuit is powered up, I'll be able to take real time measurements from there opening up the circuit for current measure. Over the last six pages I've tried to pin a value down for R7, and now understand its not possible for many reasons. So I've then tried to put together in my mind the exact mechanism of how this all works, and why some senarios are undesirable in terms of heat and exesive current.

I am grateful to everyone for helping me here, it has expanded a lot of what I didn't understand. Its not all there yet, but I'm working on it ! Hopefully more will fall into place with some practical measurements. On another forum (For radio comms) Members said this circuit was a simple fools waste of time, and build another more complex linear PSU ( One mentioned as the Matchwood project) I've studied that as well. Whilst I liked it, I was more taken with the parts count and simplicity of this emitter follower circuit.

So looking back over the last six pages, what first looks simple in this circuit is far from it, when you break it down. But I'm glad I stayed with it, and will continue to until I understand exactly what's going on in its entirety, and a final quality built unit for daily use. Just one niggle... And that would be the 12 Amps Steve mentioned produced by the ballast resistors, if I went with 25 ohms Value resistor for R7 ! Could this not be to low, just for that reason ? Its not a gripe, just a genuine question. I know I don't have to go that low in resistance value. Was this just an oversight ? Anyway, over the next couple of days fingers crossed, tounge at the right angle as Dave Jones on the eevblog says lol, it will come together with some positive results.

 

[(*steve*)]

I agree with AK

 

[(*steve*)]

Could this not be to low, just for that reason ? Its not a gripe, just a genuine question. I know I don't have to go that low in resistance value. Was this just an oversight ?

Whether the current is 12A or 5A (my calculation, in an effort to be simple, ignored some of the things AK considered) is not really the point.

The point is that you won't get an arbitrary voltage across R7, once it reaches a certain point (where there is significant conduction through the transistor's base-emitter junction) a larger and larger portion of any additional current flowing through the 7812 flows *around* R7 causing the voltage across it to rise less rapidly.

Once a relatively large current is flowing, R7 becomes almost irrelevant; indeed at high currents you could remove it (make it open circuit) and the circuit would continue to work. I DO NOT RECOMMEND YOU DO THIS.

 

[sureshot]

So where you said transient response, the two senarios, high value of R7 would mean slower to start the transistors conducting and carrying the load current. And low value resistance, faster response to the transistors conducting and carrying the load current.

Also high resistance, the regulator Carry's less current doing less of the heavy work, and low resistance the regulator is doing a larger amount of the work early on, but not exceeding its maximum ratings. Does that sound right ?

I thought where you quoted 12 Amps through the ballast resistors, this was undesirable in general. As to whether this circuit with the J11015 transistor can carry more than 5 Amps per transistor I'm not 100% sure, I would think it could being its a more powerful transistor, although I know the ballast resistors wattage would have to increase I think.

Anyway that's a hypothetical senarios for me as 5 Amps per transistor is plenty. I doubt I'll ever exceed 20 Amps for any maximum load driven, and typically 15 Amps at a 50% duty cycle will be the norm.

 

[(*steve*)]

Let me offer an analogy.

Assume you have an electric kettle filled with water at 20C. If it takes 30 seconds for the temperature to reach 50C, and another 30 seconds for it to reach 80C, How long will it take to reach 110C?

Answer, never. Something magic happens at 100C.

A similar analogy applies to asking about large voltages across R7.

The transient response has little to do with when the transistors "start to conduct". Just use a resistor between 25 ohms and 200 ohms (I recommend 100 ohms) and build the thing.

We're not achieving anything dragging this out.

 

[AnalogKid]

What he said.

 

[sureshot]

Ok yes I understand the reference and your anolagy, thanks. I was already to start with 200R for R7, But... I will start with 100R for R7.

I can see know its not critical, but yes a design applied specifaction, depending on what you want from the regulator and transistors. Yes its been a long thread, but never see these things as dragged out. Every post had a meaning for me, and maybe others following. But sure I'll build it, and as a final, post the unloaded current in and out of the regulator, and again loaded up to 50 watts. Really just so anyone reading this very long thread can see a practical conclusion. Thank you to every one again, I have appreciated the help, and its been useful to look back ! Thanks again.

 

[sureshot]

Well i have some very odd results...
I can't give current reading for the input, and output to the l7812. My output voltage with 24 Volts in is 17 Volts on the output, so i removed the collector from the output rail, and got the expected 12.04 Volts.

So i tried putting 12 Volts on the input and got an output of 8 Volts. So i went over the circuit carefully and found all the voltages (except the output) to be normal.

I went back and put 24 Volts on the input and was brave enough to power a 50 watt sacraficial 12 volt lamp, the voltage dropped to 15.90 Volts, but lit the bulb, be it very bright.

I am not sure what is going on here, but have a theory ! The MJ11015 transistor gain is so high its super amplifing the output. I can't see how else i can get 17.00 Volts out of a l7812 regulator, until i remove the collector from the output, then i get the regulators 12 Volts... but no current carrying ability. As soon as i put the collector back in circuit i get the rather over expected voltage.

Can anyone tell me what's going on here ? Any help much appreciated.

 

[(*steve*)]

there is no "super amplifying"

replace the new transistors with the old ones and see what happens.

 

[sureshot]

Its a single transistor Steve, just to see if this MJ11015 worked. If i put the TIP2955 in there it will do they same as the other unit i built with a single TIP2955 does.

As that gives the regulated 12 Volts output, less a small voltage drop at full load, well 50 watts for a single transistor. So why am i getting 17 Volts on the output with the MJ11015 transistor ? This is with an R7 value of 100R.

Why is the output voltage above that of the regulators 12 Volts output, i have no idea, but when i remove the current carrying collector, the voltage is as expected 12.04 Volts, and as soon as i put the transistors collector back in circuit, it jumps back up to 17 Volts.

I thought the transistor only amplified the output current, so don't understand where i'm getting the extra 5 Volts from. Thanks for your reply and help.

 

[(*steve*)]

change back to the original transistor to confirm you have the wiring correct.

you can also try reading the voltage across R7 when the supply is unloaded.

 

[sureshot]

Ok Steve i will give this a go, tomorrow in evening , and post the results then. Thanks !

 

[sureshot]

I removed the regulator l7812 and the MJ11015 transistor to check them, and there fine. So i put them back in circuit and the output was the same.

So i measured from ground to the base side of the R7 100R resistor, and i got a reading of 21.50 Volts. My input is exactly 24 Volts (Two xbox converted psu's in series) as i don't have a transformer yet.

I tried 200R for R7 value, it was no better. Then i have some 10R 0.5 watt resistors so tried 10R for R7 value and it worked, the output was the expected 12.04 Volts. So l measured from ground to the base side of R7 and it was 19.05 Volts. I still not sure what is going on here, and i expect the 0.5 watt power rating for R7 was not high enough.

I put 50 watt load on first and it powered it fine, dropping from 12.04 to 11.89 Volts on the output. I then tried 100 watts, again it powered it, and the Voltage dropped to 11.68 Volts.

So i'm not sure from these results what a good value for R7 would be. Every thing ran cool, well slightly warm, except the 10R 0.5 watt resistor, which ran hotter but not so hot i couldn't keep my finger on it. So from this what do you think the best way to go for R7 would be ? Thank you again for helping me with this.

 

[sureshot]

So thinking about this.... 24 Volts is my input, i'm dropping 1 Volt across the bridge rectifier. So 23 Volts going into the R7 10R resistor, then 19.50 Volts is going to the regulator l7812, and 3.5 Volts dropped across R7 10R resistor. Does that sound correct ? If it does it makes sense to me.

Edit. So if 3.5 Volts is dropped across R7 does this sound correct if i have got the maths wright. 0.350 mA is going to the base junction ? Does that sound right. Thanks again.

 

[sureshot]

No, no, yes, no. Because the effective base-emitter voltage of a power darlington is about twice that of a non-darlington, R7 will have to increase to produce the necessary voltage drop to cause the power transistors to start conducting. The transistor gain is 1000 min at 20 A, and over 5000 at 5 A according to the plot. So the total base current is only 5 mA plus a few mA through the internal resistors. This is very small, and I think it can be ignored in terms of calculating how much current is going through the 7812. That current is set by the relationship between R7 and the overall Vbe of the transistor. The datasheet does not have a chart for Vbe as a function of anything, but a reasonable starting point is that it will be around 2 V, probably less. You also have to figure in the voltage drop across the ballast resistors.

The ballast resistor has two functions. It can be used to take some of the thermal stress off of its transistor. For example, if we have 5 A through each transistor and 15 V across it, that's 75 W. If the transistor is rated for 80 W, that's way too close to its limit. But if you put in a ballast resistor of 1 ohm, the resistor will dissipate 25 W, leaving only 50 W for the transistor to handle. The idea here is that a resistor is less expensive than a higher performance transistor, and also that a big fat power resistor is more reliable at high temperatures. There is a tradeoff, in that the ballast resistor reduces the input voltage range over which the circuit will function. But you have a relatively constant input voltage from the transformer, so that does not apply here.

The second thing the ballast resistor does is assure that the total power load is distributed evenly among the transistors. No two or five transistors have exactly the same base-emitter voltages or gain, so for any particular base current or Vbe the transistors will conduct different amounts of current. In an extreme case you could have 5 transistors in parallel, with four of them conducting 1 A and one of then conducting 26 A. Not good. With a resistor in each emitter leg, the total voltage in the base-emitter leg from one transistor to the next is now the individual Vbe plus the IR voltage drop across its ballast. If one transistor starts to conduct much more than the others, its ballast voltage increases, which decreases the voltage across its base-emitter, which decreases its gain, which decreases its conducted current. This is a form of negative feedback called degeneration. So, if you have a feel for how much the Vbe of your group of transistors will vary, you can use this to set a minimum value for the ballast resistor.

R7 = Vtotal / I7812, the total of all of the voltages in series across resistor R7 divided by the current through the 7812.

If we choose a worst-case 7812 current of 0.1 A, then the question is what is Vtotal at 5 A per transistor. The transistor circuits are in parallel, so their voltage drops do not add up for this. Let's pick R1 as the typical ballast resistor.

Vballast - 5A x 0.1 = 0.5 V.
Pballast = 5 A x 0.5 V = 2.5 W

Not a bad starting point. As above, you can increase the ballasts to take some power away from the transistors and get better load sharing among the power transistors.

Vtotal - Vbe plus Vballast = 2.0 + 0.5 = 2.5 V

R7 = Vtotal / I7812 = 2.5 V / 0.1 A = 25 ohms

The power dissipated in the 7812 is its current (0.1 A) times the differential voltage across the regulator minus the 2.5 V across R7. So if the average voltage on the big filter caps is 20.5 V (to make the math easy), then:

P7812 = 0.1 A x (20.5 - 12 - 2.5) = 0.6 W

And finally (!!!), this value of R7 is much less than the 100 ohms in the original design. That value had very little current going through the regulator to the load, something I don't think well of. Power regulators work best when they have something to chew on. My example increases the 7812 current significantly. It is up to the designer's discretion; there is no one "right" number.

ak

So going back to page two, if my senario of 3.5 Volts dropped across R7 10R resistor is correct, then AK's breakdown above looks like the best option. And i'm only thinking here, but an R7 value of 100 ohms or 200 ohms is to high to get the transistor to conduct. But does the 10R R7 theory i posted above sound correct ? If it does i will order some 2 watt 25R resistors. Thanks for helping me.

 

[(*steve*)]

measure the voltage across R7. If it's higher than 2V then something is seriously wrong.

also measure it with the 100R resistor and the Darlington transistor if you want to know what's going on.

 

[BobK]

Sound to me like it is wired wrong and you have, essentially, a diode (either BE or BC) between the input and the output.

Bob

 

[sureshot]

I have measured pin 1 (base) and ground, with a 100 ohm resistor its 21.5 Volts, i did the same with a 10 ohm resistor and its 19.5 Volts.

The base has the R7 on it, and it goes the regulators input, the other side of R7 is volts + input. The emitter goes to a 0.1 ohm 10 watt ceramic resistor, the other side of that resistor has the + Volts input o its junction, making the emitter base circuit. The collector is on the regulators output, after the 1 Amp protection fuse. Like i said its working, although i'm unsure of the readings of voltage i have. Its behaving as it would if it was using TIP2955's , but a single MJ11015 transistor instead. I can only imagine what i've measured from ground to the base side of R7 is 19.5 volts to the regulator, and the other 3.5 volts across R7.

If i had put a large voltage straight into the transistor, i can only imagine that would destroy it. But with a 10 ohm resistor its running a 50 watt load fine.

when i measured across the R7 it was from ground output, and the base resistor side of the 10 ohm resistor. And i have the right pin out, pin 1 base, pin 2 emitter, case is collector.

 

[sureshot]

The input to R7 is 23 Volts, the other side of that resistor with respect to ground reads 19.5 Volts. I'm using - Volts on the output and the ground point. I will go and double check.

 

[sureshot]

Ok my mistake ! I think I've got some proper numbers now.

All the followng is with a value of 10 ohms for R7.

Voltage across R7 no load is 0.98 Volts.

Voltage across R7 with 50 watts on the final output is 1.9 Volts.

Base emitter voltage no load 0.98 Volts.

Base emitter voltage with 50 watts on the final output is 1.41 Volts.

Base collector unloaded 9.62 Volts.

Base collector 50 watts loaded on the final output 7.43 Volts.

Unloaded final output 12.19 Volts.

Loaded with 50 watts, final output 11.79 Volts.

Do these readings make more sense ? If I use a 100 ohm resistor for R7 with a single MJ11015 transistor I get 17 Volts final output. Thank you again for all your help. Hope I am getting closer now.