Transistor hfe

[sureshot]

Thanks for the description ! As much as know transistor are like a pair of diodes, they won't conduct unless a current is applied to the base junction. To have no resistor for R7 leaves me perplexed to be honest, as I've always known transistors to need a base current to function, as to if that's exesive with no resistor for R7 I'm unsure.

The only transducers I've come across is in the marine world, as I'm primarily a diver. I find transistor physics fairly complex, I know there silicone and other materials that are doped to limit the flow of electrons. My question was did I need to increase the value of R7 25R 0.5W As the other transistor has a higher maximum current of 2 Amps at its base, as opposed to 1 Amp. The two transistors are the MJ11015, and MJ11033 the latter is the transistor I was asking about altering the base value of R7 for. In the diagram there are no resistors marked 1 Amp and 30 Amps ! There are seven resistors in total, R1 to R7. R1 to R6 are ceramic power resistors of 0.1 ohms 5W, R7 is shown as 100R 0.5W. So far if I change the 6 x TIP2955 transistors to 6 x MJ11015 transistors, and that I would need to reduce the value of R7 to 25R 0.5W . But I've since happened on some surplus stock ant a very good price. These are MJ11033 transistors, there more powerful again in the specifactions sheet, and there maximum base current is 2 Amps, as opposed to 1 Amp for the MJ11015 package.

So my question was would I need to alter the value of R7 to less resistance than 25R 0.5W ? And would be very greatful of help with this ! I will continue to read and study diagrams, and learn as much as I can about transistor formula, although as I said before I struggle a bit with the complex formulas. Thank you again to everyone that has helped me in this thread.

 

[Ratch]

Thanks for the description ! As much as know transistor are like a pair of diodes, they won't conduct unless a current is applied to the base junction. To have no resistor for R7 leaves me perplexed to be honest, as I've always known transistors to need a base current to function, as to if that's exesive with no resistor for R7 I'm unsure.

You can't make a BJT (bipolar junction transistor) by connecting two diodes back to back. There is a lot more to a BJT than that. See below for discussion of R7. Transistors need a base current like a city needs a sewage system. They are both waste quantities.

The only transducers I've come across is in the marine world, as I'm primarily a diver. I find transistor physics fairly complex, I know there silicone and other materials that are doped to limit the flow of electrons. My question was did I need to increase the value of R7 25R 0.5W As the other transistor has a higher maximum current of 2 Amps at its base, as opposed to 1 Amp. The two transistors are the MJ11015, and MJ11033 the latter is the transistor I was asking about altering the base value of R7 for. In the diagram there are no resistors marked 1 Amp and 30 Amps ! There are seven resistors in total, R1 to R7. R1 to R6 are ceramic power resistors of 0.1 ohms 5W, R7 is shown as 100R 0.5W. So far if I change the 6 x TIP2955 transistors to 6 x MJ11015 transistors, and that I would need to reduce the value of R7 to 25R 0.5W . But I've since happened on some surplus stock ant a very good price. These are MJ11033 transistors, there more powerful again in the specifactions sheet, and there maximum base current is 2 Amps, as opposed to 1 Amp for the MJ11015 package.

You specified 0.5 watts for your emitter resistor wattages. Since each BJT is going to supply 5 amps, that makes each resistor dissipation 2.5 watts. Those emitter resistors are not going to last long at that rate. Each resistor should be rated at 5 watts, If you do it with one transistor, then your emitter resistor dissipation will be 30x30xR = ? You do the math.

So my question was would I need to alter the value of R7 to less resistance than 25R 0.5W ? And would be very greatful of help with this ! I will continue to read and study diagrams, and learn as much as I can about transistor formula, although as I said before I struggle a bit with the complex formulas. Thank you again to everyone that has helped me in this thread.

You want 30 amps from that transistor. The minimum beta is 400, so that makes the base current 30/400 = 75 milliamps that will go through the 7812 from the transistors when they are supplying the full current. Since the 7812 can supply 1 amp, that should not be a problem. That is well above the quiescent current of 5 milliamps, so perhaps you don't need R7. Otherwise, you could decide to use the 7812 to supply the current up to 1 amp and have the pass transistor supply the rest of the current up to 30 amps. By adjusting R7, you can vary how much current up to 1 amp the 7812 will supply. For instance, if you want the pass transistor to turn on when the 7812 is at 1 amp, and assuming the pass transistor starts to turn on at 1.4 volts, then R7 would be 1.4 ohms.

Ratch

 

[sureshot]

The ceramic resistor I plan on using would be 10 watts 0.1R. I understand your description there.
Further on in your last paragraph is interesting, I understand this as well, apart from what would stop the l7812 trying to pass more current than 1 Amp ? This is why I though a resistor value for the MJ11033 use in this circuit would be needed. Other wise I really don't understand what limits the regulators current work its doing. Ideally using the MJ11033 transistor in this circuit I'd like to limit the regulators working upper current to the 0.860mA it is where the TIP2955 transistors are used, as per the schematic. But using the MJ11033 transistor I don't know what value that would need to be. And as I've said above I don't uderstand , if there is no R7 what stops the regulator trying to do all the work. Although I know it would go into thermal shut down if it was severely overloaded. Thank you for helping me here ! I still don't know why with the absence of R7 what limits its work load.

Just looking back.. If the l7812 has to reach 2 Amps before the transistors start conducting, doesn't that mean 2 Amps through the l7812 is being worked to hard. I only mention this because its maximum working current is 1 Amp. So I would like to limit the current through the l7812 to about the same 0.860mA as in the schematic, but still biasing the MJ11033 transistors to take the current above 0.860 Amps. For this I don't know the value I would need for R7.

 

[Ratch]

The ceramic resistor I plan on using would be 10 watts 0.1R. I understand your description there.

It really should be twice the calculated wattage or 20 watt to conform to good practices.

Further on in your last paragraph is interesting, I understand this as well, apart from what would stop the l7812 trying to pass more current than 1 Amp ?

I imagine the 7812 has internal circuitry that limits the current to one amp.

This is why I though a resistor value for the MJ11033 use in this circuit would be needed. Other wise I really don't understand what limits the regulators current work its doing. Ideally using the MJ11033 transistor in this circuit I'd like to limit the regulators working upper current to the 0.860mA it is where the TIP2955 transistors are used, as per the schematic. But using the MJ11033 transistor I don't know what value that would need to be. And as I've said above I don't uderstand , if there is no R7 what stops the regulator trying to do all the work. Although I know it would go into thermal shut down if it was severely overloaded. Thank you for helping me here ! I still don't know why with the absence of R7 what limits its work load.

The specs say the quiescent current of the 7812 is 5 milliamps. I don't know whether this current comes from the input or output terminal. If it comes from the input terminal, and no R7 is present, then the 5 milliamps will be supplied by the transistor. Assuming a maximum beta of 1000, the collector current when the 7812 is off will be 5 amps. This means that the circuit cannot control the voltage when the current drops below 5 amps. With R7 present, the quiescent current is supplied by R7 instead of the transistor and the 7812 can control the current down to its lowest value.

Just looking back.. If the l7812 has to reach 2 Amps before the transistors start conducting, doesn't that mean 2 Amps through the l7812 is being worked to hard. I only mention this because its maximum working current is 1 Amp. So I would like to limit the current through the l7812 to about the same 0.860mA as in the schematic, but still biasing the MJ11033 transistors to take the current above 0.860 Amps. For this I don't know the value I would need for R7.

The 7812 will not reach 2 amps due to its limiting circuitry. Assuming 1.4 volts is needed to turn on the transistor, R7's value is 1.4 * 0.86 = 1.2 ohms. This should turn on the transistor when the 7812 current reaches 0.86 amps. Since we already calculated that 30 amps collector current causes a maximum of 75 milliamps to exist in the base circuit, there is more than enough capacity left for the 7812 to drive the transistor to 30 amps. There is no current limiting for over 30 amps, however.

Ratch

 

[sureshot]

Thanks again for your help here ! So I think I've got this now.. To limit the current through the l7812 to 0.860mA I should use a 1.2 ohm resistor, and if I've done the power calculation for this resistor its 1.63 watts, so using a 3 watt resistor should be suitable ? It is 6 x MJ11033 transistors I planned on using. And using 6 x 20 watt 0.1 ohm power resistors for the current sharing.

 

[(*steve*)]

R7 must me set such that the voltage across it when the quiescent current of the regulator flows through it, that voltage is insufficient to turn on the pass transistors.

There is no need to change it when the transistors are changed.

 

[Ratch]

Thanks again for your help here ! So I think I've got this now.. To limit the current through the l7812 to 0.860mA I should use a 1.2 ohm resistor, and if I've done the power calculation for this resistor its 1.63 watts, so using a 3 watt resistor should be suitable ? It is 6 x MJ11033 transistors I planned on using. And using 6 x 20 watt 0.1 ohm power resistors for the current sharing.

Yes, three watts or greater. I don't think 3 watts is a standard size. I think 4 watts is, but I could be wrong about that. Since the transistors are darlingtons, 0.86 * 1.2 = 1 volt sounds a little low. I would assume 1.5 volts at least. The wattage for the 6 emitter resistors should be 5 * 5 * 0.1 == 2,5 , Double that for 5 watts each. I think you got the idea of how this works.

Ratch

 

[sureshot]

I think i have... but what Steve has said has thrown me a bit, no need to chang R7 value from the suggested 25 ohms 0.5W which was for the 6 X MJ11015 transistors. i've now got my sights on the MJ11033 transistors x 6.

So if the maximum base voltage is 5 volts, and i want close to 0.500mA of current limit on the l7812, i've worked this out: 2 volts to the base transistor junction, so 2 volts squared / 3.9R = 1.20 watts / 2 volts = 0.512mA.

So 3.9 ohm 2.5 watt 5% silicone resistor for R7
Have i got that right now ? Thanks again for helping me here !

 

[sureshot]

Looking at this screen shot, reading it back through for the original calculations, i'm not sure i've got it right, its this base chain bit of text that's got me confused. I am only going back over this for the formula used. Although i still don't know if the post above workings out is correct.

 

[Ratch]

I think i have... but what Steve has said has thrown me a bit, no need to chang R7 value from the suggested 25 ohms 0.5W which was for the 6 X MJ11015 transistors. i've now got my sights on the MJ11033 transistors x 6.

If the base-emitter voltage stays the same, you do not have to change R7. Does 25 ohms make sense?

So if the maximum base voltage is 5 volts, and i want close to 0.500mA of current limit on the l7812, i've worked this out: 2 volts to the base transistor junction, so 2 volts squared / 3.9R = 1.20 watts / 2 volts = 0.512mA.

5 volts is the maximum base-emitter voltage, not what you use for nominal design.

So 3.9 ohm 2.5 watt 5% silicone resistor for R7
Have i got that right now ? Thanks again for helping me here !

What is the maximum current you desire through the 7812? What are you assuming for the base-emitter voltage?

Ratch

 

[Ratch]

Looking at this screen shot, reading it back through for the original calculations, i'm not sure i've got it right, its this base chain bit of text that's got me confused. I am only going back over this for the formula used. Although i still don't know if the post above workings out is correct.

Don't get hung up on formulas. Understand what you are doing and then calculate the values you require.

Ratch

 

[sureshot]

Does 25 ohms make sense.. it did two pages back. But this Mj11033 transistor has double the maximum base current. Ideally i would like the regulator to be working up to half its maximum rating, say 0.500mA before the transistors start conducting. So over 0.500mA the transistor switch on. From what Steve said changing the resistor value is not necessary, so i take it he means from 100 ohms ! Yet back across previous pages and post, someone said 100 ohms is to greater a resistance to turn the transistors on at all.

Even i know there's a huge difference between 100 ohms, 25 ohms, and my attempt calculation at 3.9 ohms. So conflicting comments across the thread !

I didn't expect it handed on a plate (the value of R7) yet it feels like going round in circles and getting nowhere. what's frustrating is using meters and swapping in and out resistors and measuring current and voltage is easy.

Although i don't have half the value resistors with me, i'd have to buy a few values. But having said that i should be able to get the value of R7 for this more powerful transistor on paper, working it out that is. Rather than half tie a shoe lace you teach someone to tie it all the way, then they get the hang of it ! half bits and pieces of equations isn't helping 4 pages on.

Please don't think i'm not grateful, as i am, and that's for all the help. As a diver we teach people, not throw half the gear they need and expect them to learn. I hope my comments are not taken out of context. In the diving world there are experts very experienced and novices !

Thanks for everyone's help up to here, but i'm still no closer to an R7 value.

 

[Ratch]

Does 25 ohms make sense.. it did two pages back. But this Mj11033 transistor has double the maximum base current. Ideally i would like the regulator to be working up to half its maximum rating, say 0.500mA before the transistors start conducting. So over 0.500mA the transistor switch on. From what Steve said changing the resistor value is not necessary, so i take it he means from 100 ohms ! Yet back across previous pages and post, someone said 100 ohms is to greater a resistance to turn the transistors on at all.

Even i know there's a huge difference between 100 ohms, 25 ohms, and my attempt calculation at 3.9 ohms. So conflicting comments across the thread !

I didn't expect it handed on a plate (the value of R7) yet it feels like going round in circles and getting nowhere. what's frustrating is using meters and swapping in and out resistors and measuring current and voltage is easy.

Although i don't have half the value resistors with me, i'd have to buy a few values. But having said that i should be able to get the value of R7 for this more powerful transistor on paper, working it out that is. Rather than half tie a shoe lace you teach someone to tie it all the way, then they get the hang of it ! half bits and pieces of equations isn't helping 4 pages on.

Please don't think i'm not grateful, as i am, and that's for all the help. As a diver we teach people, not throw half the gear they need and expect them to learn. I hope my comments are not taken out of context. In the diving world there are experts very experienced and novices !

Thanks for everyone's help up to here, but i'm still no closer to an R7 value.

That transistor has a minimum beta of 400. Therefore each transistor will have at most a base current of 5/400 = 12.5 milliamps. Six of those transistors will make a total base current of 75 milliamps. I did that calculation previously. Who cares if the maximum base current of each transistor can be up to 500 milliamps? All you need is it to carry 75 milliamps.

In post #70, I averred that you need to estimate the base-emitter voltage and divide it by what 7812 current you want the transistors to start conducting. As an example, 1.4 volts/0.5 amps = 2.8 ohms. That is the value of R7 you need, so why keep dragging 25 and 100 ohms back into the discussion? If different transistors have the same base-emitter voltage while conducting, then you don't have to change R7.

Ratch

 

[AnalogKid]

1. Do not eliminate R7. R7 sets the load current level at which the external transistors start to assist the 7812.

2. From the point of view of this circuit, the11015 and 11033 transistors are identical. Both have enormous gain at 5 A collector current, and approximately the same Vbe. Unfortunately, both are equally poorly documented, so your first pass at this circuit should work but the point at which the external transistor start to conduct might be off.

3. Please re-read post #26. The value of R7 depends on the value of the ballast resistors, the number of external transistors in parallel, the transistor Vbe (which is not known exactly), and your choice of the maximum current through the 7812.

ak

 

[Colin Mitchell]

"ballast resistors" They are not ballast resistors. They are BALANCING RESISTORS or EQUALISING RESISTORS. Get your terms right.

 

[Colin Mitchell]

Do not eliminate R7. The 7812 can get its quiescent current from the base of the transistors. You don't need R7.

 

[sureshot]

Thank you for these replys above ! It might have looked like I was sounding off In my "frustrated" post above. But as a novice over the last four pages we've gone from.... You don't need to change the value of R7, to you need to decrease the value of R7 to 25ohms, to you don't need a resistor for R7 so don't use one. These have been the surggesions. That is very confusing, as there's a big margin between 100 ohms and 0 ohms !

So hopefully you can see how I'm confused. As I said I do appreciate the help, and have been trying to do the maths from your formula descriptions. This is how I came up with a value of 3.9 ohms 2.5W for R7 if I want the current limited to 0.500mA on the regulator. And at this point I've no idea if that's correct, but I doubt it. And if R7 is set to 25 ohms I'm not sure what maximum current the l7812 is limited to.

I keep going back to the R7 100 value as it states the regulator Carry's 0.871mA but that's using the 6 x TIP2955 transistors. And I've gone back to looking at the R7 value at 25 ohms because that suggestion was made back on page 2 in a break down formula. As for no resistor at all for R7 I think that might be a bad idea.

Above a surggesion of 1.2 ohms for R7 was made, but Steve said the voltage would not turn the transistors on ! And the maximum base voltage is 5 volts, so 1 volt does sound a bit low, even 1.4 volts the Ratch suggested sounds a bit low.

As I need some idea myself what to use, running the regulator l7812 between 1/2 and 3/4 of its rated current limit sound reasonable. And a transistor base voltage of 2 volts sounds reasonable. So going back to the original drawing schematic running the regulator l7812 at 0.800mA sounds ideal ! And so does 2 volts at the base to switch the transistors on.

A value of 0.1 ohms 10W power resistor sounds reasonable for the current sharing resistors, maybe increasing there maximum power handling to 20W . So to achieve a current limit of 0.800mA work for the l7812, and a base voltage of 2 volts, does a resistor value for R7 of 2.5 ohms 5W achieve this ? Hope you can understand the above, thanks again for all the help.

 

[Colin Mitchell]

"
That is very confusing, as there's a big margin between 100 ohms and 0 ohms !"
No.
100 ohms and infinitely high resistance.


0.871mA ??????????????

A value of 0.1 ohms 10W power resistor 5 amps through 0.1R will produce 500mV.
This is simply an adjustment voltage to make each transistor takes the same current. The wattage dissipated is 2.5 watts so why use 10 watts. Why not use 100 watt resistor !!!!!
You are constantly running around in circles, worrying about things that do not exist and changing things that will make no difference at all.
You are much better off to use 10 transistors or reduce the transformer voltage to 18v as explained in my article on talking electronics.com website.

 

[sureshot]

"
That is very confusing, as there's a big margin between 100 ohms and 0 ohms !"
No.
100 ohms and infinitely high resistance.


0.871mA ??????????????

A value of 0.1 ohms 10W power resistor 5 amps through 0.1R will produce 500mV.
This is simply an adjustment voltage to make each transistor takes the same current. The wattage dissipated is 2.5 watts so why use 10 watts. Why not use 100 watt resistor !!!!!
You are constantly running around in circles, worrying about things that do not exist and changing things that will make no difference at all.
You are much better off to use 10 transistors or reduce the transformer voltage to 18v as explained in my article on talking electronics.com website.

I'm aware 100R to 0R is resistance to infinity, why 0.870mA ? Because that seems like a reasonable work load for the l7812. As for going round in circles this might be due to conflicting advise on the value of R7 ! I'm sure that I don't really won't to go round in circles, but just acertain a value for R7 given a current limit of 0.800mA for l7812, and a base emitter voltage of 2 volts to bias the transistors to turn on. If I can pin down a value for R7 then I could press on and build the circuit up.

 

[sureshot]

Why is it no one can advise me if a value for R7 of 2.5 ohms 5W will achieve a current limit of 0.800mA for the l7812, and a base emitter voltage of 2 volts ? With all due respect to everyone that has helped me.

All I am asking is this correct or is it wrong ?