What transistors are you using?
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To start with, you have got no heat-sinking on the diodes. The PC track-work is absolutely essential to assist in keeping the diodes cool.
Divide the output current by the current passing into the 7812 and you have the gain of the transistor.
So the output current of the original schematic is quoted as 30 Amp and the current passing through the regulator is 0.860mA this gives a gain of 34.8 but the TIP2955 has a gain of 20@4A .. It doesn't add up ! Just want to say thanks to all who have helped me this far. At this stage using the original schematic I've got an R7 value of 25 ohms at 0.5 watts, and 6 x ballast resistors at 0.1 ohms at 10 watts. This arangment is for the circuit with 6 x MJ11015 transistors. As to if they will bias and forward conduct at those resistor values, I'm not 100% sure. I have read back over several posts to try and work this out. I think at these resistor values it will work, but not certain as to variables like voltage drop under output load, and how hard the transistors would be driven.
"but the TIP2955 has a gain of 20@4A .."No between 20 and 70.
Sure I didn't say TIP2955. Had a gain of 20 - 70 anyway they work ok in this circuit, pretty good indeed to be honest. I was only interested in the MJ11015 transistor for its greater power handling capability, and where the gain is 1K that looks desirable in terms of perhaps less final voltage drop once the circuit is under load. So at this point it looks Like a taste it and see way forward. This is with changing all six transistors to the MJ11015 transistors, change R7 to 25 oms 0.5 watts, and use 10 watt 0.1 ohm ballast resistors. Hopefully this senario won't drive the transistors and voltage regulator to hard. I only need an upper maximum current output of 20Amps, and typically 15 Amps at a 50% duty cycle load. It would be nice to know the extra current would be there, even if I don't use it up to 30Amps, its a good margin in reserve.
I told you before that there is ABSOLUTELY no difference between the two transistors.
The losses will be exactly the same.
That's why very few people understand electronics. You cannot learn it from a book. It is a waste of money to buy the other transistors.
Thy are not a ballast resistor. A ballast resistor is a dumping resistor. The 0.1R is designed to get the transistors to share the load because a transistor with a higher gain will tend to pass more current. If it passes more current, this extra current will flow though the resistor and crate a slightly higher voltage drop across it.
Now there is only a certain voltage (called a fixed or generated voltage) between the input to the transistors and the base of all the transistors.
This voltage is made up of a voltage across the resistor and the voltage across the emitter-base junction. If the voltage across the resistor increases, it robs (takes away) the voltage from the emitter-base junction and this turns the transistor off slightly. This is how the transistors are forced to share the load.
The losses will be exactly the same.
That's why very few people understand electronics. You cannot learn it from a book. It is a waste of money to buy the other transistors.
Thy are not a ballast resistor. A ballast resistor is a dumping resistor. The 0.1R is designed to get the transistors to share the load because a transistor with a higher gain will tend to pass more current. If it passes more current, this extra current will flow though the resistor and crate a slightly higher voltage drop across it.
Now there is only a certain voltage (called a fixed or generated voltage) between the input to the transistors and the base of all the transistors.
This voltage is made up of a voltage across the resistor and the voltage across the emitter-base junction. If the voltage across the resistor increases, it robs (takes away) the voltage from the emitter-base junction and this turns the transistor off slightly. This is how the transistors are forced to share the load.
I know the power dissipated will not change ! The term ballast resistor is simply a term I used for resistors making transistors share current equally. My primary question was to try and assertain the R7 value via some help with the maths, as the MJ11015 transistor might not conduct with R7 value at 100 ohms. I really do understand no power dissipated through the entire circuit changes. My use of the MJ11015's was they have a higher upper power rating and would, or might be more Hardy in this circuit. And as the specifactions of the more powerful transistor his greater I was thinking it wouldn't have to work so hard for the same numbers as the TIP2955 transistors working environment. Not sure if you get what I mean. But thanks for helping.
If you are only dissipating 70 watts per transistor, it does not matter which transistor you use.
I see what you mean, but the maximum power for the TIP2955 transistor is 90 watts, and the maximum power for the MJ11015 transistor is 120 watts... So would the MJ11015 transistor have a higher upper margin on power over the TIP2955 transistor. Also as Steve pointed out to me, as its gain is far higher, then voltage sag on the final output might be less. Just things I see, as to the more powerful transistor is worth giving a go, I think yes it might just be worth trying it out. Not to much money out of pocket if it all goes south.
So at this point I'd like to say Thank you for everyone that has help me here, much appreciated ! I will have a go with this MJ11015 transistor, and have a play with resistor values, whilst measuring current and voltage with a couple of multimeters to see in real practical terms how things pan out. With a bit of luck and wind blowing in the right direction, I can achieve a reasonable result.
Many Thanks again to you all !
Many Thanks again to you all !
You might want to look at post #2 in this link. http://www.electro-tech-online.com/threads/bullet-proof.146490/#post-1240701
Ratch
Thanks Ratch ! I've done so many searches looking for people that have implemented this simple emitter follower type circuit. In my searches I have happened across the thread you posted a link for, after so many searches you forget some stuff, it only becomes aparant when your memory is jogged. Thanks for that !
After I get 6 x these MJ11015 transistors and L7812 working well I will post back some results, although I struggle a bit with the numbers on paper, I've no problems measuring parameters in real time with meters.
Just want to touch on something Colin Mitchell said on learning electronics ! As a hobbyists I've been at it a few years now, but always struggled with the numbers, "I've learned a lot from books study, although some senarios for me play out better watching tutorials from those with serious experience. Dave Jones from the eevblog is one I love watching, and learned loads from his video tutorials. Never be put off learning ! If its from books tutorials or experienced friends or colleagues. Have a go ! But at the same time stay safe and start with battery low power safe circuits. This is only for anyone new to electronics reading this thread. If you don't try your never know ! But play safe at the same time.
I will post back soon hopefully with some posative real time results. Thank you again to everyone for your help here !
After I get 6 x these MJ11015 transistors and L7812 working well I will post back some results, although I struggle a bit with the numbers on paper, I've no problems measuring parameters in real time with meters.
Just want to touch on something Colin Mitchell said on learning electronics ! As a hobbyists I've been at it a few years now, but always struggled with the numbers, "I've learned a lot from books study, although some senarios for me play out better watching tutorials from those with serious experience. Dave Jones from the eevblog is one I love watching, and learned loads from his video tutorials. Never be put off learning ! If its from books tutorials or experienced friends or colleagues. Have a go ! But at the same time stay safe and start with battery low power safe circuits. This is only for anyone new to electronics reading this thread. If you don't try your never know ! But play safe at the same time.
I will post back soon hopefully with some posative real time results. Thank you again to everyone for your help here !
Back again... Before I commit to the use of the 6 x MJ11015 PNP transistors, I've happened across some old surplus stock from a supplier. This other transistor device is the MJ11033 it looks very good for the price I can get it for.
The only difference I can see in the data sheets is the base current for the MJ11015 transistor is maximum of 1 Amp, and the MJ11033 transistor has a base current of 2 Amps maximum, and its maximum power handling ratings go up to 50Amps 300 watts.
My question is this, with help (very much appreciated) I found a value of R7 in the schematic (start of the thread) would need to be 25 ohms at 0.5 watts for the MJ11015. With this larger transistor base current of 2 Amps for the MJ11033 transistor, would I need to increase the R7 resistor value further. Thanks for reading, any advise greatly appreciated.
The only difference I can see in the data sheets is the base current for the MJ11015 transistor is maximum of 1 Amp, and the MJ11033 transistor has a base current of 2 Amps maximum, and its maximum power handling ratings go up to 50Amps 300 watts.
My question is this, with help (very much appreciated) I found a value of R7 in the schematic (start of the thread) would need to be 25 ohms at 0.5 watts for the MJ11015. With this larger transistor base current of 2 Amps for the MJ11033 transistor, would I need to increase the R7 resistor value further. Thanks for reading, any advise greatly appreciated.
Normally you "drive" a transistor. In this case the transistor "takes" or "allows to flow" the necessary base current to deliver the collector current. At 5 amps and a gain of 1,000 the base current will be a few milliamp.
Thanks for your help Colin ! So does this mean I can use the an R7 value of 25ohms 0.5W as suggested by Analogkid back on page 2 of this thread. In other words the base current is 1 Amp for the MJ11015 and 2 Amps for the MJ11033, would the same value of 25 ohms 0.5W be suitable to bias the transistors to conduct ? Thanks again.
Remove R7. It is not needed.
I think both you and several responders to this thread are in need of an adjustment in perceiving how a BJT transistor works. First of all, a BJT is not a current amplifier, beta is not the current amplification factor, and you do not control or drive the transistor with current. Without going into the physics of the BJT, let me say that a BJT is a transconductance amplifier. That means that a voltage applied to the base-emitter will control the collector current. The beta is a measure of the "waste" current that is present at the base terminal when the transistor is biased in the active region. It is proportional to the collector within limits, but it does not control the collector current. Now, a BJT can be part of a circuit that amplifies current, but a BJT by itself cannot be called a current amplifier.
When the output of the 7812 lowers, its series resistance decreases, which increases the current of R7, which increases the base-emitter voltage of the pass transistors, which increases the collector currents of the pass transistors, which increases the voltage across the load and brings the voltage back up to its nominal value. I have no idea what the resistors marked 1 amp and 30 amp do. What you have to do is make sure the 7812 can input the current of R7 and the waste current from the base of the pass transistors. Furthermore, the value of R7 must be large enough to cause enough voltage variation to control the pass transistors when the current changes in the 7812.
Ratch
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