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Resistor values.

Discussion in 'General Electronics Discussion' started by sureshot, Jan 8, 2016.

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  1. sureshot

    sureshot

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    After trying a 12 volts power supply circuit with a single TIP2955 transistor as a pass element, and a 78S12CV voltage regulator this works really well.
    This is the schematic with the single transistor.

    I would like the try and build a 10 Amp version with two of the same transistors. I think I've got the resistor values correct. I came up with 16.6R 0.5 watt for the transistor base resistor, and two 10 watt ceramic resistors of 0.5R each. Just wanted to check I've got the values correct before building this second circuit. Thanks for reading, any help appreciated.
     
  2. sureshot

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    I can't seem to upload an image of the schematics so posted a link. The 30 Amp PSU is the circuit, and I've built a single resistor version, but wanted to know if I have the resistor values correct for a two transistor version.
    http://www.zen22142.zen.co.uk/Circuits/Power/1230psu.htm
     
  3. sureshot

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    This is the circuit I wanted to make a two transistor version of, sorry about image quality, I'll try and post a better image.
     

    Attached Files:

  4. Arouse1973

    Arouse1973 Adam

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    Why on earth do you want a 10 Amp PSU? You will have big problems if you dont want 10 Amps and your circuit faults out and wants all that current. You need some current limit.
    Adam
     
  5. sureshot

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    Once I've tested this second version of two transistors, and voltage regulator I will find a suitable crow bar circuit or over voltage, over current needs only yto be a fast blow fuse. 10 Amps to drive an amplifier. I was looking to see if my values of 16.6 ohms base resistor, and two 10 watt 0.5 ohms was accurate.
     
  6. sureshot

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    The only thing I can see what I think is wrong, is R7 should be going to the base of the transistors, and not in series with the input like it shows in the schematic above.
     
  7. sureshot

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    So i can see R7 limits current into the voltage regulator, so its position must be correct, what i am really looking for is do i have the values correct for a two transistor version, of 16 ohms for a base resistor, and 10 watt ceramic ballast resistors of 0.5 ohms each two of these one for each transistor. surley the values of the resistors must be different, or can't be the same for two transistors as it is for all six transistors, any help please !
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    For a two transistor version, remove the extra transistors and their emitter resistors. That's all you need to do.
     
    hevans1944 and davenn like this.
  9. davenn

    davenn Moderator

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    yup, agreed, without changing any other resistor values
     
  10. sureshot

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    Many thanks Steve and Davenn appreciate the help. I know protection needs adding on the input circuit and the output. If the model works well in a test over time loaded, then I will add the extra protection on both input and output, and built it in a tidy chassis case. Thanks again fellas.
     
    hevans1944 and davenn like this.
  11. AnalogKid

    AnalogKid

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    A 24 V transformer secondary is pretty high for a 12 Vdc output. LOTSA heat in the pass devices.

    ak
     
  12. sureshot

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    Yes, I used a 15 volt AC secondary. But a two transistor version with the same value resistors as the six transistor version didn't work. Well not stickly true ! Its output was the expected 12 volts, but the regulator tried to do all the work. The transistors where not forwardly conducting as expected. Either the values for a two transistor version are to high, typically the base transistor at 100 ohms is to high for two transistors. Or the placement of the 100 ohm resistor in the schematic is wrong. I'm stumped, the circuit tried to conduct under a 50 watt load, but the voltage started dropping as if the regulators current limiting was cutting in. I've built a single version unit in the picture below, and that works very well indeed. Any one have any ideas ? I have posted the 6 transistor version, of which I wanted to build a 2 Transistor version, but I really am stumped. Well mobile I can't upload pictures, the circuit is in a link top of this thread, I'll try and upload pictures on the PC in a little while.
     

    Attached Files:

  13. sureshot

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    The single transistor version in the picture above works fine, if anyone can throw any light on why a two transistor version of the 6 transistor schematic in the picture above fails to carry the current, using the same resistors as in the schematic. So a base resistor of 100R 0.5 watt, and two 5 watt 0.1R emitter follower resistors, any help appreciated.

    Just as an edit..

    I found this three transistor version, whilst i see the voltage regulator is labelled different, i'm sure its essentially the same as a 7812 VR, any how the transistor on the input is very different being 10 watts 15R So unless i've missed something ! Is a 100R 0.5 watt base resistor just wrong ?
     

    Attached Files:

    Last edited: Jan 18, 2016
  14. Ratch

    Ratch

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  15. davenn

    davenn Moderator

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    so did you try the 6 transistor circuit but just using a single transistor ?
     
  16. sureshot

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    Yes I did Davenn. It works well, the resistors to the emitters in the six transistor version look different from that of the single version circuit. This is what I see.. In the single transistor version the input goes to the emitter before entering the (in my circuit, 7 watt 1R ceramic resistor) but in the six transistor version, the input goes through each ceramic power resistor before entering each emitter. Also in the single transistor version, the 10R 0.5 watt resistor goes directly to the base from the input, in the six transistor version the 100R 0.5 watt resistor, the input goes through it, as opposed to just the base of the transistors. This is where I am not getting it. The last image I posted uses a 10 watt 15R resistor to the transistors bases, that's in the b&w three transistor version. Although I believe that circuit is untested. My first attempt at a two transistor version is not able to carry the current, it seems the transistors are not forward conducting, and carrying the current. Loads up to 20 watts work, although the transistors are stone cold, but the regulator a 78S12cv is warm. Up the current to 50 watts and the voltage just drops to what looks like a safe level for the regulator. Its this that leads me to believe the transistors are failing to turn on when the current increases.
    Thanks Ratch, but not sure how diodes will help.
     
  17. davenn

    davenn Moderator

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    forget about the circuit style looking different

    just confirm that you wired a circuit EXACTLY as the 6 transistor version but just using a single transistor

    Then tried to add a second transistor. if you got a single transistor working in that style, but it doesn't in a 2 transistor version, you either have a bad transistor or you err'ed in your wiring

    in which case we need to see pix of your construction to see what you may have done wrong
     
    (*steve*) likes this.
  18. hevans1944

    hevans1944 Hop - AC8NS

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    The three power transistors derive their forward base-emitter bias from the voltage drop across R1. The three emitter resistors provide negative feedback to help equalize the current in each of the three power transistors. You can "play" with the value of R1 to see what effect that has on the ratio of current through the power transistors versus current through the regulator, but the circuit should work fine with one, two, or three power transistors if you have wired it correctly.

    [​IMG]
     
  19. sureshot

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    First thank you for the replys and help. My first build was exactly as the schematic above with the single pass transistor in it, not a single transistor version of the 6 x transistor schematic. The two schematics (colour one's) differ in the resistors placement. As seen, the single transistor version, input goes up to the emitter, as opposed to power flowing through the 5 watt 0.1R first (6 x version) and the base transistor in the single version is not in series with the input, in the 6 x version it is ? As with the black and white schematic with the 3 x pass transistors, yes essentially its the same layout schematic as the 6 x transistor version. The difference being a 10 watt 15R ceramic on its input, as opposed to the 100R 1/2 watt resistor in the 6 x transistor schematic. What I'm after is how to configure a second transistor to the single version schematic above. As the resistor placement does not look the same to me. So to sumerise a 2 x transistor version of the single transistor version schematic.
     
  20. sureshot

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    I have been over the 2 x version of the 6 x schematic carefully a few times to make sure I've missed placed nothing. Where you say don't worry about layout, if you look at the single transistor version, and the 6 x version the resistor placements are different. The base resistor is not in series with the input to the regulator in the single transistor version. And the input to the circuit ( transistors / regulator) the power goes to the emitter before passing through the power resistor (single transistor version) but goes through the power resistors first before entering the emitter in the 6 x transistor version. I'll have another look at the 2 x version I built, and if I've not wired anything wrong I will post pictures of the single transistor working unit, and pictures of the non working 2 x transistor unit.
     
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