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Resistor values.

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

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  1. sureshot

    sureshot

    234
    13
    Jul 7, 2012
    Here is the single transistor version as per the single version schematic above, and the 2 x transistor version as per the 6 x schematic above.
     

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  2. sureshot

    sureshot

    234
    13
    Jul 7, 2012
    Fresh eyes and all that the following day... I've found a wire that I soldered in to the regulators input, this lead would have shorted the base resistor leaving it redundant, and the pass transistors not conducting. "Ever felt stupid" I do at the moment. Can't be 100% sure until I alter this later, but I'm confident I've found the problem. Dam what was going on when I soldered that offending lead in. After a rearrangement later I'll test it again and get back to you, hopefully...
     
    hevans1944 and davenn like this.
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,384
    2,771
    Jan 21, 2010
    Good job. Don't feel too bad about it, we all make these mistakes and at least you're didn't result in the smoke getting out.

    There are some interesting issues regarding the choice of what you call the base resistor. In general, the higher it is, the singer the pass transistors start taking up part of the load.

    the major difference that you've noted between the 1 transistor version and the multiple transistor version is the placement of the resistors in series with the emitters rather than the base. This I'd a necessary change when moving from one to multiple transistors.

    The limitation of this circuit (in terms of current) is the amount of made current that can be driven through the transistors. This I'd limited to the max current through the 78xx device. If the pads transistors have a gain of only 25 at the operating current then the max output current would be 26A no matter how many transistors you had. In your case you wish to reduce the output current so this is not a huge issue. Note however, that any curry through the series resistor (variously 10 out 100 ohms) will subtract from the available base current and reduce the potential output current.

    Given the operating conditions, it is also probably useful to place a 0.1uF capacitor between the input and ground leads of the 78xx device.
     
    davenn likes this.
  4. sureshot

    sureshot

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    Jul 7, 2012
    Thanks Steve. I still have to reconfigure it, but I'm optermistic. There are a couple of by pass 0.1 ceramic disks directly on the 78xx leads, can't see it in the picture of the 2 x version, there are two the same again but on the board of the 1 x version, but a liitle further from the 78xx than I would like. As an after thought and to avoid voltage drops I added on the 1 x version a pair of 1000uf capacitors hoping to minimise voltage drop. By yes I understand what your saying, I'm glad you see what I meant about resistor placement, its put to bed any worry's I had on that issue. I will reconfigure in a while and post results, hoping a 50 watt load is tolerated. Its ok on the 1 x version which was connected for test over 24 hours. 1x version transistor, no load 12.15 volts, loaded to 20 watts 11.80 volts, and loaded to 50 watts 11.56 volts, stable for 24 hours before I diconected. Load was halogeon 12 volt lamps. Regulator temp max was 62°C and transistor was 74°C, current drain at 50 watts was 3.8 Amps. These figures where stable.
     
    (*steve*) likes this.
  5. sureshot

    sureshot

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    Jul 7, 2012
    Well it was my stupid mistake /-: that one wire I put into the regulators input, can't believe I made such a simple mistake. Wired up now the 2 x transistor version is at 12.07 volts no load, at 50 watts its 11.80 volts steady, at the maximum (Which I would go no higher) 100 watts its steady at 11.42 volts. Device tempreture at 100 watts is, the regulator at 58°C and the transistors at 75°C, and the ceramic power resistor are at 71°C. The heat sink is at about 53°C. That heat sink is a CPU AMD sempron one, the fan has a thermal switch on the heat sink 40°C contacts normally open, closing on tempreture rise. And no series resistor for the fan, its going full on at the supply's output voltage. That heat sink is rated for its intended purpose at about 60 -70°C I believe, but with the fan full on at that speed its holding the above tempretures. Just want to say Thank You to everyone that helped and looked in, cheers all !
     
  6. sureshot

    sureshot

    234
    13
    Jul 7, 2012
    Just a quick picture, all the light is 100 watts of halogen low voltage lamps. The final circuit will only see that load at around 50% duty cycle. As i didn't have a linear transformer rated for that current, the input (testing only) is 2 x xbox psu's in parallel, they are double insulated. Not an ideal input at 24 volts, i know heat is the enemy here, but might get a bit better regulation with lower temperature in the devices, and an input linear transformer of 15 volts before rectification and filtering. Thanks again all.
     

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    hevans1944 likes this.
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