12v Power supply





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  1. #1
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    Default 12v Power supply

    I have a 10A 13.8V power supply that I want to modify to adjust the volts between 2v to 14.6v. At the moment it is adjustable between 12.5v and 14.6v with the pot in the circuit. The pot is 200ohm and have 1200ohm resistors on each outer leg going to positive and negative. The wiper leg goes to a transistor.

    I have bypassed the pot and resistors with a 5k ohm pot, but the volts only drop down to 7v. How can I get it to go down as low as possible?
    98 Hilux 2.8D D/C 4x4

  2. #2
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    Default Re: 12v Power supply

    Quote Originally Posted by wiel View Post
    I have a 10A 13.8V power supply that I want to modify to adjust the volts between 2v to 14.6v. At the moment it is adjustable between 12.5v and 14.6v with the pot in the circuit. The pot is 200ohm and have 1200ohm resistors on each outer leg going to positive and negative. The wiper leg goes to a transistor.

    I have bypassed the pot and resistors with a 5k ohm pot, but the volts only drop down to 7v. How can I get it to go down as low as possible?
    Remove the resistor that goes to ground and replace with a wire link.

    Just be aware that you may never get to 2V and even if you do you will have dissipation problems.

    Continuously variable from 2 to 14.6 is a big ask and requires some fancy footwork with the electronics.
    Cheers

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  3. #3

    Default Re: 12v Power supply

    Unusual for a 10A power supply to use a transistor in the output stage...

  4. #4
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    Default Re: 12v Power supply

    Quote Originally Posted by Richard Mackay View Post
    Unusual for a 10A power supply to use a transistor in the output stage...
    Not if it is the ever popular 2N3055 industry workhorse configured as a series pass device in an old analogue power supply.

    Twenty years ago about 99% of all power supplies would have been made like this.
    Last edited by Fluffy; 2019/07/15 at 10:43 AM.
    Cheers

    ZS5KAD
    3 V8's
    NA TwinTurbo SuperCharger
    A V6 and an inline 4

    The frogs are starting to notice that the water is getting warm but it is already too hot to do anything about it.....

  5. #5

    Default Re: 12v Power supply

    Quote Originally Posted by Fluffy View Post
    Not if it is the ever popular 2N3055 industry workhorse configured as a series pass device in an old analogue power supply.

    Twenty years ago about 99% of all power supplies would have been made like this.
    All the more reason to recycle these hopelessly inefficient devices and move on!

  6. #6
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    Default Re: 12v Power supply

    Willie los die ding,jy gaan julle huis afbrand.
    2017 Hilux 2.4 SRX D/C 4x4 2012 Cross Polo [mamma s'n]

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    Default Re: 12v Power supply

    Quote Originally Posted by Lofty View Post
    Willie los die ding,jy gaan julle huis afbrand.
    Ek gaan by jou kom plak.
    98 Hilux 2.8D D/C 4x4

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    Default Re: 12v Power supply

    Quote Originally Posted by wiel View Post
    I have a 10A 13.8V power supply that I want to modify to adjust the volts between 2v to 14.6v. At the moment it is adjustable between 12.5v and 14.6v with the pot in the circuit. The pot is 200ohm and have 1200ohm resistors on each outer leg going to positive and negative. The wiper leg goes to a transistor.

    I have bypassed the pot and resistors with a 5k ohm pot, but the volts only drop down to 7v. How can I get it to go down as low as possible?
    Have you perhaps checked what the TRFR output voltage is?

    Do you need the full 10A or can you get along with 5A?

  9. #9
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    Default Re: 12v Power supply

    Quote Originally Posted by ekkekan View Post
    Have you perhaps checked what the TRFR output voltage is?

    Do you need the full 10A or can you get along with 5A?
    The PS is a Steed 13.8v regulated.I want to change it to a adjustable voltage PS. I need 10A at 14v ,but at 2v I will only use it to test LED lights etc. I did as Fluffy said, but it still doesnt go below 7v. The reason I think is , there is a zener diode connected between the transistor emitter and neg-. The transistor is a 1402E and it goes to a MJE3055T ,which then goes to two 2N3055 transistors.

    On second thought and to play safe I am looking at getting a DC-DC converter from Banggood that can take 15A and go down to 1.75v .
    98 Hilux 2.8D D/C 4x4

  10. #10

    Default Re: 12v Power supply

    Quote Originally Posted by wiel View Post
    The PS is a Steed 13.8v regulated.I want to change it to a adjustable voltage PS. I need 10A at 14v ,but at 2v I will only use it to test LED lights etc. I did as Fluffy said, but it still doesnt go below 7v. The reason I think is , there is a zener diode connected between the transistor emitter and neg-. The transistor is a 1402E and it goes to a MJE3055T ,which then goes to two 2N3055 transistors.On second thought and to play safe I am looking at getting a DC-DC converter from Banggood that can take 15A and go down to 1.75v .
    Best is to get a dc to dc buck converter--just check that it can handle the amps--Virtually no heat generated as the 78xx voltage regulators. and the price is low as well.
    Toyota Fortuner D-4D 4x4 AT---My "speed control en navigasie "sisteem is al oor die 50 jaar oud!Ham Call ZS1SSM

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