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I understand that what I am doing requires the utmost care and that numerous precautions should be taken in completing a project like this.

Out of necessity, I am going to fashion a car battery charger from an ATX PSU. I need to draw ~14v to do this, I'm guessing from the +12v rail.

How can I tell if this will be possible for the PSU I have?

What I know:

  • This model uses HA17431PNA shunt regulators, which I have located on the circuit.
  • I will need to modify that IC's input circuit to adjust the regulator's voltage.
  • This may also require me to adjust the output caps, but given the step-up value it also may not.

What I don't know:

  • Precisely how to tackle this -- I have a decent soldering station and resistors, but this would be the first time I worked on something other than a phone/doing rework. I found the spec sheet from Hitachi for the regulator. What other resources should I consult?
  • What generates DC from AC in an ATX PSU? A transformer? Is the transformer able to generate this voltage for me without affecting other components?
  • How this will affect the current output on the rail.
  • Any other unknowns regarding how this modification might impact the performance of the PSU.

Can someone please help me accomplish this task?

Thanks for your time.

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  • \$\begingroup\$ hmm HA17431 seems to be a clone of TL431, (17 reflected vertically even looks like TL) \$\endgroup\$
    – Jasen Слава Україні
    Feb 7, 2016 at 20:37
  • \$\begingroup\$ A lead-acid battery should not be charged using a fixed voltage. Automotive battery chargers intended to intelligently charge 12 volt lead-acid batteries should be readily available. \$\endgroup\$
    – Peter Bennett
    Feb 7, 2016 at 20:40

2 Answers 2

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Typically the feedback is taken from the 3.3V output and fed back using a TL431. There might be a second TL431 for the auxiliary power supply.

To change the voltage you would increase the series resistor from the 3.3V to the feedback terminal of the TL431/HA17431. This will increase the positive output voltages (usually the negative ones have fixed regulators).

However if the power supply has overvoltage protection, as many (and all good ones) do, then you may trigger that and your power supply will shut down. Without a schematic it is going to be difficult to determine how to deal with that.

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To increase the voltage output you need to reduce the resistor between the REF and A pins of the HA17431

increasing the 12V will also cause the 5V and 3.3V to increase

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  • \$\begingroup\$ Usually the A pin goes off to some other bit of circuitry- it's not a simple shunt regulator. In small supplies it goes to an optocoupler, in bigger ones (with a second auxiliary supply) to some discretes. \$\endgroup\$
    – Spehro Pefhany
    Feb 7, 2016 at 20:53
  • \$\begingroup\$ I would expect the optiocoupler to be above the device connected to K - A and K terminals are labeled as in a zener diode) \$\endgroup\$
    – Jasen Слава Україні
    Feb 8, 2016 at 5:54
  • \$\begingroup\$ Here is a typical arrangement with an optocoupler. In modern PC supplies there may not be an optocoupler at that position- the control circuitry isn't running at mains potential- so it will be a PNP or something like that. \$\endgroup\$
    – Spehro Pefhany
    Feb 8, 2016 at 6:23
  • \$\begingroup\$ connected to K \$\endgroup\$
    – Jasen Слава Україні
    Feb 8, 2016 at 6:49

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