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Two part question.

If I take the 11v and 5v, with diode to prevent 11v to flow to the 5v, will I get 16 volts? And will it change a 12v battery?

Last how can prevent overcharge?

Also this is an older power supply. 1999 old.

Adding: this an ATX Power Supply. Internet searching info says if you connect the +12v to the -5, you get +17v.

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    \$\begingroup\$ how are you expecting to connect the diode? .... please provide a schematic of the proposed circuit ..... how are you obtaining the two voltages? .... one power supply or two power supplies? ..... \$\endgroup\$ – jsotola Nov 24 '18 at 6:27
  • \$\begingroup\$ And check the PSU to see if the grounds are connected. If they are you have a problem. Hit the edit link to add in the missing information. \$\endgroup\$ – Transistor Nov 24 '18 at 7:42
  • \$\begingroup\$ jsotola I don't have software to make schematics. I really don't know how one would help anyway. Best description would be to place the diode inline to the -5V with the stripe facing the +12v. from there on to the battery. TY \$\endgroup\$ – Mouthpear Nov 30 '18 at 7:53
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No, it will almost certainly not work. A computer requires both 12V (for fans, hard drives...) and 5V (for motherboard...). Both these rails have to have a common ground. So, a PC power supply has all rails at a common ground. Do not connect any rails in series (or parallel) or it will destroy the power supply (possibly with smoke and loud bang).

With two separate power supplies, it may or may not work. Most PC supplies have the common ground also connected to mains earth. Disabling that may work. However, one has to be careful. Simply disabling mains earth is both risky and may be futile. The mains neutral may be connected to mains earth outside the house, and connected to chassis (the metal box) via a capacitor. One actually has to disconnect the low voltage common connection from chassis (which is usually difficult due to the way the PCB is constructed in most supplies). Typically, insulating washers are used (search on the web for intructions). However, the silver lining is that, this needs to be done on only one of the supplies. Make sure you do not disconnect the neutral to chassis capacitor (if there is any), and the chassis to mains earth connection. Make sure no part of the battery is touching mains earth (through acid leakage or otherwise).

Above all, be safe when working inside an SMPS. Discharge primary capacitors with a resistor on a wooden stick.

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  • \$\begingroup\$ Internet searching info says if you connect the +12v to the -5, you get +17v and that's with the common ground. I have yet to try it. Fearing the "loud bang" you mentioned, LOL. Seems half the people say yes it works and half say it does not and trips the safety circuitry. TY for your input. \$\endgroup\$ – Mouthpear Nov 30 '18 at 8:06
  • \$\begingroup\$ 8Ah battery will need 800mA current for 10 hours to charge (C/10 charging). The -5V will almost definitely not provide 800mA (as they existed to power op amps in soundcards). Buy a 5V 2A mobile charger, ensure it is floating, and use that instead. (2A, because they often overstate their specs). Or you can get a 6V 1A transformer. With some voltage drop on load, it may be perfect. (You will need 4 diodes and a capacitor for the transformer) \$\endgroup\$ – Indraneel Nov 30 '18 at 9:37
  • \$\begingroup\$ Did you miss the part about connecting the +12v with the -5v? \$\endgroup\$ – Mouthpear Dec 1 '18 at 17:24
  • \$\begingroup\$ Did you miss the part about the -5V not providing the required 800mA?? BOTH the 12V AND the -5V have to be capable of providing the required current. The required current being 800mA if you want to charge in reasonable time (e.g. 10 hours). The possibly 100mA that the -5V will provide, may or may not be able to float charge the battery for maintenance. If the -5V has no current limiting, attaching a discharged battery may likely fry the -5V rail.... yes even though the 12V is capable of providing several tens of amps (actually, that is what will fry the -5V). \$\endgroup\$ – Indraneel Dec 1 '18 at 18:52
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If you want to build a basic 12v charger for lead acid batteries use a hefty 12 or 16 vac transformer and 4 high-current diodes. The diodes can be from an old alternator. Cut the heatsink to give 2 diodes with common cathode and 2 with common anode. Then wire them as a bridge rectifier. A filter capacitor is not necessary, but if you use one then add a 1 kohm 5W bleed resistor. Avoid sparks near a charging battery. . . hydrogen gas.

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  • \$\begingroup\$ Point is i don't want to build one. I want to use what i got. Internet searching says it it possible. All i want the thing to do is charge a 12v 8ah sealed battery and run a 12v DC motor for very short periods of time. But thank you for the info. \$\endgroup\$ – Mouthpear Nov 30 '18 at 7:57

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