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Thread: Atari 1040 STF power supply

  1. #1

    Default Atari 1040 STF power supply

    Looking closely at the circuit board, I see 2 seperate wires (both red) that have printed next to them "+5v". I could test this power board, that I pulled from the computer, iow load test it. But was just wondering why there are 2 disparate 5vdc power circuits. If I connected both pins to a single 5vdc rail on an atx power supply, would I create a problem?

  2. #2

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    There are also 3 black wires and a blue wire...

    Both the red wires are connected together on the power supply board, the three black wires are also connected together to create a low impendance return path.
    PC power supplies have also multiple wires run in parallel to divide the load and keep the wires manageable (thick wires are harder to bend).

    Just take care when using a PC power supply: these can deliver a lot more power than the Atari needs, if you make a short circuit it will damage the Atari's board for sure.
    Better select a smaller power supply (30 to 40 Watts is fine) with a +5V (3 to 5 Amps is ok) and a +12V (about 1 Amp is more than enough) output.

  3. #3

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    Why should it matter what the atx p/s is capable of delivering. A circuit only pulls what it needs. Only the weong (high) volltage will force excess currrents. Or low voltage could damage a circuit also.

  4. #4

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    Quote Originally Posted by 2icebitn View Post
    Why should it matter what the atx p/s is capable of delivering. A circuit only pulls what it needs. Only the weong (high) volltage will force excess currrents. Or low voltage could damage a circuit also.
    If the circuit is working properly it only takes what it needs. The Atari mainboard and power supply are matched, so the traces on the board are capable of handling the currents which the mainboard needs and the power supply can deliver maybe a bit more just to be safe. If you make a short on, for example, the joystick port, the original powersupply will go into 'short circuit' mode and delivering only the maximum amount of current (say 5 Amps) in short bursts. Now if you use an ATX power supply capable of delivering 20 or even much more Amps at 5 Volt and you make the same short it will just fry up the traces with 20 or more Amps and you end up with a burned out mainboard.

    I have seen this many many times.


    It is comparable to replacing a fuse from 3 Amps to 20 Amps and causing a short.. The wiring in your 3 Amp system is not able to handle 20 Amps of current and will burn..

    If you need to use an ATX power supply make sure the outputs are fused for the maximum current your device under test can handle safely.

    Also: too low voltage normally results in a device not working or erratically, too high voltage will damage the device for sure.

  5. #5

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    +1 for gertk's assertion that PC power supplies can be dangerous when powering relatively low powered equipment because they can deliver so much current especially from the +5V output.

    As said in #3, a healthy circuit will draw only what it needs to no matter how much current is available, but as a rule of thumb it is advisable to use a PSU which can supply no more than maybe twice the peak 'normal' current drawn by the load so that the damage is limited if something in the load fails low resistance.

    It is better for the power supply output to either collapse, shut down, or be disconnected by an appropriately rated fuse due to excess current than for it to keep on manfully trying to maintain +5V into just a few ohms of load, which is exactly what a PC power supply will do.

  6. #6
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    Another vote for gertk's opinion. You really don't want to repair fried traces or burned wiring. I've recall an old P4 board that I obtained as junk because someone forgot to plug in the separate 12V CPU supply on the board. The CPU proceeded to draw its 12V from the standard ATX 21-pin conductor and obliterated the +12 trace. I repaired the trace with a length of wire-wrap wire and hooked the board up with the correct PSU connections and it turned in several years of flawless service.

  7. #7

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    Quote Originally Posted by Chuck(G) View Post
    Another vote for gertk's opinion. You really don't want to repair fried traces or burned wiring. I've recall an old P4 board that I obtained as junk because someone forgot to plug in the separate 12V CPU supply on the board. The CPU proceeded to draw its 12V from the standard ATX 21-pin conductor and obliterated the +12 trace. I repaired the trace with a length of wire-wrap wire and hooked the board up with the correct PSU connections and it turned in several years of flawless service.
    All these replies are news to me and quite instructive. I get it though.

    So oftentimes the 2nd identical supply (in the case of the 1040stf a +5vdc, in the case of the above mentioned ATX p/s, a 2nd +12vdc) is to limit the amount of current that rail can provide, so in the event of a short (somewhere) traces and/or components won't be decimated? But Chuck's board wasn't fried, or the board itself wasn't, but the components were not damaged. So components are unlikely to be damaged? Are there other reasons for a secondary identical voltage supply?

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