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PostPosted: Mon Aug 12, 2002 2:32 pm 
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I bought a 12V/5V DPDT centre-off switch a few days ago, soldered it up, and the 12V and off settings work fine. However, when I use my 5V line my power shuts off and won't start again until I unplug it and wait a couple minutes. I've checked the wiring several times and don't see any shorts, and my 5V lines read +5.08/-4.963V so they should be working fine. <BR> <BR>I've only really considered two possibilities: <BR>1) 5V isn't enough to drive two 12V fans (I use a lian-li pc10, which has two front intake fans connected to a board with three options for fan speed). However, it shouldn't shut off my psu like that. <BR>2) My switch is defective, causing a short. However, to confirm this I'd have to unsolder and re-solder some wires, which, to me, is a PITA. <BR> <BR>Can anyone else think of any other possibilities/solutions? <BR> <BR>Thanks
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PostPosted: Mon Aug 12, 2002 2:32 pm 
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It does seem like a short -- 2 ordinary 12V fans can't overload a PSU 5V line.
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PostPosted: Mon Aug 12, 2002 2:32 pm 
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Location: Coquitlam, B.C. Canada
One possibility I can think of is the PSU is unbalanced. Switcher PSU are generally not designed to operate in this manner. Several maufacturers have "feedback" windings or sensing circuits which monitor the various outputs (+5, +12, -5, -12) to provide PSU regulation. Without these feedback circuits, the PSU would not regulate and the voltages / currents would be all over the map. <br> <br>A small load of one fan 12V @ 0.1A (100mA) would probably not cause the problem. However, two fans are now around 0.2A (200mA) could cause the unbalance. Both the 5V and 12V power supplies easily have enough capacity to operate individually. This is not the concern. <br> <br>I have two questions: <br> <br>1) Does the PSU operate normally without the fans? <br>2) Does the PSU operate normally with 1 fan? <br> <br>If yes to both questions, the problem could be the unbalanced outputs. Possibly only use one fan.
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PostPosted: Mon Aug 12, 2002 2:32 pm 
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Not sure why/how you think the PSU is "unbalanced". All the 12/5V switch does is flip between the 12V and 5V rails. It is not drawing the "difference 7V" off the positive terminals of the two rails, so regardless of which position the switch is set to, it is only drawing on one or the other of the 2 voltage lines. It doesn't sound like barking_frog wired it wrong, which is why I think it probably is a fault in the switch...
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PostPosted: Mon Aug 12, 2002 2:32 pm 
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Sorry. My mistake.
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 Post subject: 3 possible causes
PostPosted: Mon Nov 11, 2002 6:38 am 
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1. The switch doesn't disengage from one contact before engaging the other. There are some switches intentionally designed like that.
2. Every switch does have some "bouncing", that is an oscillation between the two positions. Bounce depends on the mechanical construction and the force being used on the switch lever and it's in the order of a few milliseconds.
3. The fan stalls because of inadequate startup torque, so it draws too much current.

For 1-2, I suggest using two switches instead of one or even a small value resistor on each power line, to limit inrush current..
For 3, I suggest to insert a 4.7V power zener diode in series with 12V, instead of using 5V. Or simply a suitable power rating resistor.

There are more elegant solutions like a (simple) variable voltage linear regulator but then you need some elementary electronic building skills.


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