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Combining PSUs


redxine
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I'm setting up a streaming server for my church and have a nice quad core, raid 5 server to put fedora and flumotion on. The power supply is apparently redundant (http://www.rackmountnet.com/istar-ps2-400w-hotswap-redundant-eps12v-power-supply-p-316.html) but upon trying to power up the machine one will beep constantly and the other will just kind of buzz and sit there until power is removed. After the beeps get annoying and both are switched off the capacitors slowly discharge and let out a final 2 volts and a decrescendoing beep. I've traced the problem to the hard disks; they draw too much power when spinning up and the supply complains until the load is removed. The system can be started fine if all the hard drives are started up in sequence after the PSU output is stable. So it can handle the load - just not all at once. Like this one: http://www.burningissues.net/how_to/power/psu.htm

As I am not a fan of spending money right now (and after telling my church it would cost next to nothing) I would like to find a good old hacker solution instead of doing the usual and buying a new one. So someone tell me why combining ground and pin 14 on the ATX connector and running the disks off the slave PSU.

In short, disprove this comment: http://club.myce.com/f7/wire-dual-power-su...640/#post908491

***WARNING*** connecting two power supplies together may cause serious damage to the power supplies and the equipment that they are connected to.

The master/slave setups illustrated in this thread should be okay. But, connecting two power supplies so that their regulated voltages are connected to each other, is BAD.

Background) Most power supplies use one circuit to power one transformer to make all of the various output voltages. Then, one output voltage (usually the +5Vdc) feeds back to tell the input side of the transformer what to do. The other output voltages are controlled by their relationship to the feedback voltage. (They come off the same transformer using different windings) [very over-simplified] So, the input side of the power supply regulates itself to make the regulated voltage come out right; and the other voltages drift according to what is happening to the regulated voltage.

Problem) If the two different power supply regulated voltages are connected to each other, they will make the power supplies unstable.

Lets say PSU 1 regulates it's +5 line to +5.015 volts. Let's say PSU 2 regulates it's +5 line to +5.013 volts. If the two regulated voltages are spliced together, PSU 1 will pull all the load off PSU 2 by pushing the +5V to +5.015 vdc. It's other voltages (+12V, -5 V, -12V and +3.3V) will go high. PSU 2 will reduce it's output to bring the +5V down to +5.013 vdc. PSU 2's other voltages will also go low. The motherboard and other devices will be caught in the middle of the conflict.

Real power supply sharing systems use extra circuits to prevent PSU 1 from affecting PSU 2 and vice versa.

So, link 3 is bad. Don't do it. Your hardware will thank you.

Two power supplies that do not connect to each other (other than to turn on the slave PSU) are okay.

I'm pretty sure a PSU doesn't care about voltage so long as it's within a certain window and with each supply adjusting accordingly it should result in a stable output. And really all I need is the 4 pin for disks. Any comments?

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The Purpose of dual PSUs is in to have a backup in the event one of them fails. Specs of the system would be nice, since the one PSU might be too small to handle everything.

If the only thing you are powering with the second psu are the hard drives you should be ok.

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The Purpose of dual PSUs is in to have a backup in the event one of them fails. Specs of the system would be nice, since the one PSU might be too small to handle everything.

If the only thing you are powering with the second psu are the hard drives you should be ok.

It's Intel Xeon 3.00 GHz Quad core, 2gb DDR2 RAM, 4 500-gig Hot-Swap Hard drives attached to a hardware RAID 5 controller, DVD R/RW burner, Floppy drive, Ultrium LTO 2 Tape drive, (no clue about the motherboard), 5 or so mid sized cooling fans, and the above redundant hard drive. The behavior of the PSUs - how one beeps and flashes while the other just buzzes - makes me believe one might be faulty. I think I'll go ahead and try the dual PSU for the HDs.

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That could be. Ran a rough calc of total Wattage used and it's around 300W or so.

Make sure it can handle the power-on load, since it'll pull an insane amount of power to get those drives spinning.

Good luck.

Thanks. Yeah - it only really needs the power to start up. It's been running stable for 2+ days. Thanks for the help.

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The master/slave setups illustrated in this thread should be okay. But, connecting two power supplies so that their regulated voltages are connected to each other, is BAD.

Generally speaking this is true. I'll explain.

Most power supplies use one circuit to power one transformer to make all of the various output voltages. Then, one output voltage (usually the +5Vdc) feeds back to tell the input side of the transformer what to do.

Yup specifically the +5 is compared to a 5V reference. Any differences result in either an increase or decrease of the PWM duty cycle.

Problem) If the two different power supply regulated voltages are connected to each other, they will make the power supplies unstable.

Yes, but not so much as you'd really notice.

Lets say PSU 1 regulates it's +5 line to +5.015 volts. Let's say PSU 2 regulates it's +5 line to +5.013 volts. If the two regulated voltages are spliced together, PSU 1 will pull all the load off PSU 2 by pushing the +5V to +5.015 vdc. It's other voltages (+12V, -5 V, -12V and +3.3V) will go high. PSU 2 will reduce it's output to bring the +5V down to +5.013 vdc. PSU 2's other voltages will also go low. The motherboard and other devices will be caught in the middle of the conflict.

A bit exaggerated, but again it is true.

Actually what you need to worry about is that all the outputs will pull power from each other.

i.e. PSU1 has an output of +12.04V

PSU2 has an output of +12.15V.

Both are completely within spec.

However connecting the +12.04 and +12.15 creates a 0 (or very close to 0) ohm load with a 0.15V drop. This means you'll pull power from one supply to the other, until the voltages are equalized by the various regulation methods as noted by your other forum poster.

What this means is that if one power supply comes up faster than the other one - you could have a several amp or more short circuit.

I would surmise that its best to bring one up a few seconds after the other. Another complication is that as mentioned, there will be already the power present on the regulated rail before the supply thinks it should be there. You might not even be powering up PSU 2 - depending on its startup method.

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