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Dual PSUs

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HG Master

Joined: 12 Oct 2007
Posts: 119
Location: Gig Harbor, WA

PostPosted: Mon Apr 14, 2008 10:45 am    Post subject: Dual PSUs Reply with quote

When thinking about getting a new powerful psu, I thought up of making a simple dual psu that might be cheaper then getting a new one. There are other designs of how to do them and looking at what people did, mine appears to be one of the cheapest and easiest to do.

Where I learned how to design my model: http://www.wikihow.com/Convert-a-Computer-ATX-Power-Supply-to-a-Lab-Power-Supply

This link gives valuable information like connecting the brown and orange wires for them to sense the correct voltage. It also states that when the green wire is grounded, the psu turns on.

When I was messing with a multimeter, I discovered that the green wire was 2.5 voltages and uses nearly no current when it is turned on. Armed with this information, I decided to use a transistor and the molex of the other psu to turn it on.

I have yet to test it with my computer devices connected, but from what I read of other people doing it with similar methods (like using relays and splicing wires), it appears to work fine with them.

I will not give pay for any damage you encounter. But this method is safer than splicing two psu wires together. BadSweetums explains from is post at http://club.cdfreaks.com/f7/wire-dual-power-supplies-together-one-motherboard-111640/ .

***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.

My approch is the slave/master method of keeping the two supplies seperate and the power from one turns the other on.

Lol, I forgot to put a 10k ohm resistor in between the 4 pin molex and the transistor, otherwise, everything else is fine. The 4 pin one is a standard molex connector from the Master psu.

Notice the top wires (orange and brown) being connected and are required to be connected otherwise psu damage might occur
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