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Unread 06-08-2005, 08:13 AM   #1
JSimmons
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Default Two PSU's In a Case

I want to make a 2nd psu come one with my system is powered up. I know I can use a relay and a couple of connectors to do it, but I don't know which relay to use.

I've seen some people talking about only needing to short together one pair of wires on the secondary PSU, and I've seen others do 2 pairs of wires.

1) I'm going to go ahead and assume that I need to sshort two pair of wires, and I know that the relay has to be 12 volts DC, but how many amps should the relay be able to handle?

2) Do I need a single-pole-double-throw relay or a double-pole-double-throw relay?

3) Can someone recommend a Digikey part number?

4) Does the relay need to be actively cooled, or can I just button it up in a small project box and forget about it?
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Unread 06-08-2005, 09:38 AM   #2
TerraMex
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Hey.

There's actually two ways of doing this.

1) connect a relay to the PWRON (or Power Good) wire with a ground wire (GND).
It switches the PSU on as it has a small current flowing through one of it's logical circuit. That's what the shunt does.

I used a simple 5V DC- 5 V DC relay. PSU 1 feeds 5V to the relay and switches it from OFF to ON and simply powers on the PSU 2 with the above method.

The relay doesnt need a heatsink, it operates with very low currents. You can (and should IMO), use, however, a diode (1N4001 will do) for backcurrent protection in parallel with the psu1 feed.

2) Connect permanently the PWRON and GND. Use a 12v DC -240v AC relay for the mains (110V there i think, just replace the 240v) , same ideia, different approach.

This relay doesnt need a heatsink too, however it should be rated for up to 3 amps continuous use, as its secondary will feed psu2.

*3) go the extra mile and do both, bit redundant imo.

I used 2) thou. I switched to 1) when i did the "modular" upgrade but havent tested it yet (should work anyway).
Any electronics store should have those relays. Get the 5V DC, as it doesnt need more, else, get a 12v-12v DC relay. Also works fine.
I am, however, assuming your psu works as mine does, as the PWRON signal goes. You could just get a DMM and check (recheck, and check again).
PWRON is always maked on the PSU PCB, but it's usually the green wire on the 24 pin molex.
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Unread 06-08-2005, 10:38 AM   #3
JSimmons
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Quote:
Originally Posted by TerraMex
Hey.

There's actually two ways of doing this.

1) connect a relay to the PWRON (or Power Good) wire with a ground wire (GND).
It switches the PSU on as it has a small current flowing through one of it's logical circuit. That's what the shunt does.

I used a simple 5V DC- 5 V DC relay. PSU 1 feeds 5V to the relay and switches it from OFF to ON and simply powers on the PSU 2 with the above method.

The relay doesnt need a heatsink, it operates with very low currents. You can (and should IMO), use, however, a diode (1N4001 will do) for backcurrent protection in parallel with the psu1 feed.

2) Connect permanently the PWRON and GND. Use a 12v DC -240v AC relay for the mains (110V there i think, just replace the 240v) , same ideia, different approach.

This relay doesnt need a heatsink too, however it should be rated for up to 3 amps continuous use, as its secondary will feed psu2.

*3) go the extra mile and do both, bit redundant imo.

I used 2) thou. I switched to 1) when i did the "modular" upgrade but havent tested it yet (should work anyway).
Any electronics store should have those relays. Get the 5V DC, as it doesnt need more, else, get a 12v-12v DC relay. Also works fine.
I am, however, assuming your psu works as mine does, as the PWRON signal goes. You could just get a DMM and check (recheck, and check again).
PWRON is always maked on the PSU PCB, but it's usually the green wire on the 24 pin molex.
Radioshack has the following 12vdc DPDT relays:

Part# 275-218 - 10A @ 125vdc (looks like it takes solderless terminals, which is good since I have a not-so brilliant history with power tools).

Part# 275-249 - 5A at 125VAC (PCB mount and much smaller - requires soldering
that operates at 5 amps.

Radioshack does not specify whether the amperage ratings are sustained.

I'm leaning towards the first one.

Now tell me about the diode thingy (in english).
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Unread 06-08-2005, 11:49 AM   #4
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JSimmons, are both PSUs ATX? If so, the motherboard can probably drive the PS_ON (green wire) for both PSUs down to ground when the front switch is turned on.

Basically, the PS_ON line is supplied with a current limited +5V signal which can be forced down to 0V by the motherboard. If the motherboard can sink twice that current (should easily be done), then the motherboard can turn both PSUs "on" by having both PS_ON signals hooked up.

Just splice the green wire from the 2nd PSU to the green wire from the 1st PSU and have the joined signal go into the motherboard. The motherboard will then turn both PSUs on.

edit: replaced drive with sink, which is more appropriate when driving a +5V signal down to ground
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Unread 06-09-2005, 09:32 AM   #5
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Brian:

Also true, but , two things.
one is, with the above methods you can easily wire a switch and power off the secondary psu on queue, without turning off the computer. Granted that it can also be done with your's (series with wire from the secondary psu), but you only need to void the secondary psu warranty , not both, as you need to hack , at least, the mobo connector on the primary. Specially when the main one is an expensive one (... and money matters).
second one is that i haven't seen a "mod" like that (doesn't mean it doesnt exist), and , honestly, i wouldn't feel confortable with it. Just don't know how the PSU's or even the motherboard would react to it. They can be picky .

JSimmons:

The diode is only a safety precaution.
http://www.procooling.com/articles/h...pumps_-_p1.php
example.

About the relays.
Well, you can use which ever suits you better. The only thing it has to be is 12v DC on the command section (coil), which are both.
If you check the specs they state a 70mA on the 12v coil, on each.
The contact section on both can handle the mains at 5 A (at least), and a psu usually absorbs about 2, larger ones 3 but , AFAIK , never over.
If you choose to close the PWR_ON and GND, you'll be dealing with very low currents on the contact section, so, you wont have issues there.
You can use the same relay for both methods.
Either way, which method are you planning to use?
cut the mains or just wire the PRW_ON , or brian's method?

PS : if brian's, then you should get this :

http://www.digiconcepts.com/cables_power_12.htm
Keeps you from hacking in the mobo connector form the psu (voiding warranty).
and, do use a switch for emergency override.
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Unread 06-09-2005, 10:04 AM   #6
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Both PSU's are ATX.

I'm planning on using an automotive relay (12vdc, 30A). The relay will be powered by the primary PSU via a 4-pin molex connector, and the secondary PSU will be connected via it's 20-pin connector (i'll be using a 20-pin male connector coming off the relay). This way, nobody's warranty is voided, although the secondary CPU is 3 years old, so I doubt if the warranty even still exists.

Using a relay pigtail as I have described allows me to change out either PSU without having to cut wires and re-solder on a new one, not to mention keeping the amount of useless wire in the case to a minimum.


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Unread 06-09-2005, 01:57 PM   #7
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you don't need a heavy duty high amp relay for that, I used a small soldered one from radioshack when I had my computer running 2 PSU's, probably was that 5A one.

using the motherboard to turn both on doesn't always work, I had originaly tried it that way by my motherboard couldn't handle the load of both on it, it would turn on for a split second then turn right back off and it's not a cheapo motherboard.
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Unread 06-09-2005, 01:58 PM   #8
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Dandy.

Watch out for voltage drops across connectors; these Molex aren't as good as they seem.
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Unread 06-09-2005, 03:43 PM   #9
JSimmons
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I know I don't need it, but it should work, right?

The benefits of the automotive relay is that I don't have to solder anything to make it go, and being rated at 30A, there's no chance of burning it up.
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Unread 06-10-2005, 07:42 AM   #10
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yeah, it will still work fine, just they are bigger and more expensive is all.
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Unread 06-10-2005, 07:48 AM   #11
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I paid $3 for mine at Advance Auto Parts. I think the radioShack one is about the same price, maybe a bit more.
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Unread 06-10-2005, 03:20 PM   #12
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It's overkill, but it's not going to hurt.

The signal to turn on the PSU is a 5 volt TTL logic line, requiring only 10 mAmps.
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Unread 08-26-2005, 10:57 AM   #13
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Default How to with pictures

A little late but;
Thier is a great how to at:
http://www.directron.com/2powersupplies.html
with lots of pictures.
I made this circuit and used a radio shake 250V 5A dual relay so I could add 2 extra PSUs.
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