Friday, November 15, 2013

Dual Power Supply Setup For Desktop

Good morning everyone. Hope you guys are doing well. So this post is about how to use two different power supplies on a same system if you can't get a higher capacity one or have multiple lower capacity one just laying around.


Allow me to remind you some of the basics.

You probably know this already but I will just remind you one more time. Desktop computers that we use needs power and it needs stable DC voltage of 3.3V , 5V and 12V. The outlet that we have in our home provides either 110V or 220V AC depending on where you live. For us it is 220V. So the power supply has to convert the incoming 220V AC to suitable low voltage DC levels but each power supply has it's limit which you will see on the label of every power supply. Usually when we buy a system we buy a proper power supply to go along with it. Often times due to limitation of budget we go for power supplies that has little or no headroom for future upgrade, in that case what happens is if you try to connect any more device to it say a Graphics card you will run in power shortage issue because your power supply was capable before but with added hardware it no longer is. So best practice in this case is to get a newer and more capable power supply but what if you have say another power supply in hand and want to use two of them together? This post might help you with that.


You can get "dual power supply kits" or similar product online but those are kind of costly and often times not that good. Moreover if we can make our own why shouldn't we?

Possible scenario where this can be used:

Say you have two power supplies and each of them can provide 450W, but your requirement is around 550W. So none of those supplies alone is going to cut it. In that case two of them together can help. 

Dig a bit deeper:

If we want to use two power supplies on a single computer we have to understand a small thing about how they work. Thing is desktop computer's power supply is not like a mobile phone or laptop charger, yes they do pretty much the same thing and often has similar circuits but one major difference is desktop power supply won't provide power if you just plug in an AC power source to it. You have to turn it on so that it powers all the rails. So what we can do is use one power supply as the Primary which will be turned on by the user by pressing the power button of the computer which will eventually turn on the secondary one or allow power flow from secondary one to the system.


Let's do it then: 

Like I was saying the computer power supplies won't provide power on the major rails unless it is told to. So what happens is whenever you put AC voltage in your power supply it will only provide 5V and a small amount of current on it's standby rail which is the Green wire of the 24 Pin connector of your power supply. No other rail will provide any power at this stage. But if you short it out with any other Ground wire that is a signal the power supply to power up other rails, so what the power supply will do is simply put power to all other rails like the 3.3V, 5V and 12V. As long as you remove the short however it will enter the standby state pulling down all the rails to zero and pulling up the standby rail to 5V. So we can use two methods to use two power supplies in a system let's talk about those.









Method 1: 

In this method we will be using a simple electromagnetic relay that will be turned on by the Primary power supply which will short out the Green wire of the secondary power supply to any ground pin causing the secondary one to start providing power too. Problem with that is a small amount of delay because the relay will take a small amount of time to turn on and the secondary power supply has to turn on after relay being turned on which might take a small amount of time too. Although the total delay will be very small as both relay and power supply can turn on pretty fast.

Method 2:

So the other method that can be used where the secondary power supply is already on. To do so what we are going to do is short out the secondary power supply Green wire to any ground and the relay that is being turned on by the primary will be used to connect the main 12V rail of the secondary power supply to the system. Although in this method we can get rid of the delay caused by the secondary power supply as it takes time to turn on, the relay has to conduct a high amount of current and even if there is small resistance in the relay, the voltage drop across it might be something considerable. It will also keep the secondary power supply on as soon as the AC voltage is available which might waste more power compared to the first method. This method also requires larger number of cutting and joining wires.

If you want to make it however, I would ask you to go for the first method. 

Let's start with first method in details with diagram.



Like we discussed earlier in method 1 we will simply turn on the secondary power supply using the primary one. So what's gonna happen is this, when the primary power supply turns on it will provide power to it's 12V rail which will cause the relay to short out the Green wire of secondary power supply to the ground. Thus the secondary power supply will turn on and provide power to the load.

Let's move on to the secondary one. 



In here however as soon as you put the AC voltage in the secondary power supply will turn on. What the relay is going to do is simply power up the load as soon as it gets power to turn on from the primary power supply unit.

Connection:

For both methods you can simply use a Molex Cable to power up the relay from the primary power supply. The diode that I'm using here can be a simple all purpose diode.




The best way to use two power supplies is I think one for almost everything and the other to power up a certain power hungry device. For example just use one for the whole system and maybe the secondary one to power up the 6 or 8 Pin PCI express power connector of the GPU. However keep in mind that if you want to use the second method you have to do a lot of cut and joint to make it work as you need to put the relay in the middle of the 12V rail and load.

Don't forget to tie up Black wires or the GND of both the power supplies.


Advantages : 

1. Cheap. If you Graphics Card burns a lot of energy which your existing power supply can't handle, you can add another cheap power supply to provide that power. It will save you a lot of money that might be needed for buying a new heavy power supply.

2. Easy to build. This circuit is pretty simple and doesn't have to many components.

Disadvantages :

1. Lot of hassle. 

2. Looks odd.

3. Take a lot of space.

Warning :

Before you do anything make sure you have clear idea of what you are doing. Computer is a very sensitive machine, use correct polarity, don't short any yellow or red wire to black wire. Before turning on the power supply double check connections.

Don't forget to use a diode with the relay as shown in the figure.


Before using any of the methods with an actual system, do a dry run.

Good Luck everyone.

Read my other posts here.

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