New PSU wont output 12v, only getting 9-10v from rail


Aug 22, 2009
Last week I purchased a new Corsair TX650W power supply unit for my PC. After taking it out the box, everything looked fine. I took out my old PSU from my case and connected up the Corsair. I did everything the manual tells you to (although in fairness I didn't need to as installing a psu is quite a simple affair). I booted up as normal and my machine seemed to be running smoothly for about an hour. Then suddenly Vista crashed and I heard the hard drives coming to a halt. The fan on my graphics card also stopped. When I rebooted after that I got a Hardware Monitor Error during the POST, asking me to check voltages in the BIOS. The 12v reading was abnormally low. It was fluctuating between 9.40v up to a maximum of 11.07v. Also the reading was shown in red in the BIOS, which indicates there is a problem.

Is this a simple case of a faulty PSU? Or could there be any other causes at play here that are worth considering? Basically it's outputting some power on the 12v rail, but not enough to allow stable functioning of my PC. Also when I put my old PSU back in, everything works again and I get a stable 12.05v reading in the BIOS.

Right now I'm only about 70% sure the new PSU is faulty. I mean if it was completely busted it wouldn't output any power at all, right? I would assume so anyway. Also the site from which I purchased the PSU has this policy where, if you ship an item back to them as defective and they decide it's not defective, they charge £15 + vat and other charges which they do not specify. Then they ship it back to you.

Any suggestions on what to do guys?

My guess is a bad PSU. Corsair has good quality but not even they can compleatelly eliminate bad PSUs.

Contact Corsair, they will do an RMA for you. Take pics of your BIOS reading/System and send it to them as proof.

Also can you check the readings with a DMM?
First, it certainly is possible for a PSU to develop a fault and still output power that is incorrect. In your case, it appears (according to your mobo BIOS readings) there are two things wrong on the 12v rail - low voltage, and fluctuations. Shabaa's idea is a good one - if you can, verify the BIOS screen readout with measurement by voltmeter directly on the 12v supply line from the PSU where it is plugged into your mobo.If those voltages and fluctuations are correct, there certainly is something wrong with the PSU. In fact, to verify that is is the PSU and not some odd mobo factor, replace the new one with the old one and check the same voltages.

As Shadow703793 said, contact Corsair Tech Support ant give them your story and voltages. If they confirm this is a PSU fault, you'll feel more confident taking it back to the supplier. Or, Corsair might opt to replace it for you directly.


Aug 22, 2009
Thank you for your responses guys.

I don't have a multimeter, but I found a cheap one on ebay. Just a cheap pocket digital multimeter, hopefully it should be enough to do the job. I should have it in a couple of days. Also went on Corsair's site and did the form, got an email back saying they are investigating and they will respond as soon as possible.

Just got a question about the multimeter. I've been looking at some tutorials online regarding how to test a PSU using a multimeter/voltimeter. They all have advice along the lines of, "be careful doing this, and make sure you know what you are doing, there is a risk of being electrocuted etc". Which I understand. Are there any particular precautions I should take when doing the tests? Haven't actually done this before.
Simple PSU testing:

1. Remove PSU and all connections.
2. Fix a case fan on to a Molex.
3. Jump GREEN (pin 14) on the 24pin ATX connector to any BLACK ground pin. Case fan should spin up.
4. Put the + lead of the DMM to the YELLOW connector on the Molex 4 pin. Put the - lead of the DMM to the BLACK pin on the MOLEX connector. The reading should be close to 12v.
5. Do it to the RED pin also. Reading should be 5v.


Aug 13, 2007
The question that comes to surface is why did you change the PSU in the first place?
If you have a short to ground somewhere or excessive load it will pull down the voltage.

Step one at this point is to isolate the PSU and test independently like Shadow posted.
A computer PSU will NOT electrocute you unless you open up the case and reach inside. There may be higher voltages inside, but the highest voltage available on the wires that come out is 12 volts - same as a car battery. You could put your fingers across the car battery's terminals and never feel the electrical current.

The real danger in testing the PSU is to the PSU itself. If you mistakenly short out two lines with your probes you could damage the PSU. So just be careful to touch only one wire with each probe.


Jan 22, 2009
I agree with knotknut. If you test the supply and the readings are normal then you will have to test the PSU with one component attached at a time. This testing will isolate a component that is drawing too much or may have a short to ground.


Jan 11, 2009

Very true, but at 12v, even with wet hand, you cannot get the resistance of the body low enough to electrocute yourself (which is why you are allowed to submerge 12v light in a bath if you like). So generally you can go by the voltage to determine if it is safe, although technically it is the current that kills.



Even with wet hands, 12V isn't enough to drive a significant current through your skin. It's true that current kills, but you need sufficient voltage to overcome the body's resistance before the current becomes an issue.
A car battery is capable of supplying 100 to 150 amps through a very low-impedance load like a starter motor trying to turn over a stiff engine. BUT the current is determined entirely by the voltage available (yes, at low currents the car battery will output about 14.5 volts) and the resistance of the load across it. Even with wet hands to give improved electrical contact to your skin, the resistance across your hand limits the current to a few milliamps. It ususally takes currents of several amps to cause permanent damage to body parts. The most dangerous situation, though, is a current flowing through the chest where it can interfere with normal nerve signals (millivolts!) regulating heart action and cause the heart to fall into fibrillation. This can happen with a through-the-chest current as low as 10 to 30 milliamps. So if you connect to the voltage source with two wet hands you could get a potentially lethal shock from a source as low as 50 volts. But not 12 to 15 volts.

Anyway, back to OP's original problem. Knotknut and Shabaa have very good points to consider even after the PSU itself is checked. Was it overloaded by a faulty component? That needs to be answered before connecting another PSU and potentially damaging it in the same way.


Test in stages. Do the PSU with fan load. Safest place to check is one of the drive molex connectors. Remember thois represents a minimal load - less than an amp.

If you get 12 volts, plug in the main power connector (24 pins) and check. If it's low, either the PSU cannot support a 3 - 5 amp load, or there's something wrong with your motherboard.

If you get 12 volts, plug in the CPU power cable. This will represent the first real load - depending on the CPU, 6 to 10 amps. If low, either motherboard or PSU.

Continue with video card and the rest of the components.


Aug 22, 2009
Ok guys I did the test where you jump the green wire to a black wire on the 24 pin connector. It gives me a reading of only 11.20v to 11.24v on the multimeter. It fluctuates. That is with fan load only.


Aug 22, 2015

Technically, it is the amperage that will kill you, not the voltage. and yes opening a PSU while it is on can very easily kill a human, however doing a simple voltage test on its outputs will not harm you unless there is something VERY wrong with the PSU


According to William Beaty:
To stay safe, we should only play with voltages which are way less than 45 volts. Don't mess with plug-in appliances, they use 120 volts. As long as the electric charges flow through your body at a rate that is lots less than 1/1000 of an ampere (one milliamp), they're not dangerous. You can't even feel them. To stay safe, only send electrical current through your body at a rate less than 1/1000 ampere, and do this by staying away from high voltage. But this is numerical, so it's not that meaningful.

The "electricity" which flows through human bodies is charge, but it's not electrons. Instead it's charged particles: potassium ions, sodium ions, chloride, etc. Since these particles are always inside our bodies, we can't say that "electricity" is dangerous. Instead, it is the FLOW of charges which causes problems. Think like this: human beings are partly composed of movable "electricity" or charges, but these charges normally don't move very much. If something makes them move, abnormal things can happen. When someone gets a shock from touching wires, electricity WAS NOT put into their body from outside. Instead, the electric voltage on the wires pushed upon the charges already inside their body, which then moved forward, causing an electric current to appear inside. During electrocution, human beings become part of an electric circuit, and the circuit causes their own internal "electricity" to begin flowing fast.

During electrocution, the large abnormal electric current (ion currents) causes nerves to fire, which causes muscles to tense up, and this can include your heart muscle and your lungs' diaphragm (electric current can stop the heart and prevent breathing.) Also, the suddenly-tensed muscles can throw your body across a room hard enough to break bones or cause concussions. Also, electric currents directed through your heart can trigger fibrillation, which is a type of fast, quivering heartbeat which does not pump blood. Once fibrillation is triggered, it might not stop by itself. And finally, large electric currents can cause heating which cooks tissue.

As long as the value of electric current within tissues is small, it will not cause significant pain, heat, or muscle contraction. For example, if you touch the metal terminals of a 9 volt battery with wet fingertips from each hand, the voltage does push a measurable electric current in a circular path through your arms and chest! Yet the only result is that your fingertips become full of corroded metal from the electrochemistry occuring where the wet skin touches metal. In other words, 9 volt batteries are relatively safe. Now if you did the same thing with a 90 volt photographer's photoflash battery, the voltage-push is ten times higher, and the value of current would be ten times higher. You'd feel violent pain, your arms would jerk away, and some people would even go into cardiac fibrillation or "heart attack." The 9v battery creates a small current which you can't even feel. The 90v battery creates a larger current which is painful and dangerous.

Voltage is not the only thing which determines whether you get a shock. ELectrical resistance of your skin is also important. If the resistance of your skin is low, then a smaller voltage will create a larger current. For example, if you have tough, thick, dry skin on your hands, you might be able to touch even a 90-volt battery without harm (don't try it though!). Your higher skin resistance only allowed the battery to create a small current through your body. On the other hand, if you have thin, moist skin, then the same battery voltage will push a much higher current through you. If your skin is thin enough, then even a 9v battery might cause a larger current, which gives your fingers a slight tingle.

What is "safe?" I've heard that the threshold for feeling an electric current is around 1/1000 ampere. Present UL regs say that voltage below 45 Volts is not an electrocution hazard. These are general rules of thumb for everyday situations. However, voltages less than 45v aren't always safe. If you poked a 40 volt battery into a big cut on your chest, I wouldn't guarantee that you'd survive that experiment! The cut is a gap in your skin; the wet meat inside your skin has low electrical resistance, and your heart area is more sensitive than other parts of your body. On the other hand, if you accidentally touched the two prongs of an AC plug while plugging them in, the current might flow only through your fingers. This hurts a lot, but won't cause death.

One way to be slightly safer: touch a 900 volt battery instead! A very large flow of charge will appear inside you, which will fire all muscles including your heart muscle. This large current usually force your hand to jerk away from the wires, and it will will automatically defibrillate your heart. Make no mistake, this is very painful and extremely dangerous. But because of the fibrillation vs. defibrillation issues, there is a medium range of currents which are MORE dangerous than low currents or high currents. The medium currents trigger heart fibrillation. High currents do not. Still, high currents can quickly cook flesh because of heating effects, so very high currents are still extremely dangerous.

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