How to test PSU with digital multimeter?

[frozensun]

Guys,I want to test my PSU with multimeter because issues with GPU.

How do I do it?

Can I test 6-pin and 8-pin connectors with multimeter and how.

 

[RollinLower]

if you see these values you should be fine. seeing these values in a idle scenario doesn;t really mean that the PSU can still deliver these when under a heavy load tough. so it's not a thorough check.

[frozensun]

Should I leave the 24 pin plugged into mobo?

So red from multimeter goes to yellow and black to black right?

[mariushm]

The black probe goes to the lower voltage (ground in your case)

The red probe goes to the higher voltage (whatever voltage you want to check)

 

So the black probe can go to any ground wire on your power supply. Usually, the easiest is to insert it in one of the molex ground pins because it's in the middle of the connector and round and big enough that the probe can fit inside and stay there.

 

So you set your digital multimeter to Voltage (DC)  and if the meter doesn't have auto scaling, you set it to 20v or higher (because voltages of the power supply are up to around 12v). If your range is set too low, the screen will show "OL" over limit and if you leave the meter like that for long time, the meter can be damaged.

 

If you want to check the power supply while completely disconnected from pc, you can start it up by connecting the PS_ON pin to any ground pin, the easiest is to use a metal paper clip or a tiny bit of wire.

IF there's no connection between PS_ON and a ground pin, the power supply s turned off and only the stand by voltage should be present on the purple (5v SB pin), and that should be around 5v.

 

The pinout of the 24pin connector is this:

 

 

The molex connector is like this:

 

[frozensun]

Ok guys,but is there a purpose of testing PSU without components who are taking votage/current?

How can I be sure that my PSU is okay for my PC and supplying enough voltage for my gpu if I measure it without GPU.

[frozensun]

I need to check if my PSU is delivering enough voltage for my GPU.

Is there a way to know that?

On my previous thread I mentioned that 12v drops as low as 11.6V measured from BIOS.

i dont think that is norml but guys wrote is okay.

[Elisis]

2 minutes ago, frozensun said:

I need to check if my PSU is delivering enough voltage for my GPU.

Is there a way to know that?

On my previous thread I mentioned that 12v drops as low as 11.6V measured from BIOS.

i dont think that is norml but guys wrote is okay.

Software PSU voltage measurement is misleading at best, 90% of the time.

[mariushm]

Sometimes the computer doesn't start so you want to check if the power supply is dead or the motherboard's at fault or something else.

 

For example, a shorted (dead) hard drive could cause the protections inside the power supply trip and make the psu look like it's dead. Disconnecting the psu from mains for at least 10-20s or so (until the internal capacitors are discharged) will reset the psu protections... so with the psu disconnected you'd be able to measure the 5vSB+ voltage and you can power it on outside the pc and measure the voltages before inserting it back in the pc.

 

Another example... a common failure is the 5vSB circuit, which is like a phone charger that runs 24/7 inside the pc even when the pc is shut down. Because the fan of the psu doesn't work when pc is shut down, the components of the 5vSB circuit usually run hotter so they can degrade faster than everything else. On some cheap psus, after around 2-3 years, a capacitor in the 5vSB circuit can degrade so much that without any load, it will work, but as soon as the motherboard starts the chipset sucks power from this 5vSB circuit and makes the capacitor temporarily break, and this makes the motherboard shut down. You as a user would just see the computer restart itself continuously or see the pc start and shut down after a few seconds.

So in such scenario it's useful to completely unplug psu, measure the 5vSB voltage and then keep measing the voltage while the pc is started ... you would see  the voltage on the 5vSB drop to something like 4v randomly so you'd know the circuit fails and you can replace that 1$ capacitor and get a few more years out of that power supply.

An oscilloscope can be more useful in this scenario, but a multimeter is good enough.

 

Yeah, you should test a power supply with some load, but there are perfectly valid reasons why you would want to test the psu without load.

[mariushm]

11 minutes ago, frozensun said:

I need to check if my PSU is delivering enough voltage for my GPU.

Is there a way to know that?

On my previous thread I mentioned that 12v drops as low as 11.6V measured from BIOS.

i dont think that is norml but guys wrote is okay.

The video card will be fine with 11.6v .. 12.6v (even more) ... any component are supposed to tolerate up to 12v + 10% or 13.2v but in reality, you shouldn't see more than around 12.6v 

The video card basically takes this voltage and converts it down to around 1v..1.5v so the exact voltage is not that important, as long as the voltage stays relatively steady... it's the ripple (the fluctuations and drops) that upset the converter on the video card.

 

The voltage is not enough, you also need to know the maximum current the psu can supply but you can't easily measure that with the multimeter. You can measure the actual amount of power consumed by the video card (power = current x voltage) but it would be difficult, because a video card is powered through the pci-e slot (up to around 75w) and from extra pci-e connectors.

 

The current is listed on the label of the power supply but very cheap / no-name power supplies can often lie on the label.

 

The bios can not accurately display voltages, and the values are often rounded up to 50mV (0.05 or 0.1 steps). They're also measured at only one point, which may be less or more loaded than other points - if power is drawn through a wire and you measure voltage there, the voltage will be slightly less compared to other wires because there is some voltage lost on the wire itself as wires are not ideal, there's losses.

 

So for example, if the motherboard takes 8A through the two 12v wires in 24pin connector (65w going to video card, 5-10w to fans, 5-10w to onboard components on motherboard etc), you may measure 11.8v there, because those 0.2v are lost on the two wires between psu and 24 pin connector.

But, if the video card takes only 3A through the 6pin connector (so only 1A through each pair of wires), the voltage on the pci-e connector may be 11.9v because lower current means less losses on the wires between psu and video card.