Basics GPUs Pro PSU Reviews

Basics GPUs: Power Consumption, Power Supply Conflicts & Other Myths | 2014 and 2020

What actually happens on the motherboard slot?

Let's start modest and small. Not every graphics card has additional supply connections and there were even times when graphics cards didn't need any at all. Depending on the total power consumption, a graphics card can only be supplied via the motherboard slot up to an upper limit of 75 watts. These are several voltage rails, namely 3.3 and 12 volts. I already wrote that these voltages are looped through the motherboard, so that – since they are not from motherboard with affected currents – you can also comfortably measure them on the 24-pin connector.

2014: Voltage measurement for 3.3 and 12 volts

The resulting currents on the 3.3- and 12-volt line from the motherboard I measure again – as it should – between the graphics card and the motherboard and use the two loops of the riser card:

A little tight, but the pliers fit just in between.

Load peaks on the motherboard: Dangerous or not?

Over the last few months, I have been able to measure different manufacturer solutions in the supply of graphics cards. Initially, it was smaller cards such as different models of the GeForce GTX 750 Ti, whose power consumption did not really exceed the 60 watts on average, but depending on the model and the clock can generate very high power peaks. Then the 75-watt limit for fractions of a second falls quite clearly, as the graphic clearly shows us:

I have measured up to 120 watts for individual cards, which is significantly higher than the specifications! Since these are only short-term peaks, nothing will physically happen to the motherboard, because the connections lead directly to the 24-pin connection of the power supply on the motherboard with at least two 12-volt supply lines.

But what can be quite critical is the fact that in the end it is already a real high frequency, which together with other motherboard components could cause problems.

With cheaper motherboards, you can "hear" the graphics card via the onboard sound. The chirping scroll noise is probably known to pretty much everyone and now we know the cause.

Not the onboard chip is equal to garbage or the motherboard per se crap – no, here the spikes yodel their lonely and unwanted song and the graphics card is the real culprit.  Depending on the model, graphics cards with higher power consumption values still supply themselves via the motherboard slot despite separate power cables. Therefore, these cards also have two separate voltage converter ranges and the load distribution between idle and load moves back and forth between the 12-volt rails as required. Other cards – for example the R9 295X2 and various board partner designs – use the partial supply via the motherboard (almost) no longer at all.

Again, of course, I have published many more recent tests and I recommend this one as an example:

Disadvantages of onboard sound – Influence of graphics card, headphone sensitivity and motherboard layout

Continuous power and roller coaster on the PCI Express connector

Let's finally get to the show, because now it can get really violent. The map measured here is such a candidate, who hardly uses the motherboard any more, but takes up in the peaks up to well over 400 watts, in order to fall immediately to less than 40 watts:

We measure fair and clean as it should be each PCIe power connector separately. We already know the reasons and especially with maps like the Radeon R9 295X2, which we will come back to with pleasure in the next chapter, it is simply unavoidable, since each connection has to supply one of the two GPUs separately.

Clean solution: separate measurement of both PCI-E connectors with voltage and current

But back to the water-cooled Radeon R9 290X from the diagram. Their TDP is set at 250 watts, but these are not quite achieved even during the stress test. Finally, let's take a look at the table with the debit and credit account for gaming and also see that the two PCIe ports have to handle very different loads.

  Minimum Maximum Average
PCIe Total: 38 watts
(17 + 21 watts)
428 watts
(205 + 223 watts)
239 watts
(109 + 118 watts)
Mainboard 3.3V: 0 Watt 3 watts 0 Watt
Mainboard 12V: 0 Watt 20 watts 4 Watts
Mainboard Total: 0 Watt 20 watts 4 Watts
VGA Card Total: 40 watts 433 watts 243 watts

Impressive, isn't it? But there are also much worse extreme cases, as we will see on the next page.

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About the author

Igor Wallossek

Editor-in-chief and name-giver of igor'sLAB as the content successor of Tom's Hardware Germany, whose license was returned in June 2019 in order to better meet the qualitative demands of web content and challenges of new media such as YouTube with its own channel.

Computer nerd since 1983, audio freak since 1979 and pretty much open to anything with a plug or battery for over 50 years.

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