Using molex to power GPU?

[Mortis Angelus]

Not sure this is in the right thread, but here goes:

 

So my friend is looking at upgrading his pre-built system with a dedicated GPU. The original system is an ASUS pre-made machine with an AMD A10-6700 and only the APU GPU, HD 8670D. So far he's been "okay" with it, but he wants to play total war: warhammer and no man's sky, and for those tasks the HD 6870D is just not enough.

 

So we've been looking into some second hand GPUs to boost his performance (yes, we are on a tight student-budget here). We are aiming at something like the GTX 770 or an AMD equivalent, because those cards go for around 100 euros second hand here in Finland.

 

But here's the thing: Even if we bought a new GPU, the PSU causes trouble: I can only find 2x 4-pin molexes that are free - no 6-pin or 8-pin PCIE-power cables. The PSU is only 350 W according to Asus home page.

 

So: Is it possible to run a GPU off 2x molexes? I'd say not, as people say you need a minimum of 2 molexes for 1x 6-pin, and seeing how most GPUs of old will need 2x 6-pin I don't see this happening. Also, it is not generally recommended, if I interpret various threads on Toms hardware concerning this matter.

 

I would say the best thing is to buy a new PSU as well, or am I wrong here?

[Vegetable]

DO NOT RUN ANY GRAPHICS CARD BESIDES A 750 TI ON ANY 350W PSU. That is how you kill a PSU. Just get a 500-600w and you can run any single GPU. It sounds like the PSU is of bad quality anyways. 

[MattLY]

You need to get a gpu that doesnt get any power directly from psu like the 750ti or the rx 460 that take all power from pcie slot

Edited August 15, 2016 by MattLY

[Mortis Angelus]

11 minutes ago, Vegetable said:

DO NOT RUN ANY GRAPHICS CARD BESIDES A 750 TI ON ANY 350W PSU. That is how you kill a PSU. Just get a 500-600w and you can run any single GPU. It sounds like the PSU is of bad quality anyways. 

 

7 minutes ago, MattLY said:

You either need to get a gpu that doesnt get any power directly from psu. Or get a gpu like the 750ti or the rx 460 that take all power from pcie slot

Thanks guys. I was thinking this as well, but I just wanted verification.

 

The PSU is a OEM 80+ Bronze made for this specific Asus-package, so how good/bad it is I cannot tell.

 

 

[MattLY]

3 minutes ago, Mortis Angelus said:

 

Thanks guys. I was thinking this as well, but I just wanted verification.

 

The PSU is a OEM 80+ Bronze made for this specific Asus-package, so how good/bad it is I cannot tell.

 

 

Usually oem psus just meet the bare minimum requirments of the build. A 350 watt is a very low wattage. And in my og statment. I meant to write that you can either get a new psu, or a low wattage card (with no connectors) the rx 460 is actually a great card.

[Mortis Angelus]

3 minutes ago, MattLY said:

Usually oem psus just meet the bare minimum requirments of the build. A 350 watt is a very low wattage. And in my og statment. I meant to write that you can either get a new psu, or a low wattage card (with no connectors) the rx 460 is actually a great card.

But then again, there should not be that much that pulls power: the CPU is a 65 W unit, and the rest (1 fan, 1 HDD and a DVD-RW unit) should not pull that much.

 

The 460 could be a very good option, but I still feel it is a bit underwhelming compared to a GTX 770 or a 7970. A brand new 500 W PSU (from what I've learned, one should never buy a second hand PSU) + a second hand 770/7970 would be around the same price as an RX 460. Then again... the RX460 is like half of the 480's performance...

[Sovek]

I dunno around your area, but the EVGA 500w White can be had for under $40 in the US. While not being a really good PSU, it still meets the ATX standards and is pretty cheap.

[SpaceGhostC2C]

2 hours ago, Mortis Angelus said:

But then again, there should not be that much that pulls power: the CPU is a 65 W unit, and the rest (1 fan, 1 HDD and a DVD-RW unit) should not pull that much.

 

Total wattage won't really limit your rig (other than, of course, no individual component can go higher than the total), since you generally can't draw the full amount for any particular voltage (that is, you are more limited than what the total says). With that in mind, knowing the total wattage is not enough to know whether it will work or not.

 

What you need to pay attention to is the current limit for each "rail". You will have at least one rail for each of the voltages a Computer uses (12v, 5v, 3.3v). CPU and GPUs all draw from the 12v rails. If there is only one rail, you need all your 12v components combined to stay within the current capacity (it can be listed in watts or amps, with amps*voltage=watts). While you won't necessary use all components at their maximum simultaneously, I'd strongly recommend against a PSU that can't handle that hypothetical situation, because I'd advice against being too close to 100% PSU usage anyway. The 12v rails need to feed HDDs, ODDs and fans as well (all components can be hooked directly to the PSU or get their power through the motherboard, it all ends up in the PSU anyway). Your CPU only adds 65W to the count, not sure about the HDD, not much for the fan and DVD. But something like a 770 or a 7970 will require 200W and above. A OEM power supply in the 350W range will typically have a single 15-18 amp 12v rail, which limits you to 216W total, the rest being 5v (HDD, DVD, and fans again, USB) and 3.3v (other motherboard components). You need close to 300W on the 12v rail to be barely at the limit, and that's unlikely to happen.

You can check for the power requirements individually, but a quick eyeball calculation can be made on the number of connectors: the PCIe slot provides up to 75W on its own. Each 6-pin connector provides additional 75W maximum. Hence, no connector for up to 75W cards, one connector for up to 150, two connectors for up to 225W, and 8-pin connectors to push it further to 250W and beyond. 

 

When you move to the 450-550W range, chances are you can more easily feed a beefier GPU, but make sure that you can get enough power through the relevant rails: you could have a 500W PSU which doesn't provide too much 12v current, and you could have a 500W PSU providing more than enough 12v power in total, but through separate, low wattage rails. That can be a problem as well: for example, the CPU could use much less than what its rail provides, but if the GPU is on the same rail, or if it's in a second rail which doesn't support enough power draw, it would trigger the overcurrent protection of the rail despite the spare capacity in the primary 12v rail. Yes, it's tedious, but it's worth checking, unless you go overkill in which case you can overlook detail.

 

 

Personally, I treat PSUs as any other component when it comes to buying used, but that's a matter of faith and past experiences  

[MattLY]

4 hours ago, Mortis Angelus said:

But then again, there should not be that much that pulls power: the CPU is a 65 W unit, and the rest (1 fan, 1 HDD and a DVD-RW unit) should not pull that much.

 

The 460 could be a very good option, but I still feel it is a bit underwhelming compared to a GTX 770 or a 7970. A brand new 500 W PSU (from what I've learned, one should never buy a second hand PSU) + a second hand 770/7970 would be around the same price as an RX 460. Then again... the RX460 is like half of the 480's performance...

I bought a second hand 650 watt for $15 and I had no problems with it.