A short question about power consumption of old PCs / PC-hardware

[Peter z80.eu]

I was wondering how "big" a power supply must be to deliver enough power to a defined constellation of hardware.

That means:

A motherboard, a CPU ( must be ;-) ), a common/usual amount of memory (RAM), a floppy drive (3.5" or 5.25"), a Compact Flash card as an IDE HDD.

I am asking that because I got an almost empty vintage AT (clone) computer case with a 150W power supply.
My idea is to build with that case and power supply a new working PC.
I am free to select the era, I just have to use a mainboard with "baby AT size".
So I was thinking first about a Turbo XT board, a V20 CPU, 640KB (DIL) RAM, a 360KB floppy drive, a XT-IDE card with a small CF card as HDD. And an ISA VGA card like a ET4000.
BUT.
As far as I can measure it (with already build/existing PCs), there is not really a big difference in terms of power consumption, if you choose PC hardware 'til the Pentium III era.

Explanation and my question at the end:
CPU power consumption differs between 3-5 Watt for the very first Intel x86 processors (8088, V20) to somewhat between 15-25 Watt for the Pentium II era (excluding the top models with the highest frequency).
I have no values for the mainboard consumption, but the shrinking process for the circuit technology helps to reduce the power consumption, means older mainboards not really drain less power.
RAM does consume more power if the access times are going down (in nanoseconds), but this increase is moderate, too.
If you don't choose a high end PCI 3D accelerator card, instead, just an ATI RAGE XL with 8MB, this should be also acceptable.
And last but not least if you take a CF card as a HDD replacement, you will be for sure on the safe side, too.

So my idea was to take instead a Socket-7-Mainboard with a Pentium 200MMX (or similar), about 64MB RAM with FP-SIMMs, a 1.44MB floppy drive, and the already mentioned CF card (but with more capacity).
For my targeted 150W power supply, this must be still manageable, and there must be still from for an upgrade (e.g. a second floppy drive).

What do you think ?

 

[Timo W.]

First, check the power output of the +5V rail, because that is the most important one. Your 150 watts PSU may only have half as many watts on the +5V rail. The label on the PSU should tell you.

Second, for a Pentium MMX system, I would highly recommend a 240 watts PSU. 150 watts is certainly enought for anything up to a 486 DX4, but then the power requirements went up. The 200 MMX alone needs almost 20 watts, and that does not count in all the waste produced by the two split-voltage regulators and the fan(s).

A common mistake is to think a CF card (or flash memory in general) needs less power than a spinning hard disk. That's not really true. Flash memory does not need +12V, so there you save. But on the +5V rail, it can even need more current when writing than a hard disk does. On average, the load on the +5V rail is pretty much equal.

 

[DOSDays]

I would caution against using any old PSU. The last thing you need is an aging power supply that creates a ripple or overvoltage in any of the DC output lines going to your motherboard.
It may not be in keeping with a retro PC to replace it with a modern supply but you will sleep better at night knowing the rest of your old hardware is receiving clean voltage. I replaced the innards of an old AT power supply with a brand new one in order to retain the original look (it was an XT clone with the big red power switch on the outside) whilst benefiting from new circuitry/electronics, but don't do this unless you're a qualified electrical engineer! Another option is to use an 80+ PSU (must better for your electricity bill!) along with an ATX-to-AT cable adapter if required. ATX arrived around 1995/1996 so inline with your Pentium 200 MMX era.

As Timo W. suggested I'd go for something in the 250-300W range. There's some information on page 2 of my XT restoration project here.

 

[vwestlife]

FWIW, an XT-class Tandy 1000 with CRT monitor uses almost exactly the same amount of power from the AC line as a Core 2 Duo PC with LCD monitor:

 

[Plasma]

I would caution against using any old PSU. The last thing you need is an aging power supply that creates a ripple or overvoltage in any of the DC output lines going to your motherboard.
It may not be in keeping with a retro PC to replace it with a modern supply but you will sleep better at night knowing the rest of your old hardware is receiving clean voltage. I replaced the innards of an old AT power supply with a brand new one in order to retain the original look (it was an XT clone with the big red power switch on the outside) whilst benefiting from new circuitry/electronics, but don't do this unless you're a qualified electrical engineer! Another option is to use an 80+ PSU (must better for your electricity bill!) along with an ATX-to-AT cable adapter if required. ATX arrived around 1995/1996 so inline with your Pentium 200 MMX era.

As Timo W. suggested I'd go for something in the 250-300W range. There's some information on page 2 of my XT restoration project here.

Ehhh I'm not convinced that a generic China PSU is universally better than a vintage PSU...

 

[T-R-A]

Ehhh I'm not convinced that a generic China PSU is universally better than a vintage PSU...

Agreed. We have started having to use a "generic Chinese" 350W ATX supply in repair of a piece of equipment here where I work vs. the more trustworthy (albeit Taiwan) supply which is no longer available. Customer returns are up almost 300%....

 

[Chuck(G)]

OTOH, SMPSU technology has improved considerably since Bob Boschert sold his switching PSUs in the 1970s.
If you're after doing a lot with a little power, consider a RPi with an LCD monitor.

 

[twolazy]

Other option for low power be the PC/104 platform...

 

[Peter z80.eu]

I have no choice to replace the IBM XT style power supply with a more modern "generic" power supply, the case of the power supply is very special, think about the red on-off switch.
Well, I just have to try it out, I can start just with the motherboard (with CPU) and nothing else, measuring the current on the 5V rail.

 

[Peter z80.eu]

First, check the power output of the +5V rail, because that is the most important one. Your 150 watts PSU may only have half as many watts on the +5V rail. The label on the PSU should tell you.

Second, for a Pentium MMX system, I would highly recommend a 240 watts PSU. 150 watts is certainly enought for anything up to a 486 DX4, but then the power requirements went up. The 200 MMX alone needs almost 20 watts, and that does not count in all the waste produced by the two split-voltage regulators and the fan(s).

A common mistake is to think a CF card (or flash memory in general) needs less power than a spinning hard disk. That's not really true. Flash memory does not need +12V, so there you save. But on the +5V rail, it can even need more current when writing than a hard disk does. On average, the load on the +5V rail is pretty much equal.

According to https://en.wikipedia.org/wiki/List_of_CPU_power_dissipation_figures a Pentium 200 takes 15,5 watts and a Pentium MMX 200 takes 15,7 watts, not a really big difference.

 

[Peter z80.eu]

Update. I compared the technical specs of many (vintage) power supplies. I also searched for the power consumption number of a Compact Flash card. CF cards with lower capacity will not exceed max. 800mA, higher capacity around max. 1A, but typical is 200mA. I guess 150W is not too bad for a Pentium 133 system:

130W IBM PC/XT original power supply:

OUTPUT:
+5V 15A
+12V 4.2A
-5V 0.3A
-12V 0.3A
-----------------------------
145W PC AT Clone power supply:

AC Input 200-240V 4A

OUTPUT
+5V 18A max
+12V 4.2A max
(6A for 10 secs)
3.3V 10A max
-12V 0.4A max
-5V 0.3A max

Max Power 145W
Combined +5V & 3.3V Loading 90W Max
-----------------------------
200W AT power supply from "King":

AC Input 230V 3.5A

OUTPUT
+5V 20A
+12V 8A
-5V 0.5A
-12V 0.5A
-----------------------------
200W AT power supply from "dtk":

AC Input 230V 3A

OUTPUT
+5V 20A
+12V 8A
-5V 0.3A
-12V 0.3A
-----------------------------
150W AT power supply from "ASI":

AC Input 230V 3.1A

OUTPUT
+5V 15A
+12V 5.5A
-5V 0.5A
-12V 0.5A
-----------------------------
150W AT power supply from "Compaq":

AC Input 230V 3A

OUTPUT
+5V 18A
+12V 4.5A
-5V 0.3A
-12V 0.4A
-----------------------------
200W AT power supply from "HIPRO":

AC Input 230V 3.5A

OUTPUT
+5V 20A
+12V 8A
-5V 0.5A
-12V 0.5A

Max output power 200W
-----------------------------
200W AT power supply from "ILANTEC":

AC Input 230V 3.5A

OUTPUT
+5V 20A
+12V 8A
-5V 0.5A
-12V 0.5A
-----------------------------
200W AT power supply from "Seasonic":

AC Input 230V 4A

OUTPUT
+5V 20A
+12V 8A
-5V 0.5A
-12V 0.5A
-----------------------------
230W AT power supply from "Linkworld":

AC Input 230V 4A

OUTPUT
+5V 23A
+12V 9A
-5V 0.5A
-12V 0.5A
-----------------------------
180W AT power supply from "Linkworld":

AC Input 230V 3.1A

OUTPUT
+5V 18A
+12V 7A
-5V 0.5A
-12V 0.5A
-----------------------------
192W IBM PC/AT original power supply:

AC Input 230V 4A

OUTPUT
+5V 19.8A
+12V 7.3A
-5V 0.3A
-12V 0.3A

 

[Caluser2000]

My 200MMX slimline Linux system ran for decades on an 80watt psu with 256megs of ram, 4megs of vram, 1.4" floppy drive, spinning rust hdd, DVD readwriter and network card.