RISC World

Cooling a RiscPC

Dave Holden has his fans...

The RiscPC is now over a decade old and when its case and power supply was first designed the cooling was more than adequate. It had a 30 Mhz processor, and a slow Conner hard drive that produced very little heat. The addition of a PC card added some extra heat, but the original 33 Mhz 486 didn't run anywhere near as hot as the later, faster cards. In addition its location was such that there was plenty of clear air around it to ensure adequate cooling. Adding a CD ROM drive didn't produce much extra heat, other than that generated by the additional loading on the power supply, since it would normally only be run intermittently.

The later 710 processsor running at 40 Mhz produced a similar insignificant amount of heat as the 610, and when it is running if you lay your finger in the processor you will find that it is only a few degrees above ambient temperature. Faster PC cards running at 66 Mhz and then 100 Mhz did start to introduce more heat, and the 100 Mhz cards required a heatsink. However, as previously mentioned, this was in a an area that was unobstructed with plenty of clear air (about the only area of which this could be said) and in the "exit path" of the airflow so it didn't have any detrimental effect on other parts of the computer.

Adding expansion cards sometimes had some effect, depending upon their type. However to add more than two you needed a two slice machine, which would have more room for air to circulate and twice as many slots for the air to exit, so cooling was still adequate.

Had things stayed like this, well within the original design parameters, there would be no problem, but unfortunately they didn't, and the RiscPC case is often now required to cope with more and hotter components than was originally allowed for.

More about RiscPC ventilation

Air flow on the RiscPC operates in the opposite direction to most other PCs. A standard PC AT or ATX power supply has its fan on the external side. It draws air from inside the computer case and exhausts it to the outside. This allows the air inlets in the case to be positioned so that cool air is drawn into the places where it is most needed. The RiscPC power supply's fan draws air through the power supply and blows it into the case. This has the advantage of ensuring a flow of air across the motherboard and any expansion cards, but makes it far more difficult to add extra ventilation.

Before suggesting some improvements let's take a look at the standard system in greater detail.

Air is drawn into the power supply from the right hand side of the case. There are a series of slots cut in the case and careful measurement and calculation shows that the total area of these slots is approximately 27 sq. cm. Only the slots in the bottom slice are relevant. The air enters the power supply casing via a number of holes, ninety in all, and these have a total area of approximately 9 sq. cm. The "active" part of the fan aperture through which the air exits the power supply is approximately 29 sq. cm., suggesting that the fan is "throttled" by the holes in the power supply casing. Airflow through the fan is further restricted by a wide piece of plastic moulding cunningly placed right in front of it as shown below.

In this picture a strip of paper has been taped to the power supply to represent this section of the case. As you can see it passes right in front of the fan, reducing the aperture size to around 21 sq. cm. This has some effect on airflow, though not as much as would be the case were the flow not already being restricted by the inlet holes. As well as restricting the flow of air this also causes air turbulence and so adds to the fan noise, which is already higher that it should be because of the restriction due to the reduced air inlet which causes "slippage" in the airflow through the fan.

If you want to increase airflow it is therefore fairly obvious that the simplest way to do this is to improve flow into the power supply by enlarging the air inlet holes. Opening these out to 6 mm. from their current size of 4 mm. will nearly treble the area of the air inlet from 9 sq. cm. to around 25 sq. cm.

This is a fairly simple DIY job in itself, but before you can drill out the holes you will need to disassemble the power supply and remove the PCB to avoid damaging it. However, in view of the much improved airflow and zero cost you may feel it's worthwhile.

There's an even simpler improvement that you can make. Ideally all the air drawn into the power supply should come from outside the computer. However, there's a gap between the power supply itself and the computer case and this allows air to "leak" in at the top and front from inside the case. The total area of this gap is around 21 sq. cm., and so on a two slice machine quite a lot of air will simply go round and round, being blown out by the fan on one side, passing over the top of the power supply, and drawn in again. This is not good because not only will the air be warmer than fresh air from outside, but also air circulating in this way is not being circulated around the rest of the computer.

The solution is very simple. There are two alternatives. If you can obtain some of the foam strip which has adhesive on one side and is sold as draught sealer for doors and windows then you can stick this along the side of the power supply casing close to the top and close to the front and this will seal the aperture. Alternatively just stick a narrow strip of card to the top of the power supply with tape so that it projects over the side and seals the gap between the top of the power supply and the casing. Either of these two methods will prevent (or drastically reduce) air recirculation.

Adding extra fans

It's fairly easy to add extra fans to a RiscPC case but it does require careful consideration if you aren't to make matters worse. If you have a single slice machine then the apertures for the air to escape on the left hand side have the same area as the inlet on the right, so if you try to introduce too much extra air it will have little effect. Similarly poorly positioned extract fans can reduce airflow from some areas which need it.

With a two slice machine the situation is much better. There is now an extra exit path on each side, trebling the area available, so there is scope for adding additional fans blowing air into the case.

Cooling hard drives

The first place often in need of additional cooling is the hard drive in the base of the machine. This was not a problem with the Conner 210 and 420 drives originally fitted. Even after some hours running both the casing and the components on the PCB of these remained cool, so there was no question of overheating. However, most modern drives rotate at at least 7,200 rpm. and these can generate quite a bit of heat.

If you look at the previous picture you will see a thick black line drawn vertically across the paper strip towards the front of the fan. This represents the location of the RiscPC backplane. As you can see part of the fan is in front of this and this means that some of the airflow from the fan passes in front of the backplane, under the floppy disc drive mounting, in the general direction of the hard drive. However, you can see that only a small part of the fan's output is directed into this area and that is right at the back. Because the only exit for this air from this area is back over the motherboard the flow path is such that it passes over and cools the rear right hand corner of the drive but doesn't penetrate into the dark corner at the left front.

An ideal solution would be an extract fan at the front right hand corner, ensuring that the air passed over the hard drive. A small 40 mm. fan will just fit. To make room you will need to move the hard drive towards the right. This is easily done and doesn't actually require any modifications with most drives as the retaining strap will still fit and the data cable is just long enough. You will need to cut a hole for the fan. The neatest way is to use a hole saw but an alternative is to remove some of the "bars" to enlarge the air slots. You will also have to snap off the front two downward projecting lugs on the next slice as these will foul the fan.

The photo below shows the layout of this system.

Even if you don't fit a fan, moving the hard drive as shown, and opening up the air slots at the front of the bottom slice by removing every other "bar" from the front three sets of air slots will encourage airflow over the drive. Airflow will also be improved if you tape a piece of card to the rear of the CD ROM bay on the first slice so that it projects downwards between the hard drive and motherboard. This closes the shortest path for the air and forces it to take the longer route over the drive.

For most users hard drive temperature won't be a serious problem. However, I've noticed a distinct increase in the failure rate of 7,200 rpm. drives mounted in this position during hot weather, so even if you don't fancy cutting holes in your casing or fitting an extra fan just moving the drive slightly and fitting the card screen as described can make a significant difference and is easy to do and costs nothing.

Drives in 5" bays

Where a drive is fitted in one of the main drive bays it's much easier to cool. You can purchase a combined drive mounting bracket complete with cooling fan(s) for less than £10 like the one shown in the photo below. The problem with almost all of these is that they are designed to fit in PC drive bays and so require the mounting screws to be inserted from the side, which isn't possible in a RiscPC. The other problem is that the fans tend to seem rather noisy, possibly because they're very much "in your face" right at the front of the computer. If you don't want to do the necessary modifications yourself APDL can supply these altered to fit a RiscPC and with the fan slowed so that it still gives sufficient airflow to keep a hard drive cool but with much reduced noise.

You will notice that I make no mention of fitting a second hard drive in the spare floppy drive bay of a two slice machine. this is because this is something which should never be done. A hard drive fitted here is closely restricted on every side and there is no easy way that you can get even a minimal airflow into this area. A hard drive fitted here will always tend to overheat and so have a much shortened life expectancy.

Better processor cooling

Whereas the original 610 and 710 processors produced almost no heat the StrongARM processors do. Not so much that they overheat on their own, but when mounted close to a fast PC card things can get rather warm. None of the "official" Acorn PC cards had fans fitted, although the 100Mhz processors did have a heatsink, however third party 133 Mhz cards do have a fan. If the PC card gets hot then its close proximity to the Strong ARM card means that will also get warmer than it ideally should, so some form of additional cooling is a good idea.

Instead of fitting a fan directly to the processor (or in addition to it) a worthwhile addition is to fit a fan to the side of the case behind the PC card. With a single slice machine this should extract air but with a multi slice machine it should blow air into the machine as the single slice has a much smaller number of slots for the air to escape, so its effect will be diminished. In either case the effect is to improve overall ventilation and also to direct a stream of cooling air over the PC card.

The picture above shows the location for this type of fan. A 45mm fan as fitted to older Intel processors is ideal here. Once again you can simply remove some of the "bars" between the ventilation slots in the case to make an exit (or entry) path for the air. If you intend the fan to blow air into the computer rather than acting as an extractor then you may prefer to mount it on the rear of the case as the incoming air will still be directed straight at the processor card.

Fitting a PC extractor fan

These are available from PC dealers for a few pounds and are designed to fit in a spare PCI slot. A fan of this type is illustrated below. Some can be quite noisy. Their dimensions are such that they can normally be made to fit (with a little bit of modification) in a spare podule slot in a RiscPC.

This might seem like a quick and easy way to add extra ventilation and it has been advocated by some people, but I do not like this method as it may do more harm than good. By placing an extractor fan adjacent to the power supply you are, in effect, taking the airflow from the power supply fan and sending most of it straight out the back of the computer before it has a chance to cool any other components and this can diminish airflow in some areas, such as around the processor(s). However, if this was used in conjunction with additional small fans as described above which were set to blow air into the computer then this could result in a greater overall airflow which could be beneficial, especially where the ambient temperature is high.

Finally the picture below shows a product which will soon be available from APDL. This is a higher output power supply for the RiscPC, available in either 140 or 200 watt. As well as having a higher power output to cope with any additional items you may have fitted, you will see that instead of the normal single fan it has two. There is a 60 mm fan at the rear and a smaller 40 mm. fan at the front. This smaller fan blows a stream of cool air into the hard drive bay to ensure that modern 7.200 rpm. drives don't overheat. The main fan is temperature controlled so that it is normally much quieter than the normal RiscPC fan. It is also positioned so that the airflow passes over the VIDC chip. This helps keep this cool which can be very beneficial, especially if you are driving the VIDC rather hard or "overclocking" as described in an earlier article in RISCWorld Volume 4 Issue 2.

Dave Holden