Suppression of (electrical) noise from CH Thermostat

[Set Square]

OK, the title doesn't tell the whole story, because I am using a CH stat for a non-standard purpose.

I'd better explain!

I've been having problems keeping the inside of my computer, and the processor, cool - even after adding extra DC fans. So I have solved the problem by fitting one of Mr Maplin's 120mm mains-driven fans in the side of the (tower) casing - which blows quite a lot of ambient air past the electronics, exhausting through the rear.

Works a treat - *except* that it's a bit (audible) noisy, and doesn't actually *need* to run all the time. Now it just so happens that I have a spare Danfoss TWP room stat after fitting a programmable stat on the central heating. So I've mounted this TWP inside the computer case, and wired it to switch the fan on when the temperature is above the set value.

This works quite well *except* for the fact that there's quite a lot of crackling on the computer speakers for several seconds each time the stat switches on or off.

It seems to me that I ought to be able to suppress this noise with a suitable capacitor. Can some kind soul please advise on the spec/value of the capacitor required, and whether to wire it in parallel with the motor, or across the switch in the stat. FWIW the fan motor is described as "impedance protected shaded-pole motor" rated at 22w 140mA (which implies a power factor of about 0.65)

TIA.

[Martin Warby]

why not use a pot (potential devider) to slow the fan down ?

Martin Warby

[Set Square]

Do you mean insert a resistor in series with the fan - so that it *always* runs at at least low speed - and use the stat to by-pass the resistor when max cooling is required?

Would that work with this type of motor?

Wouldn't contact noise from the stat still get into the system?

[Dave Liquorice]

On a mains fan? Not to mention the fact that the dumped power ends up as heat and the power dissipation required is a bit above your average variable "pot", which generally aren't mains rated either...

I guess you could use a suitably rated wirewound resistor, slowing the fan down will help the noise, indeed just taking 10% of the speed will make quite a difference.

Suppresion of the stat, 0.1uF in series with 100R across the contacts. Either as suitably rated components or a contact suppressor like Maplin RG22Y. Note the Specification part of that items description appears blank in my browser check the device is mains rated...

[James Salisbury]

Use the correct fan, you can get them with internal / external thermisters and this regulates the fan speed.

[Martin Warby]

I was thinking of a pot (a variable resistor looks like

it up as a potential devider as below

connect outer 2 tags to supply connect inner tag to +ve lead of fan connect -ve led of fan to -ve lead on outer 2 tags

Simply turn the pot till the desired speed is obtained

Martin Warby

[Martin Warby]

DOH should have read post more throughly thought it was a DC fan but was using a mains rated thermostat.I agree using a pot would be a BAD idea in the circumstances.

I think if normal DC fans aren't powerful enough to cool the system then the general airflow in the case needs to be considered

Martin Warby

[Ian Stirling]

Lots of stuff for doing this sort of thing, very cheaply. Lots of options, from thermostatically controlled fans to little control panels that can be set to run fans at certain temperatures from multiple temperature probes, and have lots of blinking lights on.

[Set Square]

Thanks. Could you please point to a specific area on the Ebuyer site - there's a lot of stuff to wade through!

[Set Square]

I've had one or two "funnies" with the computer in the past which look as if they might be DC power related - so I'm trying to avoid loading the internal power supply any more, even though it has been uprated.

[raden]

Until you see smoke and smell burning

Have you looked at the power rating of a normal pot?

1Watt IIRC

[Set Square]

Many thanks Dave - looks like just the thing. I think the RG22Y is mains rated. I captured the picture from the Maplin website and blew it up - and although it's not wonderfully distinct, I'm pretty sure it says 250VAC on it.

[raden]

In message , Set Square writes

You could always go for water cooling, quite a few of them about nowadays

It sounds like you have some fundamental problems to sort out

I have a desktop 2 gig machine with 4 HDs, a gig of memory, quality graphics card etc in a case with poor ventilation, no problems with it cutting out with overheating

[Martin Warby]

see my earlier post I had not realised it was a mains fan (I presumed it was running from the 12V DC line).

Martin Warby

[Ian Stirling]

Sorry. You're looking for the "case mods" section. I think then the misc subsection. For example, I've got a 4 channel fan speed controller for 3.99. And 5 (sleeve bearing) 120mm fans for 72p each. (that are remarkable in that they will reliably start down at 2.4V, so can be run from the 3.3V line, in almost absolute silence.)

[Set Square]

Thanks - but does any this stuff work with a *mains* voltage fan - which is what I've got?

[raden]

Even then, you're pushing it

[Ian Stirling]

No. DC fans are pretty cheap, and may be more suitable, as they can be slowed down easily, whereas AC ones would require more complex electronics.

[Christian McArdle]

I think the idea is that at 72p, you could possibly purchase a new DC one?

Christian.

[Set Square]

True - but I've just invested rather more than that in a nice mains fan, with proper bearings - so that it doesn't make a lot of *mechanical* noise like cheap fans do.