Calling all electrics wizards !

[samwiley]

I'm building a hushbox for my Sony CX1 and I'm a total novice when it comes to electrics.

Can any of the electrical wizards out there tell me what I need to buy to run two 12Vdc fans from the mains.

Can I use an ordinary 240v to 12v transformer (do they produce 12v AC or 12v DC?) or do I need a special 12v DC power supply unit?

Any suggestions, tips, help or experience would be extremely welcome!

Thanks

Sam

[Dodgey]

OK Sam,

Firstly, you say your fans are 12v DC. So you MUST use a DC power source.

You refer to "ordinary 240 - 12V power supplies".<br />I take it you mean the type you get from Tandy, or Argos or wherever and usually plug straight into the wall and sometimes have a voltage selector (1.5, 3, 6, 9, 12v) and various plug options on the cable?

If so, then you will have no real problems. Remember to get the polarity right. If you don't then your fans will go the wrong way round. It is easy to do this, then turn the actual fan round and assume all is OK. But it is not. Most modern fan blades are designed to be much more efficient running in the correct direction. As a general rule, the middle most part of a transformer output plug will be the positive and the cable with a stripe, or the red cable on the fan will be the positive. Equally, black is almost always negative.

It is extremely unlikely you have a transformer that produces AC. Just look at the transformer and it will have the output rating in it somewhere, note the mili-amp rating, and if you see a ~ (tilde, or squiggle) mark, then it is AC (very unlikely). (Note, there WILL be a ~ mark refering to the input - 240V rating - this is OK!)

Some thing to bear in mind:

Wire the 2 fans in parallel, not series. i.e. connect the power leads of both fans directly to the power source. Don't daisy-chain the fans, otherwise they will run at half speed.

The higher the mili-amp rating of the transformer, the closer to their optimal speeds then fans will run. When you use a 240-12V transformer you cannot "give" the fans too much mili-ampage, the fans will "draw" what they need. If the power supply is not up to the job then the fans will run slower and the power supply will get hot or warm as it is being worked to the max.

You might want to consider thermostatic PC fans. You can get them from PC accessory suppliers (look in the small ads in the back of PC shopper).

They come with a thermost that you can put in the hush box, and the fans will slow down (and get quieter) as the box cools.

Are they PC fans by any chance? in which case, why not use a PC power supply (located remotely).

Hope that helps

[ 18-09-2001: Message edited by: Dodgey ]</p>

[samwiley]

Thanks Dodgey - that's exactly the info I need.

I asked the same question to a components suppier help desk and they said that I'd need to buy one of the black box plug-in type DC power supplies (like the one that you mention), strip off the connectors and wire in the fans.

I thought to myself "Why can't I just buy a 12V transformer?"

Could the answer be that the ready made power supplies cost up to £20 whereas a standard 240 - 12v clamp transformer costs £4!!

Most of the transformers I've seen also have 2 x 12v outputs, which makes wiring the two fans a doddle.

I've bought the fans already and they're 100mA, so if I understand you correctly, as long as the transformer is rated at least 100mA then it will work fine and that's all I need. Just the transformer with 240v going in one side and the two fans coming out on the other?

Is that OK?

Sam

[Ars longa, vita brevis]

yup

you can also use an ac power supply if you wire a wheatstone bridge to it to rectify it.

you can also use the power supply form a pc, which is 12v dc and should easily supply the fans as they are a couple of amps, maybe five, can't remember.

i have a multo adaptor transformer thing, in the wall type and i think that is only 100ma, so you may run into trouble with these. If you do, you can try the above.

[ 05-10-2001: Message edited by: Ars longa, vita brevis ]

[Gee]

Sam,

I'm using 4x 80mm case fans from a pc and an AT power supply again from a pc. The whole lot cost me less than £20.

[Cliff]

If you are have a bear bones transformer you will to rectify the AC to DC

[quote] you can also use an ac power supply if you wire a wheatstone bridge to it to rectify it.<br /> <hr></blockquote>

I think that was supposed to read "Bridge Rectifier".<br />Cliff

[samwiley]

Sorry guys, I'm a bit confused now, with all this talk of rectifiers, bridges and PC power supplies.

Can I put it another way;

My power source is 240vAC (mains) and I want to power 2 x 100mA 12vDC fans. I'm not using a PC (or any part of one).

What do I need?

Sam (electrical thickie) Wiley

[samwiley]

Sorry Jonno, I seem to be going round in circles here!

I want to avoid using those plug in type power supplies.

The ones that I've seen in Maplins are big and ugly. I just want an ordinary plug on an ordinary power cable coming out of the hushbox and I want to know what I need to put inside the hushbox to transform 240vAC to 12vDC.

I thought that I could just use a suitable clamp transformer. But then people started talking about rectifiers and I got confused <img src="frown.gif" border="0"> .

But thanks for your reply anyway.

Sam

[Elwood]

Sam

There is no getting around the fact that you will have to reduce the ac voltage from 240 down to a suitable level and then rectify (convert to DC) it, you just need a power supply in a box (a Personal Computer power supply would be ideal hidden inside the hushbox). Again somewhere like maplin will sell these, or indeed they may sell a kit you can build. Im afraid all the talk of rectifiers, bridges etc is bound to occur as they are essential components of ac to dc conversion.<br />Sorry about that, but as someone once said you cannot change the laws of physics!

All the best

Jonno

[samwiley]

Jonno

I'd be grateful if you would go over Dodgey's reply in this thread

[quote] It is extremely unlikely you have a transformer that produces AC. Just look at the transformer and it will have the output rating in it somewhere, note the mili-amp rating, and if you see a ~ (tilde, or squiggle) mark, then it is AC (very unlikely). (Note, there WILL be a ~ mark refering to the input - 240V rating - this is OK!) <hr></blockquote>

this implies that all I need is a transformer that will allow 240vAC in one side and give 12vDC out the other.

Is this incorrect?

Sam

[Elwood]

Sam

Dodgeys reply is not incorrect, but what he refers to as a transformer is infact a combination of components (including a transformer, rectifier etc) that makes up the power supply that you require to power the fans. So to address your final point<br /> [quote] this implies that all I need is a transformer that will allow 240vAC in one side and give 12vDC out the other.<hr></blockquote><br />This is totally correct, just re write it as <br />all I need is a power supply that will allow 240vAC in one side and give 12vDC out the other.<br /><br />and you're bang on

If you need any more info don't hesitate to ask

Jonno

[samwiley]

Thanks Jonno

The mists are starting to lift!!

They say that a picture is worth a thousand words, so I wonder could you do me one more favour, if you have time.

Could you go to <br /><a href="http://www.maplin.co.uk/" target="_blank">Maplins</a> website and take a look at product code WB02C and tell me what sort of current is output.

I'm guessing that it's AC and that I'd need a rectifier etc to use it?

Sorry to be a pain but I really want to get to the bottom of this <img src="confused.gif" border="0"> !!

Sam

[Elwood]

Sam

The transformer that you specified is a bit under rated at 100mA current output, your fans are 100mA each and it is always a good idea to have some current to spare as current required by the fans will vary due to manufacture etc. You are right in saying that a rectifier circuit will be needed to convert the ac to dc. I still think (unless you want to build your own) that a power supply from a personal computer would be the best one box solution, and cheap at about £10-20. This will give you 12 volts dc with plenty of power to spare, it also has 5 volts dc so you could even wire the fans so you can switch to a slower speed for a 'quiet mode'. It is just a case of plug 240v ac into it and out comes 12 volts dc (just be aware that PC power supplies often have a 110v\240v selector switch, so make sure it is switched to 240v, it probably will be if purchased in the UK). Try <a href="http://www.ebuyer.com" target="_blank">www.ebuyer.com</a> , they have 2 power supplies for less than £20, you will find them under computer cases.

Cheers

Jonno

[samwiley]

Thanks jonno

You've been a great help, I'll try and get the stuff you suggest over the weekend.

My knowledge on currents, transformers and rectifiers has just increased about 1000x.

Sam

[Elwood]

Sam

What you need is a 12 DC power supply capable of supplying a minimum of 2.4 Watts of power (12 volts x 0.1 Amps (100mA) = 1.2 Watts, x two fans = 2.4 Watts). A shop such as Maplin (or any other electronics shop near you, best avoid Curry's Comet etc)will be able to supply you with a plug in (to your mains wall socket) power supply for around £10. You will have to wire your two fans in parallel (connect + on power supply to + on fan 1 and then to + on fan 2 and repeat with -)to the power supply, noting polarity as mentioned earlier. This should get both fans running, as mentioned before you can't wire the fans up the wrong way round, they just won't work so well. Just don't connect + to - at any time!

Hope this helps, good luck

Jonno