Electronic Boffs ? fan 12v supply

[normspanners]

Hi guys Im just trying to learn,
just a quick one I have a 12v fan cooling a heatsink I used a 12v coz I had several from PC power supplies .
To keep it simple I thought, the tranny the fan is running off is 22-0-22 toroidal, which is part of the amplifier, For ease I used 0-22v and half waved it with a diode and capacitor of about 10uf 50v this gives me 12v dc but end up with reduced speed silent fan . All works fine, two components, simples.
But when first started the fan can be a little noisy (I would guess feq. noise) till it all 'warms', should I have included a balance resistor parallel to the fan and bigger value cap say 47uf?
Thanks

 

[PTvor]

If it's frequency noise, caused by the half wave rectification, why should it change as the fan warms up?

If the fan has been salvaged from a computer power supply, it will have a lot of hours on it and the bearings are likely to be worn. That could explain the noise until it warms up.

What's the fan like when you run it on smooth DC, say from a battery?

 

[normspanners]

Perfect , I checked it first to make sure I had a quite one. It runs harder on 12 battery IE faster. if I run it with a 1000uf it runs smooth but at 28v about the same speedish as the battery I would guess
thanks BTW.

 

[rich r]

Stick a 'scope across it so you can see whether the noise is on the supply or due to the fan itself.

 

[PTvor]

You're supplying that fan with 50 times a second pulses of about 31V (22x1.4) which isn't the way it's designed to be run.

I'd be inclined to get a 9V three pin regulator, a quid from Maplins

http://www.maplin.co.uk/2a-positive-fixed-voltage-regulators-7939

use a lot more smoothing capacitance, and power the fan with proper DC as it was designed to run with, or take the voltage from the DC power line of the amplifier and put that through the regulator.

Or you could make up a good enough regulated supply with a resistor and zener and a power transistor.

Dropping 31V to 9V, you really ought to think about the power the regulator is dropping and add a heatsink if needed.

The other way of doing it is to put an extra small transformer (and bits to provide the 8-10V you want) in the case of the amplifier.

But it's your amplifier, so be careful.

 

[normspanners]

Yeah, i should have scoped it , I have built it up afew days ago coz it worked ok in the pre build situation. If I get my scope out, I should see some kind of saw tooth? but how do I differentiate noise?
Thanks

 

[normspanners]

You're supplying that fan with 50 times a second pulses of about 31V (22x1.4) which isn't the way it's designed to be run.

I'd be inclined to get a 9V three pin regulator, a quid from Maplins

http://www.maplin.co.uk/2a-positive-...egulators-7939

use a lot more smoothing capacitance, and power the fan with proper DC as it was designed to run with, or take the voltage from the DC power line of the amplifier and put that through the regulator.

Or you could make up a good enough regulated supply with a resistor and zener and a power transistor.

Dropping 31V to 9V, you really ought to think about the power the regulator is dropping and add a heatsink if needed.

The other way of doing it is to put an extra small transformer (and bits to provide the 8-10V you want) in the case of the amplifier.


But it's your amplifier, so be careful.

Thanks for the reply, it was just that there where some spare tracks and component holes on the board right off the torroidal supply that fitted in with my two component idea and it runs perfect once it has been switched on and run for 2mins.? nothing gets warm and I was just trying to keep it as simple as poss.

 

[PTvor]

What you could do is use the 1000 uF capacitor, which runs the fan too fast on its own, and put a resistor in series with the fan, so you get a smoother supply and the voltage drop across the fan is such as to run it fast enough to blow, but not fast enough to make a noise.

 

[normspanners]

What you could do is use the 1000 uF capacitor, which runs the fan too fast on its own, and put a resistor in series with the fan, so you get a smoother supply and the voltage drop across the fan is such as to run it fast enough to blow, but not fast enough to make a noise.

Thanks.
I did try that but the resistor got VERY hot.
I may be over concerned about the situation.
I had thought about putting a larger cap and a resistor in parallel (easy job!) with the fan to give a more ballanced supply even though the fan is a resistor (it is not a steady resistance?) my thinking was so that the capacitor would smooth the pulses and the fixed resistor would help drop the voltage? but steady the ripple? as I say I am just trying to educate my self (a little) with possibilities.

 

[PTvor]

The fan will probably take 100mA and the series resistor would have something like 20V across it. So the resistor would dissiapte 20x.1 = 2W. So, if you were using a usual 0.5W resistor, it would get hot. You'd have to use a couple of 2W resistors or a single resistor of well over 2W rating. I don't claim the sums are strictly accurate because you are not dealing with DC or AC, but they should be a rough guide. You'd have to experiment with resistor values to get the fan running fast enough but not too noisily.

Ordinary meters aren't that much use when you are looking at a half-wave rectified voltage, and it would be worth using a scope if you have one.

I don't see that putting a resistor across the fan would be of any benefit.

However, if it's working apart from a noise for a couple of minutes when the fan starts, and the circuit is simple and convenient, why not leave it?

 

[Llama]

Your measured voltage with a meter will be the average, if you know the waveform it's pretty easy to work out the max/rms from there.

 

[normspanners]

The fan will probably take 100mA and the series resistor would have something like 20V across it. So the resistor would dissiapte 20x.1 = 2W. So, if you were using a usual 0.5W resistor, it would get hot. You'd have to use a couple of 2W resistors or a single resistor of well over 2W rating. I don't claim the sums are strictly accurate because you are not dealing with DC or AC, but they should be a rough guide. You'd have to experiment with resistor values to get the fan running fast enough but not too noisily.

Ordinary meters aren't that much use when you are looking at a half-wave rectified voltage, and it would be worth using a scope if you have one.

I don't see that putting a resistor across the fan would be of any benefit.

However, if it's working apart from a noise for a couple of minutes when the fan starts, and the circuit is simple and convenient, why not leave it?

Thank you, Of course I hadnt thought about the actual voltage / wattage that the resistor was dissipating also I didnt want to possibly pull one side of the tranny down which might effect the performance just to run a little fan, yes I have a scope but I would need to get it out, I wasnt sure what I would be looking for as regards a noisy signal, I suppose I could scope it then look for a pattern change as the noise reduces?
I totally agree re letting it work as is, it has run for days now with no ill effects no heat and eating hardly any power, I was just curious as to why it was a little noisy, and to be honest it is just a tiny amount, more of a slight rattle?
But I like to try and get things working as well as possible, even if it is in my head so that I can learn from my experience and ideas.
It is with help from the vast knowledge base on this platform that other people can learn, I had a friend/ mentor who has since died who was a truck driver interested in electronics and taught himself to repair televisions and ended up fixing them for all the 'repair' shops in the area, even the main dealers with all there back up ! I learnt a lot from him and him from me over car repairs.

 

[dannyp]

for the coupleof quid they probaly are id be tempted to get a 24v fan, hate seeing power /energy been dumped to reduce the voltage.

or a DC-DC converter module much nicer solution imo

 

[normspanners]

for the coupleof quid they probaly are id be tempted to get a 24v fan, hate seeing power /energy been dumped to reduce the voltage.

or a DC-DC converter module much nicer solution imo

Yes you are right but I would have had to go for at least a 30v to get it virtually silent, I had the fan sat there and as said there was space on the board for the two components also I wanted to try and JUST DO IT! if ya catch my drift.

 

[dannyp]

aye know whay you mean, do the same sort of thing myself often enough

 

[normspanners]

If the fan has been salvaged from a computer power supply, it will have a lot of hours on it and the bearings are likely to be worn.

Just an update for those that helped (Thanks again BTW) the system was running yesterday all day fine then for no reason the fan started making a rattle'y noise for no apparent reason I gave the speaker a slap with my hand (I have mounted the amp inside one of my speakers to make them active its 400W X 2 with the heat sink at the top, hence, I feel, the need for the fan) and the noise immediately stopped, I have now run it all night and is still silent, so it does look like it may be just a slightly dry bearing rearing its head.