Ampege for driving fans

connect 2 fans in series

I am always trying to learn about electronics but two things always bog me down 1 the math im terrible at it, 2 i have dificulty understanding the relationship between amps and volts and watts.

So please bear with me and my possibly dumb questions.

I have several fans scavanged froman old pc they have different info on them as to their power characteristics which i will list below,

Fan 1 DC 12 V 1.4W
Fan 2 DC 12V 0.15 A
Fan 3 DC 12V 0.32 A This is just what is written on the fans Am i rightin thinking that i could work out the amp rating of the fan marked 1.4 w by testing its resistance and then doing V/R = I

Now my problem is i now have a laptop and wish to use the fans to make a laptop cooler, i have a spare mains transformer which can out put from 1.5 V to 12 V at 1200Ma (max). What i need to know is this, can this transformer be used to run one or more of these fans. i have a hunch that i can if i wire the fans in either series or paralel (to divide the amperage?) but im not sure, one problem is im not sure of the units involved I.E is 0.15 A the same as 1500Ma.

I do have some experiance making circuits but mostly from just following circuit diagrams, and i have read a number of books on electronics forrest m mims etc. but ive never been able to really discuss what ive read with anybody to see if ive understood it correctly.

If a device (in the above case a fan), says 12v 0.15A dous that mean, It needs 12volts available to run at its top speed,(maximum power watts?) and that it draws 0.15 of an Ampere (15% of an Ampere?). What implications does this have for the device supplying the power, say a transformer rated as the above or say, a nine volt battery. So if 0.15A is more than my transformer is rated for what is happening in the transformer, is the fan basically over pulling juice from the transformer or battery for instance, is this even the right way to think about it. I am really really bad at math so please take this into acount if answering my question, like if you were explaining to a very small child

Also im a sculptor and i want o make a peace which has a slowly pulsing light in it , An LED light would be perfect. I know there are ways to do this using the way a capacitor discharges and im sure i ussed to have abook somewhere with acircuit diagram in it i could use, i would really apreciate it if somone could help me with this as well.

Cheers.

slowing down a dc 12v 1.4w pc fan

Hi,

Only one question then


Wattage is volts x current , so 12v x .15A = 1.8w

0.15A is 150ma - 1000ma = 1Amp

Your 1200ma ( 1.2A ) transformer will handle all three fans PROVIDING if has a rectifier fitted to convert the AC to DC -??

By variable output voltages, do you mean different taps on the transformer or an adjustable dc voltage regulator ?

( when you say transformer - do you mean one of the encased wall socket plug in types ? )

The fans would normally be connected in parallel, if you connect 2 fans in series they will run at half speed.

The fans rating are at the max voltage of 12v and then take the stated current, although this can go a bit higher during start up.


If your transformer was so badly under rated it would get very hot, and fuses should be blowing to save it going up in flames.


You can buy leds with built in flashers at Maplins, ( in the high street of most large towns ) they do various sizes and colours, you just need a battery and possibly a resistor.

can i run driving lights off an ac transformer

Thanks thats helped a lot especially clearing up that 0.XXA means XXMa.

The transofrmer is a commercially bought dc rectified unit i bought from radio shak to power a radio instead of using batterys. I think its maybe over rated as when i run the 0.15 A and 0.32 A fans off it in parralel the unit gets very hot and noticably cooler if i just run one fan off it, I know transformers all produce heat but it juts seems a little excessive so im not going to risk running both of them, in fact i have a 1.5 A transformer off an old mobile phone ).

Im still not clear on how the device drawing power effects the device supplying power. If i had a fan (for instance) rated at .5A and another at .25A is the .25A probably offering more resistance to current flow hence not drawing as much current from the supply say a transformer rated at 1A.

Also (sorry i know im going on a bit) is it always better to have a good margin of over capacity, so drive a .5A device with a 1A supply?

As for the led flashers maybe i shold have been clearer, i would like to use a LED light becouse they are small and they run cool and last longer. What i need is not a flashing light but a slow pulse I.E a slow rise in brightness followed by a slow fall. I have made plenty of flashing lights and running lights using 555 timers in the past. Oddly i find making digital circuits with IC`s easier on the brain as most circuit digrams for such are easier to understand becouse it is all so obviously logical (obviously . Its gettinng my head around analogue circuits and understanding how current voltage and power is all controlled and exploited to get a desired effect that ive always found hard.

output 12v~1200ma 14.4va

Hi,

The transofrmer is a commercially bought dc rectified unit i bought from radio shak to power a radio instead of using batterys. I think its maybe over rated as when i run the 0.15 A and 0.32 A fans off it in parralel the unit gets very hot and noticably cooler if i just run one fan off it, I know transformers all produce heat but it juts seems a little excessive so im not going to risk running both of them, in fact i have a 1.5 A transformer off an old mobile phone ).

Click to expand...

If the transformer is rated at 1200ma that is surprising - it probably has a voltage regulator of some sort and they use a metal heat sink which is designed to get warm.
If its the actual transformer that gets hot then sometihing is wrong, either the transformer rating is too high or the fans are faulty and drawing too much current.

My view of hot - touch something with the tip of your finger and you immediatelly pull it off - warm is something you can hold your finger on for 5 -10 sec.

Have you measured the actual current being used by each fan - one of then my be partially shorting and drawing way more current than its rated at.

Im still not clear on how the device drawing power effects the device supplying power. If i had a fan (for instance) rated at .5A and another at .25A is the .25A probably offering more resistance to current flow hence not drawing as much current from the supply say a transformer rated at 1A.

Click to expand...

Most power supplies are rated in VA, that is 12v at 1amp = 12va

According to your figure your transformer is 1.2A at 12v =14.4 VA.

This means it should be capable of delivering that amout of voltage and current; any demand above that then the power is reduced and overheating will occur.

So does your transformer have a VA rating ? - if it is 14.4VA then it should easily handle all three of your fans.

Your point about the different power ratings of your fans, each one will draw what power it needs from the power supply - if one overloads the power supply then the power supplys output is lowered and all fans will receive a reduced supply.

Think of it like a tap with one or more hose pipes connected .

Also (sorry i know im going on a bit) is it always better to have a good margin of over capacity, so drive a .5A device with a 1A supply?

Click to expand...

Down to personal interpretation but my transformer loading is on that same basis, to ensure the transformer runs coolish over time.

As for the led flashers maybe i shold have been clearer, i would like to use a LED light becouse they are small and they run cool and last longer. What i need is not a flashing light but a slow pulse I.E a slow rise in brightness followed by a slow fall

Click to expand...

What you need is some form of ramp generator - to slowly build up the voltage to the led.

There are a couple of ways to do this, from discrete components to little micros.
A popular way is by PWM, pluse width modulation - simply put, a stream of high and low pulses - the frequency of the pulses is changed which in turns brightens or dims the led.

The good old 555 is astable mode with a pot to alter the frequency will do just that - there are plenty of such circuits around for that on the web , plus some additional parts to make it an automatic dim and bright cycle

555 led slow pulse

Thanks that was a whole lot more help than i expected lots of good advice, especially clearing up what VA means i had been thinking of putting a post up to ask about that.

can i use 12v 1200ma instead of 12v 1a

Hi

my need is not a flashing light but a slow pulse I.E a slow rise in brightness followed by a slow fall.

Click to expand...

Here is a cheeky little 555 circuit - might still be a bit fast for your needs ?

http://www.youtube.com/watch?v=_Blg8T0DWtU&feature=response_watch

This circuit shows the basic method but needs addditional parts to make it automatic up and down.
http://www.reuk.co.uk/LED-Dimmer-Circuit.htm

fan label current different from actual current

Thanks i will just have to find all the bits now

fan current voltage amperage resistance

One more comment:

In most cases you cannot measure the resistance across the wires of a fan to calculate how much current it will draw from a given voltage. Ohms law will not apply to almost all small fins.

The reason:
Small fans normally use transistor oscillator circuits to sequence the current through the motor coils. Measuring across the wires just tells you the resistance of the oscillator circuit and doesn't really tell you much about the power drawn when the oscillator is running.

If you have a faulty fan which is no longer of use, remove the label from the rotor and beneath it you will find a circlip which holds the rotor to the shaft, break it off or cut through it and pull the rotor out of the frame. Inside you will see a printed circuit board with one or more transistors and possibly a hall effect device (magetic sensor).

Brian.

how long lasts 0.15a battery

Hi Orbilius

I sent you a PM yesterday - still not opened by you...

Hi Orbilius

I sent you another PM the other day..

I was away from the pc for a time. Its ok you dont need to send a copy of that book, I got a 555 circuit book through from amazon and its got plenty to go on. As for a pulsing light ive found a circuit that produces a sine wave and im going to try driving a led off it through a darlington pair. thanks for the offfer though.

It is all about electrical resistance.

If you got a 10m long copper wire. Lets say this wire is in fan as a motor coil. The resistance of wire (coil) is ... lets say 8 ohms.

So if you will apply a voltage (+ side of battery) to one side of wire and ground (- side of battery) to the other side of wire ----- potential a difference. A electrons will start to flow.

If you will apply voltage, amperage will appear .
Remember the job is done by current (amps) not by voltage. Voltage is needed to create current (amps)

And about resistance. Different fans consume different amount of amps. Bigger fans got low resistance coils, due resistance bigger amount of amps will flow = more work is done coz bigger fan blades spin harder

I(amps A) = U(voltage V) / R (resistance ohm)

You want to make a laptop cooler, then why dont you run these fans off the USB. It will give you 5Volts so the fans will run a little slower. Unless though you get power through two USB ports which theoretically can give you 10V so it will run almost at full speed.

Most USB coolers though have 5V fans which is why they run off the USB.