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Running a 5vDC fan on 9vDC

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moody07747

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I'm making a mini guitar amp, (AKA the Ruby amp) and want to put a small fan in the case to add a little style.

http://www.runoffgroove.com/ruby.html

Anyways I'm running the circuit at 9VDC and the small 25mm fan I have runs at just 5VDC

Now I know I have to use the ohms law calculation and I'm great at it......When the number of amps is at or over 1...

So I'm just wondering what rating resistor I should use to get this fan running at the correct voltage

Also, would it be safe running it at 9VDC?
The most this amp would be running at one time is around 1 1/2 hours.


Power supply:
9VDC, 800mA

Fan:
5VDC 0.3W

Fan link:
http://www.allelectronics.com/cgi-bin/item/CF-248/search/25MM_5VDC_COOLING_FAN_.html
 
hi,

As the fan is rated a 5Vdc 0.3W.

I= 0.3/5 = .06A

You want to drop 4Volts so, 4/.06= 66R, nearest preferred value is 68R.

Wattage of 68R, 4V * 0.06 = 0.24W, I would recommend at least a 0.5W.

Dont run the 5V motor from 9Vdc, it will generate about 1W and overheat!
 
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The fan needs a high current to start running because it is stalled. A simple series resistor limits the current then it might not start.
Add an electrolytic capacior across the resistor so the fan gets its charging current for starting.
 
hi,
If you add the electrolytic cap as suggested by 'agu',
make sure you connect the '+' positive of the cap to the +9v end of the resistor and the '-' to the motor end of the resistor.
 
unless... it can cope running at the higher speed ;)
V ~ speed
I ~ torque

Torque would of increased (cube law and bearings)
speed as more then doubled
 
My parts just shipped out today and will be here in three days.

My local radio shack said the MPF102 FET and resistor I needed were in stock at the store but after looking they seemed to be, "missing at the time".....
I'll try another shack tomorrow.
If all else fails I'll just order them online.

Any general stats for that cap?
I have a few in the electronics bin that I can try. Thanks for that info.
 
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hi dave,
>> Any general stats for that cap?
I have a few in the electronics bin that I can try.


Try without first, then try 10uF[16Vwkg], then increase the value until you get a consistent motor start at switch ON.
 
Ok I have a problem...The power is getting to the circuit but the fan won't turn on.
I have tried with the resistor and the 10uF/50V, 22uF/50V, 33uF/50V caps.

Now my understanding is this cap will give the fan a surge of current to get it spinning but without it I should be able to spin the fan by hand and it should run after that..The caps just for the start up.
however when I powered on and flicked the fan I got nothing at all....no movement or signs that it wanted to.

I have another problem with the power jack shorting right now so I'll be running a meter over the circuit tomorrow and do some tests to see whats really going on.


edit
I got the power jack working and with the meter I read around 10VDC at the motor leads after the 68 ohm resistor and a 10uF 50V capacitor....same reading with that bypassed.

Before I had shorted a (+) lead to ground in the circuit unknowingly with my metal power jack set to outer (+) and the case it was mounted on was ground so I'm not sure if I blew anything up but I cant get any sound out of the amp now.
I've replaced the FET and amp chip as I have a few more but still now sound...everything looks find in the circuit as well.
I will have to trace it out with an audio probe some time today.
 
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The motor should be connected only after the capacitor has charged up. If the motor is across the capacitor at switch on then the capacitor won't be able to get charged as the motor is shorting it out.

Have you tried using a low current bulb is series with the motor?
A 60mA indicator bulb like**broken link removed** or **broken link removed** may do the trick. They are available in 6V and 12V versions. It may need some experimenting with different bulbs. Normal torch bulbs will be too high current.
Maybe a 12V 40mA ?
 
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It could be that the fan you have has an electronic controller onboard which is rated at 5 volts. By running it through the resistor you are still presenting it with a 9 volt supply. As the electronics control the motor speed (there is normally a small hall effect switch in the motor and a controller) there is a very very low current draw until the controller starts the motor fields.

At the best it will work, at the worst you will have fried your controller.

A much better solution is to use a L7805 regulator - anything from 0.1 to 1amp at 5 volt will do.
 
You measure 10VDC across a 5V fan (that doesn't run) that is in series with 68 ohms resistor.
Then the motor is burnt out.

The amp is also burnt out maybe because the polarity of the 10V is backwards.
 
audioguru said:
You measure 10VDC across a 5V fan (that doesn't run) that is in series with 68 ohms resistor.
Then the motor is burnt out.

The amp is also burnt out maybe because the polarity of the 10V is backwards.
Not necessarily

If you have a fan with onboard controller it may only take 0.5ma when its powered up - until it decides to turn on the fan motor windings (some of them have onboard speed controllers) then you're not going to be taking much/any current hence no voltage drop across the resistor.

If the controller sees 9-10 volts instead of the expected 5 then it might refuse to do anything and keep itself turned off (or burn itself out - try applying 10 volts to a Pic Microcontroller :p )
 
As of now the fan is just there for looks and I could really care less about it.
I'm more into getting the amp circuit running now...........however this fan would be nice to get working....

Thers only POTs, resistors, caps, a FET, and a LM386 chip in this circuit.
replacing the FET and LM386 leaves just a few caps or resistors to be burned and they don't seem to be at all.

Last time I turned the amp on I unhooked the fan.

picbits said:
Not necessarily

If you have a fan with onboard controller it may only take 0.5ma when its powered up - until it decides to turn on the fan motor windings (some of them have onboard speed controllers) then you're not going to be taking much/any current hence no voltage drop across the resistor.

If the controller sees 9-10 volts instead of the expected 5 then it might refuse to do anything and keep itself turned off (or burn itself out - try applying 10 volts to a Pic Microcontroller :p )

Yea I figured the fan was just shutting down due to the high voltage.

It looks like ill need a regulator like the L7805
 
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The circuit for the 7805 regulator is correct but don't use it for the LM386 amplifier because the power output will be very low with a 5V supply. The power with some clipping is 835mW (not very much) with a 10V supply into an 8 ohm speaker. With a 5V supply the power output is only 175mW.
 
With the switch and wiring I have in this amp I will be able to mound the components on the board where the amp is now but will be able to feed 9V to the amp/LED and use the 5V source to the fan with just one switch.

I'll stop by IEI and radio shack later to see if they have any 7805 chips and if not ill make another order out to Jameco.

I'll also work on making an audio probe to find the problem with the audio section of the amp.
 
Connect the fan to the 5V output of the 7805 voltage regulator.
Connect the power source to one terminal of the switch and connect both the amplifier and the input to the regulator to the other terminal of the switch.
Connect the 0V of the power source, the anplifier, the voltage regulator and the fan together.
Then when you turn on the switch both the amplifier and the regulator/fan will get power.
 
You may be able to get away with one of the to-92 package regulators if you put it in the airflow path of the fan. But you may not need to since it's only 60mA and those packages can do up to 100mA i believe.
 
OK update time.
I got out the audio probe to trace the audio line and think i found the problem with that...bad solder joint on the transistor socket.

Anyways I also got out my power supply and fed 5VDC to the fan and its still working fine so that's not blown up.

Radio shack and the other stores around here didn't have the 7805 so I'll make another order in a few days and start that build.
 
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