Resistors for fans in series?

[beakie]

Hey

I have some questions.

1) If I connect a 12V pc fan (0.12A, say) to a 7812, do I need to connect a resistor? If so, how do I work out what value of resistor?

2) I understand that the 7812 supplies only the amount of current being pulled from the rest of the circuit, but does this apply to fans?

2) Assuming the answer to the above question is that I do need a resistor... If I want to connect multiple fans in series, do I need a resistor for each or will one connected to them all do. Each fan is 12V but they each require differing amounts of current.

Thanks!
G

 

[MikeMl]

Why do you think you even need a resistor? If it is a 12V fan, hook it to 12V. DONE!

Think about it like this. Go get the 12V battery out of your car. It has a Cranking Amp Rating of ~800A. Now hook your 0.12A fan to the battery. How much current will flow through the fan, 0.12A or 800A?

 

[MrAl]

Hello there,

You also have to realize that you can not feed +12v into a 7812H regulator ic and expect to get +12v output to the fans. You need at least 14.5v on the input side to get +12v on the output side. You can then connect all the fans to the output directly without any resistors.

 

[beakie]

Why do you think you even need a resistor?

My confusion comes from working with LEDs. What says that a device like a fan in a circuit doesn't need a resistor when an LED does? What is it about the diode that makes it require a resistor?

 

[MikeMl]

A fan is voltage operated device.

An LED is a current operated device. The resistor limits the current to the required value.

 

[ChrisP]

If I want to connect multiple fans in series, do I need a resistor for each or will one connected to them all do. Each fan is 12V but they each require differing amounts of current.

I am sure that you realize that the fans in your illustrations are connected in parallel - not in series per your question. Additionally, connecting multiple 12VDC fans in series, should that be what you really mean to do, will cause them to each drop a portion of the circuit voltage...

 

[beakie]

I am sure that you realize that the fans in your illustrations are connected in parallel - not in series per your question.

I meant parallel. Sorry, it was late.

 

[ChrisP]

No problem - I simply wanted to remove any confusion for any newcomers who may not understand the difference.

 

[karloangel]

Hey

I also need some help about 7812, my 5v CPU fan in my laptop is not working, I've called VAIO people for replacement and it could take weeks. I have a 12v(0.5A) fan that could fit inside and I'm thinking of using 7812 to drop 19v from AC Adapter to 12v. Do i need to have capacitors or resistors connected as well?

 

[Reloadron]

The AC adapter provides 19 VDC to the unit to charge the battery and run the unit. You should have 3.3, 5, and 12 VDC available in the unit. You mention the CPU HSF assembly fan which is 5 volts? Most laptop HSF fans I have seen are 12 volt fans but maybe Sony uses 5 volt fans. While 12 volt 500 mA seems high for a simple 12 volt fan for this use, you should have plenty of 12 VDC power in the system already.

Ron

 

[koolguy]

A fan is voltage operated device.

Why.....................??