# Controlling voltage to a load with a single battery..

Discussion in 'The Projects Forum' started by arkadelic, Jan 23, 2016.

Jan 23, 2016
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Seems like a simple question, I have several PC fans (12 V, 1.35 A), all in separate loops and powered by a 12 v battery. The fans are wired to transistors being used as switches. My issue is that I don't want the transistor to supply 12 V of power, I want to immediately and always supply the fans with a certain voltage below 12 V (e.g. 6 V) and then when my button is pressed, it activates the transistor, which allows the other 6 V to the fan, allowing it to reach full capacity. I'm not much for electrical engineering so excuse my lack of knowledge, hope I can get an answer soon!

2. ### GopherT AAC Fanatic!

Nov 23, 2012
6,297
4,019
You need a diode from your 6v supply which will prevent current from flowing into the 6V supply if a higher voltage is present. Put a switch on your 12 v supply. If the 12 v requires high current, let me know, I can add a transisitor to that so you can use a small (low current) switch.

Jan 23, 2016
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This is exactly what I was looking for thank you! The 12 V battery will also be supplying the 6 V, so if I place a resistor before the diode that reduces the voltage from 12 to 6 will this idea still work?

4. ### Dr.killjoy Well-Known Member

Apr 28, 2013
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What kind of fans are these ??

5. ### crutschow Expert

Mar 14, 2008
13,469
3,356
No.
Put a resistor to ground in series with the fan connected to 12V to get the lower voltage across the fan.
Connect the collector of the transistor to the junction of the resistor and the fan to connect the fan directly to ground for 12V when the transistor is ON.

6. ### hp1729 Well-Known Member

Nov 23, 2015
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How much current does the fan draw at 6 Volts? Ball park estimates you are looking at about a 4 Ohm to 8 Ohm 10 Watt (or more) resistor. I have ran 12 Volt 2 or 3 wire CPU fans at 9 V before with no problems, but never tried 6 Volts.