Controlling voltage to a load with a single battery..

Thread Starter

arkadelic

Joined Jan 23, 2016
2
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!
 

GopherT

Joined Nov 23, 2012
8,012
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!
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.

image.jpg
 

Thread Starter

arkadelic

Joined Jan 23, 2016
2
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.

View attachment 99283
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?
 

crutschow

Joined Mar 14, 2008
24,327
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?
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.
 

hp1729

Joined Nov 23, 2015
2,304
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?
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.
 
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