Controlling voltage to a load with a single battery..

[Arkadelic]

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!

 

[alec_t]

Welcome to ETO!
Do you want all three fans to come on at the same time with the 6V/12V drive?

Edit:
Do the fans have a tacho output?
Do they have a PWM input?

 

[4pyros]

I would call this a High/Low switch. I can think of a few ways do do this.
If you can describe what you are trying to do with the fans better, maybe we can help you to pick the best option.
I noticed you tagged arduino in this post. You you trying to control these fans from one?

 

[ronsimpson]

What is the fan current at 6 volts?
12V X 1.35A X 3 = approx. 48 watts.
6V X ?A X 3 fans = maybe 20 watts.
One option is "linear" and there will be 20 watts lost in heat.
Another option is to "switch" the fans and have little loss in heat.
....There are many ways to switch the fans. (PWM)

 

[audioguru]

The 12VDC motor in the fan might not have enough torque to start running if you reduce its supplied voltage to only 6V. That is why DC motors have their speed adjusted with Pulse-Width-Modulation (PWM). The pulses are all 12V and only their width is changed to control the motor's speed. Then the torque is high but the speed can be low.

 

[AnalogKid]

Since the switch transistors already are in place and working, the most simple method is to put a resistor in parallel with each transistor switch. When the transistor is off, the resistor limits the current through the fan to a value that guarantees starting but does not allow full speed. When the transistor comes on it shorts out the resistor and you have full power to the fan.

Read the label on each fan. Divide the voltage (12) by the current (1.35 A). In this case the result is 8.9. That is the equivalent resistance of the fan at full speed, in ohms. Placing an external resistor of the same value in series with the fan will put approximately half the 12 V across the fan, for approx. half speed. In fact, a fan at half voltage draws more than half it's normal current, so whenever you are rounding things, round down. Buy that resistor or the next smaller standard (8.2, 7.5, etc.) value in at least 5 W. This is a starting point; there are lotsa fan variations. If low speed is too low or the fan does not start reliably, decrease the resistor value.

ak

 

[Nigel Goodwin]

Since the switch transistors already are in place and working, the most simple method is to put a resistor in parallel with each transistor switch.

I completely agree with you - nice and simple, and works well - it's essentially how the heater fan in your car is controlled (if it's switched, like most are).

 

[4pyros]

If the OP is controlling the switching transistors wit an arduino he can PWM the fans in software with no additional hardware!