Transistor Resistor Calculating

How would I calculate the resistor I need for a transistor? I am building a computer, and the fan controller I have isn’t powerful enough. It is built into the case, so I can’t get a more powerful one. I know you use a 1k for 5 volts, but what would you use for 12 volts? Also, what is the best transistor to use for 2 amps or so at 12 volts?

Thanks in advanced,

Using 1k automatically (or any value) can get you into trouble. The key is knowing how much power you want to control (current and voltage are both important) and also the gain of the transistor. For example, I recently made the same assumption, and then discovered that the transistors (2n5550) were getting very hot, although they were only carrying 425 mA, much less than their rated 600mA. Turns out that I wasn't driving the transistors into saturation, dropping the base resistor to around 500 was what I had to do. 470Ohms is what I had in the bins, so that's what I selected...

If you are using the transistor as a switch (saturating):

Otherwise, the more complex:

I'm not sure if I understand fully. I just want to use it to basicly amplify the current I can use with the controller. The controller is only raded for like .5 amps, and I need closer to 2. I need to know what resister I would need to use the transistor with a 12 volt base. I also need to know what the best transistor is to use with 2 amp loads.

You need to select a transistor that will carry the current you need. Look at the data sheet and note the gain, that is the ratio of base current to load current. So if you have a power transistor that will take say 3 amps with a gain of 100 then you will have to pass about 30 mA into the base to pass that load of 3 amp. Usually you add a bit more current through the base to make sure the transistor is fully turned on. So if you are using 12v supply the resistor for the base will be roughly 12v/.03a = 400ohm. That is for switching, if you will be varying the speed then a larger heat sink will be required for the transistor. If you only want to switch on and off then a power mosfet would be a better choice. Also a darlington configuration would be better at that level of current.

Why a darlington?

And it is a speed controller, so I need it to be adjustible.

Then you should manage the speed via PWM (which basically turns it off and on very quickly to control the speed, rather than try to feed it a varying amount of power). Since it will be on or off (in terms of power) it still uses the transistor like a switch.

A darlington is where two transistors are used, with the first one feeding the second one.

Since it's a ratio of how much you want to control vs. how much you apply to the base, at high currents, it makes sense to use a darlington arrangement, because then the amount of current you need to apply to the base is dramatically smaller. Logic devices like Arduino can only supply a certain amount of current, it's best to use as little as possible to get the job done.

You can but transistors already set up, and you use them just like any other transistor, they just have HUGE gains. TIP-120 is a common Darlington power transistor.. they tend to be fairly cheap and easy to find. The gain on TIP-120 is typically a MINIMUM of 1000.

It might be a fairly good choice for your project, they are available at almost any electronics store, even Radio Shack.

Well, I have no clue what the fan controller uses. The case is a NZXT Phantom, with a built in fan controller. I don't even have the case yet, and if I did, I don't have an o-scope to check the output. I assume it's true analog, because I think it uses slider pots.

The bottom line is that the fan controller goes 0-12 volts, at a max of 20 watts.

I need to up that to 0-12 volts at about 2 amps. I have no clue what watts that is.

I need to know how to hook up a transistor to the fan controller, so that the voltage will be the same, but the amperage is higher.

Ok mebbe you should do a bit of research on the basics of transistors and components :)

On the other hand it's not rocket science either !!

I reckon for speed control of a fan ( 2 amps) just use a power mosfet, which are really cheap on eBay and control that with the Arduino PWM output. AFAIK there are schematics in the tutorials to do just that.

You could use the direct output of a digital port to drive the mosfet since very little current is required to switch the mosfet. Really a better way to do what you want. Check this link below to see what I mean about the mosfet. or search for some there.

Cheers ........ Blakus


I need to up that to 0-12 volts at about 2 amps. I have no clue what watts that is.

Roughly: power = 12 volts * 2 amp = 24 watts

Choose the next standard value above that.