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How to safely jump a transistor? Answered

So I want to default the fans in this Xbox 360 I'm fixing to run at a faster speed.  Some people just borrow 12 volts from someplace else on the board and wire it directly to the fans.  I'd like to avoid this, since A) it seems like kind of a hack job, and B) I'd like the fans to run somewhat less than full bore since it's noisy, maybe 7-9 volts.  I've seen some vague mentions of either jumping or grounding transistors to achieve this, but I don't really know that much about transistors to do it safely.

This thread here says that by grounding Pin 1 of the transistor (which is on the far left I believe?) with a resistor, you can change the voltage of the fans safely.  So where should I ground to?  Or does that simply mean bridge Pin 1 and Pin 4 with a resistor?  This guy here just bridged the pin on the far right to the top pin (Pin 3 to Pin 4 I believe) with no resistors involved.  Would that method be better? And if so, how could I step down the voltage, just a simple resistor as well?

Also, I've figured out that B, E, and C refer to Base, Emitter, and Collector.  But what does that really mean in terms of practical application?  And the BCP51 transistor that is used in the Xbox has two pins labelled as "C".  I'm confused.  Here's the datasheet for the BCP51 for reference.

Anyways, any help is appreciated.


For the essentials try looking at this instructable https://www.instructables.com/id/How-electricity-and-electronics-works/

For a PNP transistor to conduct you need to make the base more negative than the emitter. this can easily be done by connecting the base to a negative supply. Without seeing the specific circuit it;s hard to guess where this may be. BUT the collector should be connected to a negative supply. .

The 2 pins labelled C are for heat sinking purposes and are the same.

As it happens, someone drew up a circuit diagram in that first thread, here's a link.  And thanks for the link to that Instructable, it was very helpful.

So I think I get it... by grounding the base with different resistors you're diverting some of the voltage being applied, and therefore you're making it more or less negative, which then affects the output voltage of the transistor, correct?

So referencing the chart made by "RBJTech" in that first link, I could ground pin 1 with a ~10K resistor and get close enough to my desired voltage.  Does his method seem sound?  I'd have jumped in already and started experimenting but I'd like to avoid damaging this Xbox since it isn't mine.

Yes more or less. transistors are current drive devices, i.e. they control the flow of electrons through the transistor.

There are 2 types PNP and the opposite NPN For PNP the base needs to be more negative than the emitter for NPN it's the other way round.

You can with reasonable safety take the base to ground through a 18,000 ohm (18K) resistor (according to the link you gave) . Electrons will flow into the base and turn the transistor on.

One thing that just occurred to me is that the Xbox would normally throttle the fans on its own (albeit very unsatisfactorily), so would it still attempt to do that even while the base is grounded? I could see that causing some issues.

So if hypothetically I were to just go for the full 12 volts, then I could just bridge the emitter and collector, thereby bypassing the transistor entirely, right? And then I wouldn't have to worry about the Xbox's own fan control interfering with my modifications, unless that proves to be a non-issue.

correct. although it is impossible to really tell you what may happen as I don't have a diagram or xbox to play with.

OK, I'll give it a shot then. This all may prove to be pointless as the darn thing keeps locking up... But thanks for the help! Much appreciated.

Another 'hack job' is to connect the fan across 5V and 12V which will give you 7V across the fan.  There are technical reasons why you shouldn't do this, but in practice it works a treat.  (But don't use it on a fan where you need to use speed sensing.)