I'm having a little problem getting enough current to switch my relay off the 3.3v source I have (normally 14ma). I have an NPN transistor of Hfe 100 but I can't seem to get any amplification.....here's what I have:



At point a I measure 3.3v, 1ma, like I want. At point b I'm still only getting 3.3v, 14ma. Now, this is my first time ever working with transistors, but I've modeled mine after a tutorial and it seems like I should be getting approx 100ma. Anyone have any ideas where my problem is? The model I'm using says using the same 3.3v source is ok, but could that be my problem? Thanks.

I'll be able to help, but I'll need some more info. May I ask exactly what your trying to do (in the grand scheme of things?) even a block diagram is ok.....

I'm using the XIR kit to have remote hard drive switching. I have the relay wired to switch between powering the two hard drives and that works fine when I use something with higher current like a 9v battery. I have the XIR installed and have isolated pin 17 and ground so that when I hit select I have 3.3v between the two and when I hit display I have 0v. However, I've only got 14ma off that line, which isn't enough to switch the relay, so I'm simply trying to amplify the current of that line. Thanks.

if your existing source is only 14mA, then your plum outta luck.... transistors can amplify your current, but they cant exceede the original limitations. In that respect, your not going to get more than your original current from that transistor output, no matter how hard you try.....

What you can do in this case is to wire in a small powerpack and hookup to a Voltage regulator... you should have room in your box for these, and since they'll be spliced in before the power supply, it will give you a nice healthy 500mA -> 1A of current to work with at all times, even if the XBOX is powered off.

Hmmm, alright, thanks for the help. I already have an atx psu, so there's plenty of power, but I'll have to look into a voltage regulator. What about using the transistor as a switch for a higher current source? Like having the 3.3v 14ma go connected to the base so that when it's 3.3v it allows the connection between the collector and emitter, but when it's 0v it won't flow? I've found out a little about this approach when researching the amplification, but don't know if I'll run into any hurdles with that as well.

this is a better situation for vona to figure out than me...

lol, making your apartment track your every move is easy, but this is complicated stuff.

jk, thanks for looking.

ok, but the question still stands as to where you are going to get this higher current source from?

Im assuming that the 3.3V your using is the standby power, right? If thats the case at the time when the system is powered off, all the other more convenient power sources are off anyways..... which is why the only other way your going to get anything is with an outside powerpack/regulator....

Now, on the opposite side of the fence:

if you do want to switch your HDD's when the power is on (which i really recommend against, but thats just me) then its real easy.

put the 3.3V source into the base of the transistor (resistor not really needed)
connect the collector to your new power source (enough to power relay)
connect the emitter to your relay.

Now, you have to understand that you wont have the full 3.3V coming into the relay, as the Vdrop is about .65V so into the relay will be about 2.85V or so. and when the base is 0V, the relay gets 0V.

Yes, the 3.3v is from the standby. However, as soon as the select button is pressed, pin 17 immediately jumps to 3.3v....meanwhile, the xbox is just starting to load the bios....if the relay switch is almost instantaneous, would there be a problem not having it actually change til the power goes on? It seems like the relay would switch as soon as the box turned on, so there wouldn't be a problem. If I were to use the transistor as a switch, I would use either a 5v or 12v source from the atx psu into the collector for the relay.

Edit: sorry, posted this before your second post showed up.

Well, if thats the way your going to be using it, then try hooking up the collector of the transistor into the 5V supply (12V is overkill, especially since the transistor itself will only output 3.3V - 0.6V and see if that works ok for you.



EDIT: added diagram.