ok, i need somethign that when a TTL 5v 5ma signal is recieved, will switch a molex's 5v line and 12v line. I was wondering weather 2 NPN tips with their bases joined together would do this? so collector of one goes to +5v and collector of the other goes to +12v and emitters going to output 5v and output 12v, then the bases joined together and when they get the 5v signal, out goes the power. Another way might be to switch the groung perhaps, so usnig a PNP transistor / tip have it switch the ground, but i dont think that is ver professional. Any ideas?
First, if you decide to go with bipolar transistors, you will have to put a current limiting resistor (approx. 1 K) in series with each transistor's base or you'll toast them. Second, because you're switching on the "high side" (the positive rail), you'll have to use PNP transistors, with the emitter connected to the positive rail and the collector going to the load. And third, because PNP transistors turn "on" when you apply 0 volts and off when you apply a positive voltage to their base, you'll need another inverter transistor. Also, +5 V input might not even be enough to block the 12 V rail switching transistor... Maybe if you give more details about the load that you are trying to switch, you'll get better answers. BUT: I would recommend a P-Channel MOSFET transistor as a switch (like IRF 9530. Switches up to 10 Amps or more, has a low Rds-on resistance (which is good, it means a low voltage drop across the transistor)
You're going to lose at least 200mV across each transistor with bipolars, can you afford that? And if you want to switch high currents with a logic signal, you'll need a 2-stage switch or a power darlington (which will lose twice that) or a logic-level mosfet. What current do you want to switch?
ok, i am bidding on some 1amp 30v dc DPDT relays that operate on 4.5V dc, so i will use a BC327 transistor for that! (and a 1n4001 diode - of course) thanks anyway!
