Transistor Selection for Switching Relay Coil-with a little bit of info on my "ARSC"

[Pic_User]

Hi Adam,

....what transistor would be best to use in my case and how do I wire up the collector, base, and emitter of said transistor for use as "driving" a relay-installed ...

2N3904, 2N4401, 2N2222, PN2222 are suitable to actuate a small relay.

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... I do have quite a few 2N3904's, 2N3905's, 2N3906's, and a great deal more of what some would consider "common" transistors. Some of the more common ones should have the same pin-outs regardless of the brand, shouldn't they? I do have at least one manufacturer's datasheet saved for almost each transistor I have, but most of them don't have the manufacturer's logo or name printed on their T0-92 cases...

Transistors of the same number almost always have similar pin placements, not always!

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....Also, if anybody knows any good tips or tricks on how to determine whether a transistor is an NPN or PNP style (or even determining/deciphering whether the transistor in question is bi-polar, an FET, BJT, Darlington, etc.) and how to figure out which leads are the collector, base, and emitter (that's a definite need for the cheap off-brand assortment of transistors I have on-hand), I would greatly appreciate any of them. ....
Sincerely,
-Adam Collins

The best way to tell the type of transistor, is the transistor number. The best way to tell the “pin-out” is the data sheet.

My personal choice, at the tinkering stage, would be to use a plain old 2N7000 MOSFET to drive each relay (with flywheel diode, of course).

Multiple device drivers are handy and certainly have their place, but they also have some inherent limitations, one of which is the total power dissipation available in the DIP package. For example, see the thermal curves for the ULN2004.

True, plain old 2N7000 MOSFET, or plain old BS170 MOSFET is a good choice if you have them on hand.
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I wouldn't have any Relays on the PICs 5v Rail if I could possibly avoid it.

Have a 12v Relay on the 12v line if you have one available, use any medium power NPN Transistor (Emitter to 0v, Collector to Relay). The other end of the Relay is connected to +12. Drive the Transistor Base from the PIC's I/O via a suitable current limiting Resistor (value depends on selected Transistor). You will still need a Diode reverse connected across the Relay coil (Kathode to +12, Anode to Transistor Collector).

If you are driving multiple Relays, it's less hassle to put in something like a ULN2004. No Resistors needed, you can connect it directly to your PIC, and the suppression Diodes are built-in.

If you are using ADC or Comparators, and if you are using the 5v rail as VRef, put NOTHING on your PICs 5v line that will unnecessarily load it, spike it, or in any way abuse it - and a Power Relay will.

The ULN2004 is a real space saver for multiple relays.
-Adam-