Quote Originally Posted by mackrackit View Post
What is the coil rating is amps of the relay coil? If not known then what is the resistance?
If the amperage is less than 20ma then there is know problem driving it from the PIC as long as you have a fly-back diode installed. You need the fly-back for the transistor too.

The above info is needed even if you use a transistor.

Simplify things a bit. What is the amperage if the valves that the relays will be controlling?
Then we pick a MOSFET to drive the device directly doing away with the relay and transistor.
Omron's datasheet for the relay specifies "360mW coil consumption (max)", and the resistance of the coil averaged around 67-Ohms for the four relays I have mounted on the PCB. The amp rating printed on the top of the relays is 10A at 250VAC (N.O.).

Here's a link to the same valves we're using: http://www.coastpneumatics.net/displ...VXD2130-03N-6G
They're the NC 12VDC VXD21XX parts. A link to the PDF datasheet is listed on that page...

Even that datasheet for the valves doesn't give a lot of information, and they're not in front of me at the present time to check with my Fluke meter. I found a 6W holding power consumption rating in the datasheet for the style of valve we're using; that would put the amperage near 0.5A, wouldn't it (with a 12VDC automotive electrical system)? That would still be over the PIC's 20mA output, so I'm assuming that I would need a MOSFET (or transistor array IC, transistor of another kind, etc.)...

Quote Originally Posted by Melanie View Post
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.
I do have a 12V line on the PCB; it supplies power to the 5V regulator (7805's are the only dedicated 5V regulators I have on-hand, but future versions may incorporate either an LM2931T-5.0 or LM2940CT-5.0 automotive 5V regulator) and will be the tie point for the incoming switchd ignition hot power wire.

I don't have a problem ordering a few 12VDC relays, I had just foolishly bought about fifty of the 5VDC versions for this specific purpose. I wish I would've found this forum before placing that order...

The ULN2004 you mentioned-is it similar to the Motorola ULN2068B (quad high-current Darlington array) or the TI ULN2803A (eight NPN Darlington transistor array)? I don't have any of those on-hand, but I can order whichever would be the best for my project with the 12V relays, most likely a quad array because I really only have room for four relays on the prototyping PCB this circuit is being built on...

I can see why you've recommended what you have-mostly for protection of the PIC and it's A/D Vref (which will be connected to the 5V rail through a resistor-a value yet undetermined-that will reduce the voltage to 1.525V for greater accuracy within the psi range we'll be working with-100psi and less). It makes sense, and I can (for the most part) comprehend and picture it's lay-out. This method would still require a CMOS "high" output from the PIC to the base of the transistor (or array), correct? That would make the transistor act as a switch, grounding the collector side of the transistor (which is connected to the 12V relay coil) through the emitter when the CMOS "high" is applied to its' base from the PIC's output pin. I still get confused with "anode" and "cathode" terminology for diodes (but not LED's), so the +12 would connect to the striped end of the diode (to block +12V from passing through to the other side) and the opposite end without the stripe would connect to the negative (0V) side of the relay coil (for the same purposes, only with negative current), right?

I do believe I'll order a transistor array-that way there's no confusion as to which lead is which and I can make sure I get the proper datasheet from the same manufacturer. I'll need four 12VDC "sugar cube" PCB mounted relays, a quad NPN transistor array (like the ULN2004 or the ULN2068B), and I'm assuming it would be OK to leave the 1N4001's in place as long as they're connected correctly, right? I do believe I'm going to order a good "transistor tester", like this one from www.electronickits.com: http://www.electronickits.com/kit/co...eas/dt100k.htm