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

[rxforspeed]

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.

In all honesty, I'll most likely build two, three, or four prototypes of this project. I've installed the air ride suspension and all the related components on the vehicle that this project will control, but I've got a couple of other friends wanting to go the digital route with their air ride suspensions and have push-button control over their ride height/psi level as well. So instead of using up my solder braid to remove the 5V relays, I'll start fresh with a new prototyping PCB (www.anykits.com #K003, the only thing I dislike about it is the single-sided copper clad) and build the new one for the "Phantom" (the vehicle I installed the air ride on-a Johnson Motor Company re-worked '88 Pontiac Firebird, Nova/Chevy II front frame clip, and custom fiberglass work to resemble an older Rolls Royce Phantom) with Melanie's suggested parts. When I placed the orders, I did happen to find a good deal on a few IRF540 N-channel MOSFETs in a T0-220 package, so I picked up a few of those in hopes to use with either the 5V relay PCB or for a different future project when the need arises (and I learn a bit more about transistors)...

I'll build the important prototype-the "Phantom's"-with Melanie's suggested parts, but I don't want to discard the other. If nothing else, I'll be able to teach myself something about transistors along the way with it. I have enough 16F690's to be able to fry one or two without worry...

Remember-I'm still a "noob" in the general area of electronics, but I'm a quick learner. Thermal curves on datasheets aren't usually something I study at the time being, but I WILL compare the ones on both of the transistor array datasheets I've ordered (the ULN2004 and the ULN2803A) and the IRF540 and 2N7000 MOSFETs.

Are you familiar with the IRF540, Mr. Martin? Regardless, I know what I need to do to figure out if it's interchangable with the 2N7000 you've recommended-download and read the datasheet. The most visited site on my notebook (and the one I download the most from) is probably www.datasheetarchive.com...

Thanks for all the input, info, and help you guys have given me, I sincerely appreciate it. It seems I've learned something new with almost every post on this topic, and of course there's always more than one way to skin a cat, SOOOO...even if I do use the board with the 5V relays it'll most likely go to a buddy of mine that only drives his air ride equipped truck for a few hours on nice, sunny weekends. I don't think that would contribute a great deal to melting the polar ice caps...

Back to "The Art of Electronics"...
-Adam Collins