# Relay suggestion for use w/ microcontroller...

Joined Feb 9, 2012
122
Hi all.. I'm thinking about working on a project that uses a microcontroller to run a motor that needs 12VDC at 1A max for about 30 seconds twice daily and will be in-use outside (it's for a pet door). The microcontroller I'm planning to use is a TI MSP430 based device (not sure of the exact part quite yet). In doing some reading earlier on this forum and elsewhere I gather I need to watch the coil resistance to ensure I don't overstress the μc's output port(s). Whatever relay is chosen, I'm leaning towards a type that will take the weather pretty well (meaning sealed from external exposure as much as possible).. I was thinking of a reed switch but cost is an issue as I'd like to keep it as low as possible. Any suggestions?

P.S. in poking around on mouser.com I see that most reed relays that I believe meet my criteria are about $10 or higher per relay. Hopefully I can find something for much less...? Also -- if a relay is indicating a contact rating in AC volts only does that mean it's not usable for DC? I'd think it would work but the rating is just different -- is that a valid assumption? Thanks!! #### mhastie1234 Joined Feb 10, 2012 29 You could use a npn transistor as a switch and use pretty much any relay you wish. #### Attachments • 4.2 KB Views: 25 Thread Starter #### osx-addict Joined Feb 9, 2012 122 Thanks.. I'll keep that in mind.. Now I've just got to fire up Eaglecad and remember how to start laying out my schematic.. #### SgtWookie Joined Jul 17, 2007 22,221 Why don't you use automotive relays? They're fairly inexpensive, and are designed to work under automotive conditions; automotive environments are among the worst on the planet. Use a couple of single-pole, double throw relays. #### panic mode Joined Oct 10, 2011 1,877 this is$1 and rated about 10A, sold by piece and there is plenty of stock:
http://ca.mouser.com/ProductDetail/Song-Chuan/833H-1C-C-12VDC/?qs=sGAEpiMZZMt9EC5K82p3yqvjYGeuH8QZ

you are right about rating, AC rated relays have much lower (if any) DC rating. problem here is that DC burns contacts more than AC. similarly there are many relays (automotive for example) that only have DC rating. problem here is usually compact form which limits voltage rating. there are some really small cube relays rated amazing 30A or so but only up to 16VDC (check DG20 from Durakool for example). The best way is to read the datasheet and evaluate rating for your application. Just because relay is rated 10A, it does not mean that it will survive reasonably long 1A current if load is inductive and running on sufficiently high DC voltage.

for example, relay in link above is rated "10A" but that is at 277VAC, it can handle 15A at 125VAC or 7A at up to 30VDC according to UL/cUL. TUV rating is only 7A at 250VAC and 12A at 125VAC. As you can see limits can and do vary.

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