help relay question again

wonka

I have a 5 vdc RSB-5 relay that operates with a 120 ohm coil. The I/O pin will not activate it but the Vdd will. The problem with the Vdd pin is that i can not use it in my could. Any more great ideas from you poeple will be great cause i am in a bind it for my senior project that do friday. I just need to be able to operate the solnoid valve.



thanks Adam

Post Edited By Moderator (IT Guy (Parallax)) : 4/17/2005 11:28:48 PM GMT

Jon Williams

The reason the pin won't work is that the coil needs about 40 mA to operate -- this is WAY beyond what a microcontroller pin can handle.· A simple buffer circuit will take care of things.· There's a great article in a past Nuts & Volts that will be very helpful.· Click on the link below.

http://www.parallax.com/dl/docs/cols/nv/vol1/col/nv6.pdf

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Jon Williams
Applications Engineer, Parallax
Dallas, TX· USA

Jim S.

Adam,
·
Jon is correct.· The best way to design it “cleanly” is to add a buffer transistor as shown in N&V.
·
You may consider another “quick and dirty” fix.· If you have a few more spare I/Os on your Stamp, you may try· paralleling them together (my guess is that 3 or 4 would do it).· Not sure if you’re sourcing or sinking the relay, but generally outputs have more drive when sinking.· Therefore, connect one side of the relay coil to +5 and connect other side to the paralleled outputs (your logic will now be “active-low”; a LOW will activate relay).
·
Also, make absolutely sure you add the back-EMF diode across the relay coil regardless which circuit you choose.
·
Jim S.

Jon Williams

Please don't do that ... if you didn't already know about buffering high-current devices, then programming multiple ports as paralleled outputs is also going to be a challenge that is likely to lead to a damaged BASIC Stamp.

Use one pin and a suitable buffer circuit to prevent damaging your BASIC Stamp.

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Jon Williams
Applications Engineer, Parallax
Dallas, TX· USA