Wallpack and relay and ???

jefm

This is electronics 101 stuff but someone asked me and i wanted to be sure...
Project:
A smart 110vac outlet will power up a wallpack. 12v, 500ma or whatever we can find, it's irrelevent. The wallpack runs a small relay, 1amp or less. That's basically it, the relay contacts will dis/connect another piece of equipment that i've already worked out.

Should I put a 10 ohm resistor or something in the coil circuit to reduce current usage? I just worried if the relay or wallpack will get messed up if it's doing an almost short circuit for days on end. I played around with some resistors here and can get to around 40 ohm before the relay quits contacting. Thanks for any help!

jefm

Righty-o, my righty-o's....
Going to recommend a dozen or so ohms in line. It's easy enough to figure out when the coil stalls without a meter, and i figure too little resistance will be better then none at all.

bill190

Relays for different uses have all sorts of different names and terminology. Like also solenoid·or contactor.

And relays are designed to be momentarily on, or on for long periods of time. One term used for rated to be on for long periods of time is "Continuous Duty" relay. I seem to recall other terms used, but can't think of them now.

Another set of terms they use is "definite purpose" "contactor" (relay). The idea behind this is if the part fails, it should only be replaced with the same part.

Also relays which are on for long periods of time·can be quite hot. Might want to enclose in a well ventilated fire-proof box.

Look at the "datasheet" for the relay and this will show coil current/amperage as well as the "duty cycle/rating". Here is an example...

http://www1.futureelectronics.com/doc/TYCO%20ELECTRONICS/3-1393237-1.pdf

FYI -·All about·relay contacts, "Relay Contact Life"...

http://relays.tycoelectronics.com/appnotes/app_pdfs/13c3236.pdf

Mike Green

In addition, relay coils have to provide enough force to move the contact and provide enough contact force for the application. If you limit the current to the coil, there may not be enough force for the relay to function as designed. Bottom line: Use a relay as it was designed. Drive it with the voltage it was designed for and provide the amount of current expected to the coil. Limit the voltage and current on the contacts to what's specified. In most cases, the specified values have a lot of margin, but don't cheat on that unless you know what you're doing and unless you're prepared to test the relay yourself for proper operation under the conditions you plan to use (and you understand what you're doing).