Relay question

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Old 06-13-09, 01:07 PM
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Relay question

The scenario is a relay that has the coil side energized 99+% of the time. The coil can be energized with either low voltage DC or 120vac, if it makes any difference in the answer. The contact side is only used as a dry contact. No current ever goes thru the contacts. The contacts are gold flashed. The relay is enclosed, but not in a temp/humidity controlled environment.

If I only care about a state change, i.e., I don't care whether the contacts go open or go close when the coil is de-energized, would a NO or a NC relay be preferable?

I wonder about things like the contacts corroding/oxidizing/sticking(?) if they are in the same position most of the time.

It's not a critical situation where failsafe considerations need to be considered.

Would it be a different answer if there is low voltage DC going thru the contacts?

Thanks,
Ira
 
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Old 06-13-09, 05:01 PM
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Originally Posted by ibroussard View Post
If I only care about a state change, i.e., I don't care whether the contacts go open or go close when the coil is de-energized, would a NO or a NC relay be preferable?
Doesn't matter since you aren't concerned with a failsafe design. Most relays have at least one set of SPDT (single pole double throw) contacts which allow you to choose whether you want to use the NO or NC contacts.

I wonder about things like the contacts corroding/oxidizing/sticking(?) if they are in the same position most of the time.

Would it be a different answer if there is low voltage DC going thru the contacts?
The gold contacts are to guard against corrosion for milliamp or very low current going through them, like for inputs to a PLC.

Here is one explanation from a switch manufacturer:

GOLD CONTACTS
Gold contact switching elements are characterized by high corrosion resistance and high reliability in switching low voltage and amperage circuits. They are recommended for intrinsically safe and computer interface circuits.

willis

 
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Old 06-14-09, 12:28 AM
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I am amazed at the reliability of the typical relay. The failure rate is miniscule, and they are used in Life Safety and Critical Life Safety systems all over the world.

Still, if it was me, I would not design a circuit that powered the relay coil all the time. A much better choice would be to power a solid-state circuit -- a comparator that activates a transistor to power the relay.

Although your circuit calls for a "dry" contact, all that means is your relay's contacts won't be powered from your end. There is a voltage running through the contacts that originates from the far end. The voltage and current will dictate whether gold contacts should be used. As stated above, a small current and low voltage could be affected by corrosion.

If the relay will be used to switch audio, video, TTL, or other very low voltage/current, consider strapping a reverse-biased diode across the coil to shunt back voltage when the coil releases. Some of that back voltage can show up as a voltage spike in the contacts, so it's best to shunt it to avoid damaging the downstream components.
 
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Old 06-14-09, 07:18 AM
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You can get "reed relays" where the contacts are sealed in a little glass tube and I think they fill this with some sort of gas which protects the contacts.

Other than that, following is everything you ever wanted to know about relay contacts...

Relay Contact Life...
http://relays.tycoelectronics.com/ap...fs/13c3236.pdf
 
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Old 06-14-09, 11:25 AM
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I'm working on two projects. The one that has no voltage going thru the contacts is a project to monitor the "state" of POCO power and genset power at my home. I'm logging the date and time of POCO outages, genset exercise cycles, etc. using a data logger like this one. It's just looking for a dry contact opening and closing. The relay coils are powered by a 120Vac tap on the POCO line and the genset line. So, one relay coil is normally energized and the other is normally not energized.

The second project actually does have voltage going thru the contacts. In this case, I'm using leak detectors in a well pump house to automatically shut off power to my water well pump. I have a thread here in the Electronic Alarms and Home Security Devices forum because some searches turned up similar topics in that sub-forum. That thread gives the details of the project as well as a suggested solution that looked good to me. One relay will have low voltage DC going thru the contact to control the double pole relay which will have 120Vac going thru each contact set to the well pump.

All of the relays I use for both projects will probably be some variant of the relay in a box because they are easy to implement. If there is something as easy to use that is better than the relays, I will certainly consider it.

Thanks,
Ira
 
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