# Relay Use Rule Of Thumb

Thread Starter

#### hobbiest_ee

Joined Dec 5, 2014
4
Hey everyone, I have a real basic question. I am using a SPST (single pole single throw) relay on my test fixture. I have 1.5V as the COM line. I want to switch this 1.5V to the NC (Normally closed) or NO (Normally Open) lines out. Basically I have just using the relay to switch the 1.5V supply to two different lines.

My question is: Is there a rule of thumb on whether to enable the 1.5V supply first and then switch in the relay (actuate relay) or switch in the relay and then enable the 1.5V supply? Or does it even matter the order? Would the relay have a longer time adjusting to the voltage on the line if the 1.5V supply was enabled at first and then the switch was switched in?

The same goes for shutting down the relay that is actuated. Would I first turn the supply off and then unactuate the relay? Or does it not matter? Will the relay be ok if I unactuated it while the supply was still on?

What is the rule of thumb for relay use in this aspect?

Thanks!

#### gerty

Joined Aug 30, 2007
1,303
If you switch the 1.5 volts on/off I see no real need for a relay at all. You can tie the NC connections together, and when you shut off the 1.5 volts, it would appear like a NO connection. If the relay has additional uses the sequence shouldn't matter.

#### Bernard

Joined Aug 7, 2008
5,788
The carry current rating of a switch is usually greater than make or break current, but at lower currents no difference.

#### MikeML

Joined Oct 2, 2009
5,444
With two different power supplies (1.5V, relay coil), you have four different states and two outputs:

Code:
[FONT=Courier New]
State                  NO                NC

-----------------------------------------------------------

Both Off               0V                0V

RelayOn, 1.5VOff       0V                0V

RelayOn, 1.5VOn       1.5V               0V

RelayOff, 1.5VOn       0V               1.5V
[/FONT]

My conclusion, it matters not which order you turn on the supplies. Follow the reasoning, and learn from this...

#### Alec_t

Joined Sep 17, 2013
12,248
If the power is off when the relay changes state there will be no chance of arcing at the contacts. This should prolong relay life.

#### MikeML

Joined Oct 2, 2009
5,444
If the power is off when the relay changes state there will be no chance of arcing at the contacts. This should prolong relay life.
Not necessarily. Some types of contacts depend on switching a current to keep them clean... You have to dig into the details of the relay's data sheet to answer this.

#### djsfantasi

Joined Apr 11, 2010
7,865
Is it a SPST relay or SPDT relay. You say the former but describe the latter . ???

Thread Starter

#### hobbiest_ee

Joined Dec 5, 2014
4
If the power is off when the relay changes state there will be no chance of arcing at the contacts. This should prolong relay life.
That's basically what I was looking for. The most efficient way to use a relay for circuit safety and long life. For SETTING UP, Should the switch be closed first or the live voltage applied first or does it not really matter? Should SHUTTING DOWN, should the switch be opened first and then the power disabled or vice versa or it really doesn't matter?

Thread Starter

#### hobbiest_ee

Joined Dec 5, 2014
4
Is it a SPST relay or SPDT relay. You say the former but describe the latter . ???
SPST

#### alfacliff

Joined Dec 13, 2013
2,458
a spst relay would not have both nc and no contacts. only one set of contacts. with both, you have a spdt relay.

Thread Starter

#### hobbiest_ee

Joined Dec 5, 2014
4
a spst relay would not have both nc and no contacts. only one set of contacts. with both, you have a spdt relay.
Sorry I meant SPDT. My error...

#### Alec_t

Joined Sep 17, 2013
12,248
For SETTING UP, Should the switch be closed first or the live voltage applied first or does it not really matter? Should SHUTTING DOWN, should the switch be opened first and then the power disabled or vice versa or it really doesn't matter?
Post #5 covers both situations.

#### cmartinez

Joined Jan 17, 2007
7,454
Your question is more interesting that it appears to be at first... When you open or close a switch, its contacts normally "spark" due to the sudden start or end of electrical current. These sparks with time tend to erode the contacts and in turn develop more and more resistance, making the problem worse with each cycle, until they eventually fail.
At the voltage that you're you're working with that shouldn't be a problem, since 1.5V is too low to cause damage to the contacts (although it might if you were to run an insanely amount of current through them)
I've been using mechanical relays to switch low current, 5V TTL circuits for a long time, and even these stop working properly after awhile, since the resistance at the contacts increases too much (due to either electric erosion or rust because of normal humidity) for them to deliver a stable and reliable signal.
I try to avoid using mechanical relays if I can, and will always go for solid state instead... But if the use of a mechanical contact is unavoidable, then I use a magnetically activated reed switch, since they're completely encapsulated in glass and are impervious to moisture and dust, the disadvantage being that they can only handle a minuscule amount of current.

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