Understanding Relay Spec's.

motor.

situation?

I like the knife blade switches because the grasp the entire blade of conductor with more conductor.

I just posted this because that is what came to mind when I envisioned contact bounce events.

It was the first thing I thought of. Frankenstein.

They also do not exhibit the same bounce characteristic, idiot.

So, you just shot yourself in the foot, dumbass.

Try for the head now, boy.

OK, explain to us why the contact bounce pattern depends on the load impedance.

John

situation?

You don't need worst case to stress the contacts that way. Any time the contacts open L*di/dt can - and often does - create a high voltage which can arc and stress the contacts.

Ed

AC doesn't sustain an arc like DC does. I don't think (mostly guessing here) that the inductance of a motor makes much difference to relay contact arcing on opening.

Relays often have three contact current ratings, for resistive loads, incandescent, and motors. The resistive is highest, and incandescent loads don't have much inductance.

DC contact voltage ratings are usually much less than AC.

John

OK, I figured out where my relationship with Thompson went wrong. I sent him a 6-pack of Widmer. It was the wrong beer.

ftp://jjlarkin.lmi.net/Jim_Beer.JPG

John

You explain how an inductive load can cause a different current signature to be present at the time of initiation, which is when the contacts bounce.

Don't forget to describe what the high current influx does as the conductors that make up the contacts bounce open and closed several times in the first milliseconds of operation.

With a resistive load, the contacts almost meld together (molecularly speaking) at the moment they contact. It is a "controlled short". Ever seen the points on a lawn mower engine? They only 'bounce' once, each time they are operated. They bounce 'open',and there is an arc and a material transference.

For AC, regardless of load, the best way to fire a relay is to time it such that it closes while the voltage is crossing through zero volts.

Hmm, is Jim Beer anything like Billy Beer?

"John Larkin"

** In the time span of a few mS - there is no big difference.

How much a pair of contacts arc depends on how much current has to be broken and the voltage available after opening - all AC power does is provide a good opportunity for the arc to self extinguish within 8 to 10 mS. The amount of damage done to the contacts in that time may not be great - but if repeated 10,000 times it is often fatal.

** With incandescent loads, the inrush surge is higher ( around 10 to 12 times the normal Ipk ) - but comparatively brief at only a few cycles instead of a few seconds. Contacts generally have longer thermal time constants than a few cycles. ** Yep - typical relays rated for 240VAC switching at 10 amps are also rated for 24 or 32 volts DC at 10 amps. When tested, there is a dirty great flash from the contacts when doing the latter and a total failure to break the circuit if the voltage is raised above about 40 volts.

I tried this few times with a 50 volt DC supply (cap filtered) and a 4 ohm resistive load - a self sustaining arc developed across the relay contacts

• first shot * and destroyed them in a few seconds.

Using a relay with a larger opening gap helps somewhat as does using a strong magnet near the contacts - but is still not usable over many operations.

.... Phil

motor.

situation?

With AC, the current goes to zero every half cycle, so it generally doesn't sustain an arc for long.

John

A baseball bat would make a decent impression...

--
You can't fix stupid. You can't even put a Band-Aid? on it, because it's
Teflon coated.

motor.

situation?

--
You obviously either misread me or you're just trying to be combative.

The point is that, if there were no bounce, the only stress the
contacts would experience would be the momentarily increased I²R
losses at startup; no big deal. 

However, since the current is so much higher during startup, bounce
will result in a plasma capable of doing much more damage to the
contacts than the one created when breaking a resistive load.

--
Nope; mea culpa.

--
See the platitude,
dressed to the nines, posing as
An epiphany.

Period!

Sorry, no. It's part of the decision when replacing a relay, how often will I need to do this again? Last time I used relays, I socketed them, and none of the relays failed. Good magic ;)

I've been looking into this! I find the following on several sites, which is why I inserted the apostrophe. "We use an apostrophe when letters are missing. I will look at the obvious cases first. These are where we deliberately shorten a word

or phrase and then use an apostrophe to show that letters are missing."

HOWEVER, all the examples I see are when letters are missing AND two words are combined.

So, I made a horrible grammatical error and won't do it again!

Until I do.

Mikek

Ok, my new relay has two sets of contacts, If I parallel the two sets, am I doubling the chance that a set of contacts will weld themselves together?

Is it less than a doubling?

Is it paralleling the contacts a good idea to increase MTBF?

Someone suggested a snubber across the contacts, seems to me it was big enough to help, it would allow "quite" a current to flow in the motor.

Mikek :-)

"quite" a word to use when you don't have a clue.

On a relay, one places a 'snubber' on the relay mechanical actuator coil to clamp the back EMF spike when power is removed from a DC energized coil. It has nothing to do with the switched contacts or circuitry thereof.