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Is it possible to use zero-crossing with electromechanical relay?

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EngIntoHW

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Say that you have a circuit which identifies zero-crossing of the line voltage, and you want to switch an electromechanical relay exactly when that zero-crossing happens.

Is it possible? (taking its bouncing into account).

Thanks.
 
Yes but there will be several millisecond closing time associated with mechanical relay.
 
Since a relay coil is inductive, and the risetime of the current di/dt is primarily dependent on the Inductance, switching it at a zero crossing vs any other random time during an AC cycle is not going to change the pull-in behavior much. It takes several tens of ms for the armature to pull-in.
 
Currently the company has decided to stay with the electromechanical relay.
I'd like to ask 2 questions please:
1. Would it be wiseful to measure the average switching time (ON and OFF) of several such relays, and take it into account when using the zero-cross signal?
That way, if the scattering around this average is +/-3ms, then I only miss 3ms when switching the relay.

2. What is the ideal relay to use in order to be able to use the zero-cross signal for switching the relay exactly at zero-crossing?

Thanks a lot.
 
Use a solid state relay - thus no switching time*.

Andrew

*Well, practically no time
 
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Thanks!

How about question #1?
How do you see this suggestion, as in the meantime, the electromechanical relay will be used.
 
It seems a waste of time, a relay is too slow and too inaccurate to make it worth trying.

If you want to use zero point switching, then use a TRIAC.

Hey,
I actually measured the switching time of several relays, and the scattering of their switching times around the average switching time was +/-3ms as I said.
An inaccuracy of +/-3ms is something I can live with.

So why is it such a bad idea?

+/-3ms => the armature is switched at +/-75V (Max).
 
Hi,

If you have a spread of plus or minus 3ms isnt that a lot? If your period is 20ms then the half period is 10ms, and 3ms is plus or minus 54 degrees, and that means the sine wave is at about 80 percent of it's peak voltage anyway, so you'd be switching near the peak for plus or minus 3ms.
It's not a bad idea it just doesnt work too well :)

BTW, do you have to turn on or turn off near the zero crossing?
You might be able to use controlled current but that's a lot more complicated.
 
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Hi again,

Sorry, plus or minus 3ms is much higher than that. It puts the turn on/turn off time at 180*0.003/0.010=54 degrees, of which the sine is 0.81 which is 81 percent of peak, and that's assuming 50Hz (60Hz would be even worse). I think you did something wrong there in the calculation, although if you want to explain i'll certainly listen and compare your results to mine.

Controlled current is where you control the current rather than the voltage. Let me explain a bit more...
The relay coil is rated in voltage but really the physics of the coil has a current limit and only a voltage limit if that voltage is constant. The manufacturer assumes a constant voltage, so they rate it in voltage units rather than current. It's easy to get the current rating, just measure the current while it is operating at nominal voltage. I assume also that you uare using DC coil relays, not AC coil relays as i dont think the AC coil relays would work very well for this.
Ok back to the current rating...
Say you measure the current at 100ma. That would mean the max current should be kept to around 100ma or maybe a little higher for short periods if the relay does not have to turn on and off very frequently. Holding the current at 100ma means you can either apply 120vdc (or whatever the coil voltage is) and wait for the current to build up, OR, you can apply a current. Interestingly, the magnetic field is related to the current, not the voltage, and the armature pulls in due to the current causing the mag field, not the voltage. This means if we can get the current to build up much faster then we can get the relay to pull in faster. You still didnt say if you need to turn it on or off at the zero cross though, but i'll assume turn on for now. To get the current to build up very quickly, you would use a current generator rather than a voltage supply. Unfortunately the current generator has to be constructed and the voltage will be limited so we cant get 1us turn on for example, but we can improve it. With twice the voltage (as a limit that is) we might get 1/2 the response time, which could reduce 3ms to 1.5ms, which would be a pretty good improvement. It also depends partly on the inertia though.
So it all boils down to how far you are willing to go to get this to work. You'll need to make a power supply that can put out at least twice the voltage and you'll need to make it current and voltage limited. It could be as simple as making a voltage doubler with regulated output, but that's a bit of a task too, so i'll have to wait until you get back here and mention what you think you would be willing to build to get this to work.
So until then a couple questions too:
1. Are you turning on or turning off?
2. How many relays do you have to operate?
 
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Hey,
I actually measured the switching time of several relays, and the scattering of their switching times around the average switching time was +/-3ms as I said.
An inaccuracy of +/-3ms is something I can live with.

So why is it such a bad idea?

It just seems pointless, either just switch the relay randomly (as has worked perfectly for over 100 years), or use a TRIAC to zero switch.
 
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Hi MrAl,
Thanks a lot!

1. I'm turning it both on and off.
2. I have to operate one relay at each single unit.

It was lovely reading about the current controlling.
I became more wise :)

I'd wait until we get a non-electromechanical relay for using the zero-crossing.
 
Maybe you could share with us the problem you are trying to solve. I think we are all thinking it is some type of noise or arcing problem. Are we on the right path?
 
Hi MrAl,
Thanks a lot!

1. I'm turning it both on and off.
2. I have to operate one relay at each single unit.

It was lovely reading about the current controlling.
I became more wise :)

I'd wait until we get a non-electromechanical relay for using the zero-crossing.

Hi,

Very good idea :)
The current control method is not easy, mainly only for use when you absolutely have to use a relay and can not use some solid state solution.
 
A compromise would be to use two relays, with a diode in series with the contacts of each. You could then turn each relay on or while its diode is reverse biased. That way the relays never make or break the current, and your timing only has to be in the correct half cycle.
 
One of the early PIClisters solved this one back in about 2000, and if I remember right put it into production. He measured the pull-in and pull-out times of the relays used in their product on a storage scope and used a PIC to switch the relay at the correct time to give the contact open/close at the zero cross point or extremely close to it.

I can't remember if it was myself or one of the other people offered the idea of using a lower and known relay current (I think he used a regulated 12v DC and a resistor in series with the coil). The reduced current gave a more repeatable release time as the release time was the more variable of the 2 times due to short term residual magnetism in the steel relay parts.
 
Even if you DID have a relay that switching time is good enough, a reactive load will still cause [insert correct word here, google translate didn't find it] lightening between the relay contacts and thus reduce it's life time.

A pure resitive load would be the best. But as said before at switching moment, the peak voltage might be quite high.
 
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