[SOLVED] switching at current zero crossing and switching at voltage zero crossing

[mamech]

hello

I am in confusion about when to use the signal of current zero crossing and voltage zero crossing.
I am sure that when I switch on a relay, it is best to fire it at voltage zero crossing, but regarding switching off the relay, should I switch off it at voltage zero crossing or current zero crossing?

 

[KlausST]

Hi,

https://www.edaboard.com/threads/42795/#post194863

A relay usually only has a couple of mA --> it doesn´t matter much.

Zero cross or not often is discussed with triac or SCR. Bot automatically switch OFF at zero current.


For a mor detailed answer we need more detailed information.
--> show your circuit and give additional information.



Klaus

 

[mamech]

Hi,

https://www.edaboard.com/threads/42795/#post194863

A relay usually only has a couple of mA --> it doesn´t matter much.

Zero cross or not often is discussed with triac or SCR. Bot automatically switch OFF at zero current.


For a mor detailed answer we need more detailed information.
--> show your circuit and give additional information.



Klaus

sorry for not explaining with enough details. I know already that thyristor or triac are different in their firing theory than relay. Relay only get the current in its coil to switch on the high side, but this is not what I am pointing to.
my question was about harmonics. To make the relay produce lowest harmonics possible, it is needed to switch it on at voltage zero crossing. my question, when switching it off, is it better to switch it at voltage zero crossing or current zero crossing?

 

[asdf44]

So you're trying to sync the relay turn-off to its load? Assuming 50/60hz are you sure your relay turn-off time is well enough specified to make that worthwhile? I'm familiar with specified relay timing on the order of 10mS if not more which is more than a 60hz half cycle. So even if you aim at the zero crossing the uncertainty is high enough you might actually hit the peak instead.

 

[mamech]

So you're trying to sync the relay turn-off to its load? Assuming 50/60hz are you sure your relay turn-off time is well enough specified to make that worthwhile? I'm familiar with specified relay timing on the order of 10mS if not more which is more than a 60hz half cycle. So even if you aim at the zero crossing the uncertainty is high enough you might actually hit the peak instead.

actually the relay is part of product that I am designing, and harmonics have bad effect on other electronics. Regarding uncertainty it is not my issue, because I have curve in relay datasheet related to switch on and off times, so I can adjust this . Also I may use oscilloscope for fine tuning.

still my question remains, I need to switch at zero current or zero voltage? which produces less harmonics and gives longer life for relay?

 

[asdf44]

I find it unlikely that you can reliably tune the turn off that accurately in a way that will remain stable over time, temperature and component to component variation. If this is critical for your application I have a few suggestions
1) Solid state relays. They're 'soft' and respond quickly so you could actually hit the zero crossings if you want. They're more expensive and larger but available for almost any application.

If you continue with your plan
2) You can add an RC snubber across the main contact which significantly reduces di/dt and dv/dt on turn off and may solve your problems alone.
3) You may find that the pick-up time and variation is less than the drop-out. In that case you may want to use the NC path and energize to open.
4) If you do use the NO path use a zener clamp instead of a forward diode for a snubber on the coil, that will squelch the current and open the contact more quickly.


Between voltage and current I'd say current because it neutralizes inductance which would otherwise create an arc. But again, I don't personally think you can achieve what you're trying