Two 12A 250VAC inrush relays in parallel?

[cupoftea]

Hi,
Do you think itll be ok to pllel two of these, (12A relays) for a 4kW SMPS off 240VAC?.....

https://www.te.com/commerce/DocumentDelivery/DDEController?Action=showdoc&DocId=Data+Sheet%7FRZ%7F0920%7Fpdf%7FEnglish%7FENG_DS_RZ_0920.pdf%7F5-1415899-5

(will ry and make the tracking to each relay of similar resistance)
How will these relays fayre when we reduce VAC to 100VAC and each relay has to carry 23Arms? (they are only 12A rated). How long till they fail?

 

[dick_freebird]

Until one of them starts to develop contact resistance, and
then current-sharing goes out the window (maybe this will
lead the other to "catch up", but maybe not). Similarly if the
actuation is mismatched in time, you might not get the
sharing you'd hope, and one takes the inrush current / turnoff arc
punishment while the other takes it easy.

You'd have to ask the mfr for overstress life test data, and
prepare for disappointment (customers used to pester me to
follow them down their rabbit-hole and tell them violating
abs max / rec max conditions was OK; but do it once, and
why have ratings at all?).

Hard to believe there's not one suitable 25A relay out there.
Maybe you are now looking for "contactors", but still - the
job is getting done, somehow.

 

[KlausST]

Hi,

Paralleling relays doesnt work like expected.
There is timing mismatch making the fastest "ON" contact to carry all the switch ON current for a short time.
Also the slowest turn OFF contact caries all the switch off current .. and most probably builds the arc.

Klaus

 

[cupoftea]

Thanks, as such, we will see about putting in zero cross detect and only switch them at mains ZX...so no current flows through them when they switch.
Also, for when the relays are closed...do you think it would be worth adding series resistance with each so that they share current better?....eg one of these 10milliOhm resistors?.....

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[dick_freebird]

Can you actuate a relay fast enough to stay anywhere near zero crossing? At 16ms (or 20 for youse on the right side of the pond) full cycle time) it's maybe 4ms from zero to peak. You may need to "lead the duck" to hit it.

 

[cupoftea]

Thanks, great point, i was forgetting that......though i still wonder if the "sharing resistors" are a good idea? (i think with the switching, we will have to only switch the relay when the caps are totally full...then zero current through relay when they actually switch.

 

[betwixt]

I use a triac with a relay connected across it. The triac is fired by a ZCD opto isolator and the relay contacts close a few cycles later. When switching off, the relay contacts open a few cycles before the triac turns off. So the surge is dealt with by accurate zero crossing and the triac but the relay contacts carry all the current immediately afterwards. It's a low cost, simple solution that also removes the strain and arcing risk on the contacts while the triac dissipates virtually no power.

I use it for switching large single phase pumps that draw about 30A at start up. Been in use for about 20 years, switching maybe 50 times a day with no problems.

Brian.