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Non-aero app: 120/240 AC through 24vdc switch?

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jonlaury

Joined: 06 Nov 2006
Posts: 336

Posted: Tue Aug 22, 2017 5:07 pm Post subject: Non-aero app: 120/240 AC through 24vdc switch?

In a machine that I produce, I have been using a 24vdc switch to control 120/240 AC and the contacts are sticking when the switch is de-energized. I thought that the problem was poor quality but a customer return and more frequent complaints got my attention to look further and I found the 24v designation on the switch.



I use another switch in the  machine by the same mfgr and it works unfailingly. I'm wondering if my "poor quality" assumption was a poor quality one   and, simply, a design flaw.



Thanks,

John

jonlaury

Joined: 06 Nov 2006
Posts: 336

Posted: Tue Aug 22, 2017 5:38 pm Post subject: Re: Non-aero app: 120/240 AC through 24vdc switch?

Maybe a false alarm re voltage causing contact sticking.



 The 24v stamped on the switch is the integral LED voltage. Further digging revealed the switch to be rated at 250ac (at) 5a, with most of my applications using 120vac. So perhaps "poor quality" is a possibility



See specs here:

https://tinyurl.com/y965y2wo

user9253

Joined: 28 Mar 2008
Posts: 1908
Location: Riley TWP Michigan

Posted: Tue Aug 22, 2017 6:43 pm Post subject: Re: Non-aero app: 120/240 AC through 24vdc switch?

Does the switch control an inductive load?

_________________
Joe Gores

ceengland7(at)gmail.com
Guest

Posted: Tue Aug 22, 2017 7:29 pm Post subject: Non-aero app: 120/240 AC through 24vdc switch?

You didn't mention the current being switched...

 Charlie

 On Aug 22, 2017, at 9:38 PM, jonlaury <jonlaury(at)impulse.net (jonlaury(at)impulse.net)> wrote: 	  Quote: 	
		    	  Quote: 	
		  --> AeroElectric-List message posted by: "jonlaury" <jonlaury(at)impulse.net>Maybe a false alarm re voltage causing contact sticking. The 24v stamped on the switch is the integral LED voltage. Further digging revealed the switch to be rated at 250ac (at) 5a, with most of my applications using 120vac. So perhaps "poor quality" is a possibilitySee specs here:https://tinyurl.com/y965y2woRead this topic online here:http://forums.matronics.com/viewtopic.php?p=472076#472076http://www.matronics.com/Navigator?AeroElp://www.matronics.com/contribution">http://www.matronics.com/contribution

jonlaury

Joined: 06 Nov 2006
Posts: 336

Posted: Wed Aug 23, 2017 8:14 am Post subject: Re: Non-aero app: 120/240 AC through 24vdc switch?

Joe and Charlie,



The switch controls an electronic lamp ballast driving two 75W UVC producing. I don't know if that is inductive or resistive  load. Steady state amperage is 800ma



And found this white paper(excerpt) on electronic ballasts. :



"Design Considerations for Electronic Ballasts



Inrush currents at turn-on, produced from some electronic 

ballasts can cause premature relay contact failures. 

These inrush currents can be much higher than those 

experienced with traditional tungsten or magnetic ballast 

loads because many electronic ballasts employ large 

energy storage capacitors. Such capacitors can charge up 

to 400V for a 277V line voltage, and will briefly draw high 

current from the line to achieve this voltage. 

As mentioned above, these typically short inrush currents 

can climb as high as 100 times the nominal operating 

levels. 

. 



Type of Load 

Inrush Current vs. Steady State Current



Incandescent Lamp 

10 - 15x 

Fluorescent or HID Lamp with  Magnetic Ballast

Up to 15x

 

Electronic Ballast (Capacitive Load) 

 Up to 100x



The inrush current magnitude and duration depends upon 

the value of the capacitance (measured in μF), along with 

circuit and power distribution network impedances. This 

current may weld relay contacts and other switching 

devices or possibly fuse semiconductors. "



Below is an excellent, graphic explanation of how fluorescent lamps work. It describes a building magnetic field during start up that is collapsed to produce an inductive kick. Is that the smoking gun?



http://www.edisontechcenter.org/Fluorescent.html

user9253

Joined: 28 Mar 2008
Posts: 1908
Location: Riley TWP Michigan

Posted: Wed Aug 23, 2017 9:36 am Post subject: Re: Non-aero app: 120/240 AC through 24vdc switch?

Switches can bounce when closing.  If the inrush current is 80 amps (100 times  normal) then that explains why the 5 amp switch is prematurely failing.  A switch rated at 15 or 20 amps would last longer.  A double pole switch might help also.  Connect the two halves of the switch in parallel.  That will divide the current between the two sets of contacts.  While the switch is bouncing, maybe one half will be closed while the other half is closed.  Or maybe not.  Anyway, a bigger double pole switch should last longer.

_________________
Joe Gores

jonlaury

Joined: 06 Nov 2006
Posts: 336

Posted: Wed Aug 23, 2017 12:43 pm Post subject: Re: Non-aero app: 120/240 AC through 24vdc switch?

Joe,

Would installing a flyback diode across the Line (diode banded end) and Load terminals on the switch snub the inductive spike?

Aesthetically, I'm partial to the switch I'm using. It may have a DPDT cousin, but not sure.

jonlaury

Joined: 06 Nov 2006
Posts: 336

Posted: Wed Aug 23, 2017 1:42 pm Post subject: Re: Non-aero app: 120/240 AC through 24vdc switch?

Joe,

Your idea re splitting the load on a DBDT is the right direction for a po' boy solution. I looked at Crydom Zero-Cross relays for my app but they're pretty expensive. Going to look at hi amp latching relays too.



This from Crydom, mfgr of Solid State Relays re Zero-Cross relays:



Electronic Ballasts typcially do not present an inductive characteristic, so an inductive spike at turn-off is not the issue n your application.   Rather they are more like "switching" power supplies and present more of a capacitive characteristic, particularly on power up.   The inrush current at turn-on can be many times the normal load current for a few milliseconds, especially if the power happens to be applied at the peak of the AC sine wave.   That is what is likely causing your switch to stick on when you try to turn it off. 

 

You're thought of using a zero-cross SSR is actually the way to go, but not for the turn-off.   Rather it ensures that the AC line power is connected at the zero-cross point of the AC wave, minimizing the inrush current to the (essentially) "discharged" capacitive circuitry of the electronic ballast.

user9253

Joined: 28 Mar 2008
Posts: 1908
Location: Riley TWP Michigan

Posted: Wed Aug 23, 2017 3:56 pm Post subject: Re: Non-aero app: 120/240 AC through 24vdc switch?

Since the problem is inrush current to charge a capacitor, not inductive generated current, a diode is not going to help.  I see two possible solutions: limit the inrush current or use a switching device that can handle the current.  The switching device can be a bigger switch or a mechanical relay or a solid state relay.

_________________
Joe Gores

jonlaury

Joined: 06 Nov 2006
Posts: 336

Posted: Thu Aug 24, 2017 10:36 am Post subject: Re: Non-aero app: 120/240 AC through 24vdc switch?

After further investigation, suspicion has grown about the inrush current to the power supply that I'm using. 



The ballast mfgr says any inrush to the ballast is not a problem for the garden-variety installation. The thing that is different about my install is the inclusion of a step-down power supply that specs 40A inrush current...8x the switch spec of 5A. 



Stay tuned

John

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