Note: All relays should be energized prior to connecting loads. No hot switching.

This statement needs further explanation because the "relay" is quite often used to switch loads on?

 

Question, what are you doing still working with electromechanical relays? Shouldn't it be far better if you were to substitute them for SSRs ??? I'm just sayin'

Because the cheapest 30 amp SSR I can find costs $55 and a similar mechanical contactor costs less than $20. Because an SSR can be hurt by lightning transients on the power line and a mechanical contactor is much more rugged. I have seen a motor that took an almost direct hit that blew the power connectors off it, and the contactor still worked.

http://www.mouser.com/Electromechan...lays/_/N-ay0ti?P=1z0x2rkZ1yvsbas&Ns=Pricing|0

 

Because the cheapest 30 amp SSR I can find costs $55 and a similar mechanical contactor costs less than $20. Because an SSR can be hurt by lightning transients on the power line and a mechanical contactor is much more rugged. I have seen a motor that took an almost direct hit that blew the power connectors off it, and the contactor still worked.

http://www.mouser.com/Electromechan...lays/_/N-ay0ti?P=1z0x2rkZ1yvsbas&Ns=Pricing|0

Reliability... with a dash of economics ... both of them are good points, if reaction speed is not an issue

 

I have never seen anything in the 20 to 30 amp category that cared about a few dozen milliseconds. I'm probably showing my ignorance now.

 

Question, what are you doing still working with electromechanical relays? Shouldn't it be far better if you were to substitute them for SSRs ??? I'm just sayin'

I agree SSRs would be better. But you would be surprised that Mil-Aerospace contractors use what they have on hand when it comes to testing. We were using programmable switch banks that were made in the early 70s for testing. The worst thing was getting parts for repairing them and frequently had to add delays to the software so that they could keep up.

 

What is the power rating of your heating elements?

Have you considered using solid state relays?

It should pull about 5.2 A (624W). I do have an SSR laying around but I also have a bunch of brand new electromechanical ones laying around also. The switching time isn't important and I'd like to save the SSR in case I needed a higher rated relay. The SSR I have is 10 A and the electromechanical is 6 A, also the SSR needs a 24V control signal and the other is 120 V. Less parts and a bit cheaper.

 

It should pull about 5.2 A (624W). I do have an SSR laying around but I also have a bunch of brand new electromechanical ones laying around also. The switching time isn't important and I'd like to save the SSR in case I needed a higher rated relay. The SSR I have is 10 A and the electromechanical is 6 A, also the SSR needs a 24V control signal and the other is 120 V. Less parts and a bit cheaper.

So you are controlling (turning Off / On a 120 Volt approximately 624 Watt heating element which works out to be about 5.2 Amps. That is nothing. The heating element is also a resistive load making for easy to do. You can buy an off the shelf relay for about $17 rated at 10 Amps 240 VAC on the contacts in about any control voltage you could want. Heck, I likely have some 120 VAC coil versions laying around, if you want I'll send you one (free, I have plenty and postage is on me). Even a lowly 10 amp SSR would work in any control voltage you want, including 120 VAC.

Ron

 

That's why I babble on about this stuff. Noobs are watching.

Noob reporting for duty sir!

Seriously though, thanks for posting your previous comment. It gave me an "Aha" moment there. I think that may have solved one of my biggest setbacks on a future project of mine.

 

Noob reporting for duty sir!

Seriously though, thanks for posting your previous comment. It gave me an "Aha" moment there. I think that may have solved one of my biggest setbacks on a future project of mine.

Good to hear... now go buy some PNPs and then give me 20!

 

I have never seen anything in the 20 to 30 amp category that cared about a few dozen milliseconds. I'm probably showing my ignorance now.

Well, I'm refering more about their capacity of reacting to input signal during just half a mains period (1/120 8.3 ms) but probably I'm exaggerating a bit, if you compare it to 20ms

 

You have to figure an AC relay or contactor is going to get its magnetic field stable in about 17 milliseconds, then the mechanical part is delayed by inertia. A little 6 amp relay has a moving arm that only weighs a few grams. A 30 amp contactor has to move a couple of ounces. I reach into my memory for the delay time between clicking a switch and hearing the contactor slam shut. My smallest increment of perception is about a tenth of a second, but I can't detect any delay with my ears.

This datasheet says a DC driven, 30 amp relay makes contact in 20 microseconds.
Even if that's a misprint, 20 milliseconds isn't too bad.

 

You have to figure an AC relay or contactor is going to get its magnetic field stable in about 17 milliseconds, then the mechanical part is delayed by inertia. A little 6 amp relay has a moving arm that only weighs a few grams. A 30 amp contactor has to move a couple of ounces. I reach into my memory for the delay time between clicking a switch and hearing the contactor slam shut. My smallest increment of perception is about a tenth of a second, but I can't detect any delay with my ears.

This datasheet says a DC driven, 30 amp relay makes contact in 20 microseconds.
Even if that's a misprint, 20 milliseconds isn't too bad.

You got your numbers about right... but don't relays get damaged more often during the turn-off cycle than during the turn-on one? And do both cycles last the same?

 

Well, I'm refering more about their capacity of reacting to input signal during just half a mains period (1/120 8.3 ms) but probably I'm exaggerating a bit, if you compare it to 20ms

You got your numbers about right... but don't relays get damaged more often during the turn-off cycle than during the turn-on one? And do both cycles last the same?

Damaged? I thought we were talking about activation time.
I need you to explain your question.

 

Damaged? I thought we were talking about activation time.
I need you to explain your question.

You're right, I digressed. (still curious, though)
Anyway, my last question remains. Aren't the turn-on and turn-off cycles significantly different in most electro-mecahnical relays?

 

The turn on is electrically powered and the turn off is spring driven. The spring can't be stronger than the magnetic field or the relay would never make contact. Therefore we can assume that a weaker force takes more time to accelerate the mass. However, the powered stroke has to move full length to activate and the spring only has to make a gap.

That's as far as I can logic this out. Is 10 milliseconds of difference significant? If you really need to know, you're going to have to measure one. This is not in the datasheets.

As for damage, I have seen the same number of relay contacts welded together when they made contact and close but not conduct current. (That would mean they were damaged while opening.) In my experience, it's 50-50.