relay point contact lifespan IF AC versus DC

[sdowney717]

I have a KA-11AG DPDT relay. it has a 120 volt coil and contact points are rated 10 amps at 120 volts.

I take the relay and use it to switch 12 volt DC power to a small 12 volt DC bait well style pump that draws 3 amps.

Since there are 2 sets of points (DPDT), I gang them together to 'help' with the current flow, the points switch together at the same time.

Since this is a 120 - 240 volt AC relay, how long wil it work switching the DC power off and on.?
The is a free part, no cost to me. If it failed it would not cause any trouble for the boat or anyone on the boat. The pump will be used to pump water through a marine Cruiseair heat pump.

Originally, these Cruiseair units used a 120 volt AC pump, using a rubber impeller, which are very pricey and expensive to repair!
but you can substitute a centrifugal bait well pump and get the same water flow. (And I have all the items already)

The coil on the relay will still be energized with 120 volt AC.

I would not have bothered asking this, but I am here on the PC

 

[ForceFed70]

With only a 3amp load, I would think that relay would last quite some time. To the best of my knowledge, the contacts arn't made any different in an AC vs DC relay.

Typically they are rated AC or DC simply for the relay coil itself.

 

[mrb]

usually relays will list voltage and current ratings for the contacts in AC and DC. I checked the datasheet for that particular relay and it does not have a DC contact rating and as such would be unsuitable for DC use.

Here is a proper DC relay with 12vdc coil for $2.40
http://www.allelectronics.com/make-a-store/item/RLY-351/12V-SPDT-30-AMP-AUTOMOTIVE-RELAY//1.html

 

[sdowney717]

thanks mrb, I will bookmark it.

 

[sdowney717]

pictures of the salvaged box with lid.
I epoxy coated this and then painted grey
I got this out of a boat that was scrapped. The relay box used to have 2 relays inside.
Something made by Raritan from the 1960 era.
Steel is nice and thick, box is tough.

relay board, very thick phenolic board on that.

 

[ddawg16]

About the only time relay contact material matters is in very high current/voltage applications and very low voltage/current applications....neither of which share very much in common with regards to contacts.

The high voltage/current stuff is pretty obvious....

For low voltage (typically signal stuff), you want a contact rated for is low current operation....typicall these are gold plated contacts and/or wiping contacts (every time they make, they rub...thus wiping)

Relays alone is a whole seperate area of electronics.....

Sorry....brought back some old memories.....

So to answer your orignal question.....yea.....your fine....

 

[sdowney717]

Thanks, I figured i would try it and see how it works.

 

[Torque1st]

For power loads there are contact materials that will handle either AC or DC usually at different levels for both resistive and inductive loads. You will also see Tungsten load ratings which are for incandescent lighting applications and HP ratings. Other materials are for AC or DC only. This KA series relay is for AC loads only. It has no DC rating and is unsuitable for DC operation. The ratings usually given are maximum and my result in shortened service life. See the datasheet for the product for details. There is a KRP relay available that is a plug-in style that is available with AC coil and DC contact ratings. I would use a KRP with a 10ADC G or N contact code since this is a DC motor load. This relay is available from DigiKey or other electrical/electronic supply houses.

Paralleling contacts to increase contact rating is a major NO-NO. Even though the contacts are switched by the same armature differences remain and one contact will always open or close before the other. This will lead to rapid erosion of the contacts.

Relays and contactors are something I have spent a lot of time specifying.

 

[Gary S]

Go ahead and to it. I expect the relay will last a long time the way you want to do it. You aren't near the max current rating of the relay so you should be OK.

I built a similar setup 25 years ago to switch 120AC on a lightbulb that I heat my garage fridge in the winter to keep it from freezing. It runs 6 months of the year cycling on and off every few minutes. It still works with the original relay.

 

[nehog]

You can also substantially improve contact life by placing a (small, but you must size it correctly) capacitor across the contacts. Points are damaged on opening (break), not closing (make).

The capacitor then actually allows current to flow (through the capacitor) while the contacts are opening (it is very fast), and by the time the capacitor has fully charged, the contact are opened enough that there can be no arc over.

This applies to contacts that have DC applied to them, contacts that have AC applied have other rules, the capacitor won't help on an AC contact usually.

 

[sdowney717]

Yes I had read about arc suppression on points using capacitors.
You mention capacity sizing is important, so what size is ideal for that small load?


OK, thinking about this and old car distributors using capacitors across points. I likely have a capacitor leftover from those yesteryears in the garage somewhere. I suppose it wont hurt to do that.

 

[nehog]

Knowing the load would be necessary to figure out the capacitor size. But, I'll see if I can find some typical values and post them.

 

[ddawg16]

I would not use just a capacitor.....it will almost make the problem worse.

Here is a very good write up on contact material and contact life....it even has the equations for selecting a RC (resistor/capacitor) network to reduce contact arc.

Relay Contacts

Presonally.....I wouldn't do anything

FYI....if we assume that it is going to switch off and on about 10 times a day....and operate 365 days a year....assuming a minimum relay life of 100,000 cycles....it will last a little over a year.....but since your not pulling rated contact load....it will most likely last a lot longer.....

 

[sdowney717]

Even if you were on the boat all the time, typically they dont switch that much per day. Generally when you want it to run it runs all the time.

Then you mostly will only run it at the dock and mostly only at night.
And you only might run it occasionally when your at the boat when it is real hot.
Mostly it wont be running.
If it is really cold, you dont go boating

I tried running this on the seas with the gen set with the sun beating on the boat and it does no good, too much glass and sun creates too much heat. I found it really worked best for sleeping at night.