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Posted By Topic: Relay arc suppression

Wiredone
Jan 10 2019 18:03

Howdy

Having a bit of an issue with relay contact burnout. The relays are rated correctly for their load, but are switching thousands of times a day. I have installed diodes across the contacts and relay coils, to dampen the arc, and have had some success. But im after any further or better ideas to make my relays last longer. They are standard Finder 24V single pole relays, pulling in field solenoid valves. Thanks for the help.
   

Someone
Jan 10 2019 18:27

DC or AC load? DC presumably, as you're talking about diodes.

Go solid state?
   

Someone
Jan 10 2019 18:29

A typical mechanical relay is only good for <200k cycles. So lasting a few months is probably on spec anyway.
   

Wiredone
Jan 10 2019 19:07

Yes, it's a DC load. Any inherent issues with SSRs?, cannot say I've used them too frequently. Would they be ideal with my situation where a Plc output is switching a relay to pull in a solenoid valve?. My current relays can operate every few seconds.
   

Wiredone
Jan 10 2019 19:13

I'm well aware of relay life cycles, I'm really just after what best practice is, to get the most out of my equipment.
   

DougP
Jan 10 2019 19:56

Maybe an RC, or RCD snubber?
   

YeahNah
Jan 10 2019 21:39

SSR's have merit, but if you run them close to their limits and they get hot they have a habit of latching (not releasing) even if the source is removed.
Goggle schematics for ones used for motorbike electrics.
Double pole relays with in/out in parallel will increase the circuit life X 2 and even 4 times.
   

YeahNah
Jan 10 2019 21:42

I meant SSR's in DC mode, closet to their limits have a habit of latching.
   

YeahNah
Jan 10 2019 21:59

Also Google DC relay, arc suppression.

Some pretty simple inductor/capacitor suppression diagrams.
   

YeahNah
Jan 10 2019 22:05

AND......... if you're interested.

Take a close up Slow-Mo video with your iPhone or Android of the contacts opening and closing.

If you replay it in super slow mode you can see the contacts opening and arcing.

Doesn't fix anything... Just mildly interesting.
   

dbuckley
Jan 10 2019 23:16

Switching DC inductive loads "thousands of times a day" is going to end up a crapshoot of which of the lifetime ratings of the relay you violate first.

SSRs are the only answer to this problem, they have none of the disadvantages that mechanical contacts have when switching inductive DC loads, even up to currents of thousands of amps.

A SSR will cost many times what the relay it replaces costs, but will outlast a boxful of those relays. Many DC SSRs have integrated snubber networks and thus are rated for driving inductive loads directly with no need for additional transient absorption things.

You mentioned you've "installed diodes across the contacts and relay coils and solenoids"; diodes are only of benefit across the coils, their purpose is to provide a path for the back-EMF once the circuit is broken. However, that doesn't stop the arcing at the contacts that occurs while the contacts are opening and the current continues to flow, which is what does the majority of the damage to the contacts.

I should probably also mention that the wording that describes relays in the catalogues is often quite misleading, it specifies a DC voltage and current the relay can switch, but omits to mention it can't switch that voltage at that current. You need to dig into the data sheet to find the derating curves to find out what current can be switched at the voltage of interest, and the derating is sometimes very dramatic.
   

ppaw1965
Jan 11 2019 01:25

A capacitor across the contacts is what you use for DC. Remember the old points on car distributors? They all had a capacitor across the points to stop them burning out.
   

Wiredone
Jan 11 2019 01:53

I'd have to add a resistor on series with that capacitor though, due to in rush currents. Thanks for all the replies, very helpful. I'm going to purchase and install a couple of SSRs to see how they go.
   

YeahNah
Jan 11 2019 08:24

" A SSR will cost many times what the relay it replaces costs "
There are dozens of SSR's on Trade Me for $0-$20

There are single and 4 channel SSR kits for Arudino for as little as $15.

https://www.trademe.co.nz/electronics-photography/other-electronics/electronic-components/other/listing-1904679168.htm?rsqid=17d0c3fe95e7452a9813a9da4061fa0b
   

dbuckley
Jan 11 2019 11:01

ppaw1965 notes:


> A capacitor across the contacts is what you use for DC. Remember
> the old points on car distributors? They all had a capacitor across the
> points to stop them burning out.

Yep, I'm old enough to remember when we had contact breakers and distributors on car engines, and they did indeed use condensors (as they were called back then) to reduce the sparking across the points.

That was then, this is now. Distributors and contact breakers have all disappeared because we have better ways to get spark timing to cylinders done. Ditto with suppression of back-emf generated by inductive loads. Diodes do the job better.

When the current flowing in the (solenoid or relay) coil is interrupted, the collapsing magnetic field causes the coil to act as a generator, and so a voltage is generated, the back-EMF, and depending, with a 12V or 24V coil, this can often be hundreds of volts. This voltage has to go somewhere, and in the absence of anywhere else to go, will arc across the opening contacts of the switch or relay. By placing a diode or capacitor across the coil, the back-EMF has somewhere to go.

The difference is that the diode acts as a one-way short and thus absorbs the back-EMF, whereas the capacitor stores the back-EMF, and the releases it back across the coil, and thus continues the current flow in the coil longer than necessary, hence slowing down the release of the relay or solenoid, albeit for only a short period of time.

No-one would have cared much about the difference, condensors were used on car spark gaps from the beginning to their end, and were never upgraded. When points were invented, there were no diodes that could have been used in car applications, these didn't exist until the 1960s. When it started to matter was when inductive devices started to be driven by transistors, as back-EMF would immediately destroy a transistor device, and so diodes were used to protect the transistor, and have become the gold standard of back-EMF protection.


   

SaintAlan
Jan 12 2019 20:37

Ah, the old ignition circuits; I did my project on that subject as a student some years ago.
The 'condenser' is not for spark suppression on the 'points', its purpose is to form a resonant circuit with the inductance of the coil.
Resonance causes a peaky ac current in the primary and several thousand volts on the secondary for the spark plug.
Such circuits do not spark if the capacitor is open-circuit.
Those were the good old days!
   

jasmoon123
Jan 13 2019 10:56

Hi Wiredone,

As suggested above SSR (solid state relays) are your way to go. They are electronic and have no moving parts so will switch forever.

You should be able to use the same relay base.

The thing to watch out is you need to keep the diode on the load (solenoid coil) as the SSR's are sensitive to the back emf voltage. You should be able to buy the solenoid plugs with built-in flywheel diodes. The other issue is they have a small leakage current (ie not a perfect switch). This won't be a problem if the coil is a big enough load (ie requires decent current to turn on) but I would check with the solenoid supplier on the specs.