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Old 31st May 2015, 2:22 pm   #1
ValvoStef
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Default Relay Problem

Afternoon, odd thing with a relay. It is a 5V dc coil relay, the switch contacts are rated 16A 250V ac. I want to switch a 5V dc 1A load. After a few operations the contacts welded together. This happened with two relays. I thought the relay should be able to handle 1A dc loads or am I wrong? The load by the way is resistive only.
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Old 31st May 2015, 2:50 pm   #2
vidjoman
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Default Re: Relay problem

Possibly arcing with a DC load. Contacts are probably too close together.
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Old 31st May 2015, 3:06 pm   #3
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Default Re: Relay problem

Quote:
Originally Posted by vidjoman View Post
Possibly arcing with a DC load. Contacts are probably too close together.
You are probably right. thanks.
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Old 31st May 2015, 4:17 pm   #4
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Default Re: Relay problem

Can't read what your relay part no is but a similar looking specimen from TYCO has no rating for DC switching, only AC ratings.

http://uk.farnell.com/te-connectivit...16a/dp/2060829
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Old 31st May 2015, 4:51 pm   #5
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Default Re: Relay problem

Odd, I use the exact same relays in my linear amplifier as drivers for contactors, 24v DC at around 1 A

They are also used in some control circuits of generators we have at work - very strange.
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Old 31st May 2015, 4:52 pm   #6
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Default Re: Relay problem

Ah, the good old AC/DC ratings catch'em...

Any set of contacts, either in a relay or a switch, has DC current/voltage ratings which are generally very much lower than the AC ratings due to the tendency for an arc to form when the contacts break. In an AC circuit any arc will be self-extinguished by the nature of the AC waveform whereas arcs in a DC circuit will only extinguish when the contacts have separated by a given distance. This distance will be dependant on switching voltage and carried current. Once an arc is established, the air in the vicinity of the contacts is ionised causing it to become conductive and the arc is maintained for a surprisingly long distance. The result is that contacts which only have an AC current rating stated are intended ONLY for use on AC current as their contact spacing tends to be smaller than those with reasonable DC current ratings.

For example high power industrial contactors especially those used to control large DC drive motors are often fitted with some form of arc-suppressor mechanism which blows a jet of gas across the contacts as they release (Sulphur Hexaflouride SF6 springs to mind from my old heavy engineering days).

The contact material also has some effect on ratings, but the point above is more relevant in this case unless the contacts are Gold (Au) in which case they are only intended for AC/DC signal switching of around 50mA or less.
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Old 31st May 2015, 5:17 pm   #7
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Default Re: Relay problem

To add to the above, for DC switching contactors are often used. Not only is twice the contact gap achieved for a given armature travel, but more significantly, the rate of increase in gap as the armature releases is doubled.

Switching a big 24V motor from a stiff supply with a conventional relay - suitably rated - produced a continuous and destructive arc from the contacts. As the job was a vehicle mounted application, a sudden jolt could have produced the same effect.

Specifying a small contactor of about the same volume as the existing relay solved the problem. Ag/CdO contacts are also resistant to welding - plain silver is not.

Leon.
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Old 31st May 2015, 6:17 pm   #8
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Default Re: Relay problem

If its a sealed relay with just a vent for flow soldering, you could try filling it (almost) with oil and sealing it up.
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Old 31st May 2015, 6:36 pm   #9
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Default Re: Relay problem

Well, a relay rated 240V 16A, AC should have no trouble with 5V 1A resistive load.

Generally I have found that DC ratings, when given, are limited to 48V (might even have been 24V), but I can't remember anything as wimpy as 5V - except gold-flashed signal relays which no way would have an AC rating of 16A 240V.

Are you sure the load is resistive? 5V just isn't enough to sustain an arc, you need a couple of tens of volts for that, easily. But 5V 1A in series with 20H of inductance would quickly put paid to slowly-opening contacts. However, they wouldn't be welded together, they'd burn away. Welding together sounds more of a huge capacitance dumping energy as contacts close.

Can you replace the relay with a fast-opening, chunky switch, 'scope across the contacts as it makes or breaks, and check for anything unexpected?
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Old 31st May 2015, 8:29 pm   #10
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Default Re: Relay problem

Well, if you're sure that it's just a resistive load, and it is a 16 amp relay (SPCO contacts, but wired with two PCB pins to each contact and so the same footprint as a DPCO relay), then 5V / 1A sounds entirely within the ratings of that relay. In fact, it shouldn't even hurt one of the DPCO relays which usually have contacts that are good for about 5 amps.

You could always just have struck unlucky and got a faulty relay in the first place.
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Old 31st May 2015, 11:48 pm   #11
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Default Re: Relay problem

That really sounds ok to me. I've used smaller relays to switch larger loads (eg 24V dc solenoids which are very inductive!) on industrial machinery where they switch hundreds of times an hour. Maybe you have a large inrush current from a downstream capacitor and a particularly low impedance power supply? What's the application?
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Old 1st Jun 2015, 8:21 am   #12
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Default Re: Relay problem

Thanks for all the replies. The application is a power supply, it switches 5v dc from an 7805 regulator . The switched 5V feed a couple of TTLs and some LEDs. The TTLs control 12v relays (with BC107). The problem could be 5V and 12V share the same ground.

The age of the relays is unknown but should not matter as they are completely sealed.
They appeared to be unused, don't know where I got them from.
Maybe - as suggested above - I was very unlucky and picked two faulty relays.

I have two more of the same type, just for the hell I make some tests and switch a 12v lamp.
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Old 1st Jun 2015, 8:21 am   #13
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Default Re: Relay problem

I agree that the relay should be fine switching DC at such a low voltage even if only rated for AC.
A "rule of thumb" is that DC is about 10 times more destructive of contacts than AC and that therefore most contacts intended for AC should be fine on DC provided that the voltage be reduced to about one tenth of the AC rating.

I would expect relay contacts intended for 240 volts AC to be fine at up to about 24 volts DC.

I would suspect either defective manufacture or a fault in the circuit being switched, or less likely a huge inrush current.
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Old 1st Jun 2015, 6:09 pm   #14
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Default Re: Relay problem

You don't say what type of load the relay is switching, if its inductive then there will be arc at the contact as they open. Have you thought about fitting a flywheel diode across the load. Also some DC relays have a diode built in and so are polarised.
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Old 1st Jun 2015, 6:36 pm   #15
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Default Re: Relay problem

But surely arcing would tend to cause contact erosion, as opposed to welding?

For the contacts to weld up, the load would have to be highly capacitive, and so resemble a dead short when fully discharged.
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Old 2nd Jun 2015, 1:13 am   #16
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Default Re: Relay problem

That's what I reckoned too - see post #9.
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Old 2nd Jun 2015, 6:59 am   #17
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Default Re: Relay problem

Without seeing the circuit, I can't really say much, but if the relay contact which is arcing and welding is powering both an inductive load (other relay coils) and any other circuitry, then I'd be worried about the voltage transient being applied to the other circuitry as well as the welding of the contact. It may be that the arc and weld is clamping a transient, and if you switched to contacts which didn't weld, you might replace one problem with another.

Whenever you switch an inductive load, you have to worry about where stored the energy goes.

Unless you need isolation, there are some very good low-resistance MOSFETS which can switch DC with less voltage drop than a pair of contacts and no contact erosion, but you still have to manage the transients.

David
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Old 2nd Jun 2015, 11:14 am   #18
ValvoStef
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Default Re: Relay Problem

Meanwhile I discovered my mistake: I am switching the 5V regulator with the load, this must cause a surge when the regulator powers up. I altered the links on the board so that the regulator stays on and the relay switches the load only. The 5V regulator is actually a 2A type not a 1A as I assumed originally. Well, one more experience richer. Thanks guys for all your input. I think this weekend I am going to experiment with Radio Wranglers idea using a MosFet.
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