A question about relays

I know that queasy feeling. I just bought some 110VAC (coil and contacts) relays. Contacts rated at 10A! Flying lead from contact to spade connection looks like #20 :-( ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
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              by understanding what nature is hiding. 

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The wires do look small. However they are short so the voltage drop will not be very much. Wires in free air can carry a lot of current,but if very long will have lots of voltage drop.

But mainly it's the very, very short 'duty cycle' - a one-time 10s press or so.

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I don't ever remember seeing duty cycle as a spec on any relays.

Current and voltage for the contacts, nominal voltage for the coil and must make must release voltages, and mechanical/environmental, number of cycles, are all the specs you usually get.

** The total number of cycles and time spent in conduction are both low in this case. What you call it is not important.

** Relay contacts can pass more current than they can reliably switch - and even modest DC voltages are a big problem due to the likelihood of contacts arcing.

.... Phil

That's what gets me about these automotive types. The coil on something like the AC clutch, and even the solenoid for the starter motor, are relatively large inductors and those contacts seem mighty small for the current and inductance.

Comparing that to the ratings on standard industrial switching types versus size of the contacts I have to conclude that the auto types are designed with cost as the predominant concern, or the industrial types are very conservatively rated and could easily replace the auto type and probably outlast them.

The auto parts store is charging me the same for the cheaply made relay, as I can buy industrial control relays for.

Ebay, of course, has the auto relay much cheaper. Brand name industrial relays are about the same on Ebay as I pay through a distributor, unless they are used, or auctioned.

I'm wondering if it would be wiser to use an industrial relay in the truck - I've had to replace the fuel pump and AC clutch relay in the truck already.

nd even modest DC voltages are a big problem due to the likelihood of conta cts arcing. "

Yes but they're still full of shit. They play the numbers. The A/C clutch r elay obviously works for much longer, but that draws less current. In fact even the solenoid does not pull 40 amps. The starter itself maybe but that is switched by the solenoid.

If he has 1/8th" diameter contacts, well that's not so bad as long as they' re nice and clean. I think it is bigger than # 12 wire. (on AWG) But those fingers that carry the electricity to the contacts... They are not very thi ck or wide. those little relays what size ya figure ? Maybe 0.02 thick and

0.25 wide ? I doubt it is the equivalent of 16 gauge. Maybe just barely, an d # 16 will not carry 40 amps for long, I mean like a few seconds tops.

Sounds like they're using car stereo amp math for more than just car stereo amps.

Auto manufactures want top reliability from their vendors but at a very low cost. The engineers that design the relays play it very close and play the odds.You probably would be surprised at what you would go through if one of your relays failed in the field.

But small area means high contact pressure for a given solenoid-applied force; unless you know WHY the old one failed, just get a replacement, and you're good. Automotive goods are just as good (or bad) as industrial, but for different service conditions. Shopping around won't substitute for engineering.

They fail because the contacts burn. Dressing the contacts with a needle file will get it working for a time. When the fuel pump relay goes out the vehicle won't start. That is annoying when your sitting at a gas station 90 miles from home on a Sunday.

Philosophy won't substitute for engineering either :(

That's why I put it in inverted commas! The essence of it is, you will have no trouble provided you energise your solenoid in well-spaced, short bursts. Used that way it should last for years. If you hold it constantly on, however, it'll burn out within 2-3 minutes.

Some people think they can save a small fortune by using auto solenoids in place of the correct ones in applications like winches. They don't realise it's the extended 'on' time capability that you're paying for. Auto starter solenoids are NOT designed for what amounts to continual duty and the manufacturers simply assume (not unreasonably) that you will not use them for anything other than their stated purpose!

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Starter motors are another example. When Robot Rumble (or whatever it was called) first hit the screens over 20 years ago, some green-as-grass wouldbe fighting robot builders tried to use scrap auto starter motors for propulsion. Probably seems like a great idea for any keen novice to get started with. Again, however, these are not designed for extended use and quickly burned out. Think about it: they have none of the necessary cooling in terms of air vents and fins that you find in true propulsion motors. They had the torque all right; all the power, low-voltage/high current, but.... no good for use outside of their intended purpose.

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Where I worked several motors of about 25 or 50 HP were burnt up when they were put on inverters for speed control. Before that a magnetic clutch was used.

I solved the problem for them. The motors were running very slow so the fan on the motor shaft could not cool the motor. Extermal cool air was blown across and into the motor and the motor lasted many years.

Yes, in so many areas, keeping things cool and longevity are heavily correlated.

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Oh.. I only mention the solenoid because it is on the load side of one of the ice-cube size small relays. I realize the actual starter solenoid is a whole 'nother thing.

The tiny relays are rated for a 40 amp load and the contacts eventually burn. They switch the starter solenoid, AC clutch, and fuel pump. Strangely enough, the headlight and horn are wired direct without relays.

Oh, right. Sorry, I didn't read your original question correctly, then. What they *may* have done is to silver pad the contact points. Silver has a marginally lower electrical resistance than copper, but you would not notice without specifically looking at the contact areas.

Horn, no surprise. Doesn't use much current. Headlamps, yes, that is 'unusual' shall we say. I can't say I've ever come across that.

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Something like 3.5 % only. While I was unable to get a specific figure on t he electrical conductivity of silver oxide, it is said to be almost as cond uctive as silver, and is self limiting in thickness. Gold is not really a g reat conductor but it almost can't oxidise or anything, so if the coating i s thin enough it works fine. Copper oxides and compounds are pretty much in sulators so that, why there is plating. In my recent internet travels someo ne mentioned the palladium would work well. That may be but I don't know wh at it costs. Gold is not necessarily the most expensive substance on Earth.

gold, palladiun, and platinum all cost about the same, currently gold seems to be the most expensive of the three.

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     ?

** The tarnish that forms on silver is "silver sulphide" - an insulator.

Being soft, it polishes or wears off fairly easily.

.... Phil