How does a relay work?

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I think a relay is a simple device that facilitiates the switching of a heavy current by using a small current to effect the switch. The small current simply moving a conductor to switch the heavy load at the relay which is remote from the switch. So I could use say 200ma to switch an 8A load. Obvious saving in using thinner cables for the run to a convenient switch location as well as shorter cable runs for load cables thereby reducinjg voltage drop. The relay will therefore have four connection - two for the light load terminals and two for the heavy load cables.

Is all of that correct or utter nonsense? If the above is right I want to use a relay to switch a 10A supply to an holding tank pump where the switch is significantly remote from the 10A supply route. Fully accept that I might be talking nonsense.

If I'm about right - where does one by a relay, how are they rated and how much might they cost? Are there good and bad quality? Can one by "tinned" relays?

TIA

Rob
 
You are correct, and a good explanation.
Best relays for this use are quality automotive ones. The likes of AC Delco are good.
They are rated in amps at 12v.

it is advisable to buy a relay of twice the current you need. The larger ones tend to come with tinned terminals. They are designed for automotive use so are pretty robust, providing you keep them dry you should have no worries.

Cost should be around 5 gbp or so.. or visit a local scrap yard.. then about 50 p each... get plenty.. ! /forums/images/graemlins/smirk.gif
 
You can use solid state relays as well. Same function, no moving parts. Have to be matched to load voltage and current
 
Hi Rob, tother way round, the silvered are the load.. this is a single pole change over, perfect, it has 5 terminals. but you would only use 4. Yes, the terminals are marked, usually a little diagram under the relay.
It is easy with a multimeter to tell, but you should have no problems.
As in all electronics the switch cirsuit AND load should be fused.
 
Take a look at this one from Maplin instead. You'll see that there's a little diagram printed on it, showing the terminal connections. Relays are classified according to what they do, and are described as, for example, SPCO which means Single Pole Change Over (applying voltage to the coil will change a power supply from one terminal to another). SPCO is sometimes also called SPDT (Single Pole Double Throw). Another description is SPNO (Single Pole Normally Open - the main power contacts are normally open, but close when a voltage is applied to the coil). For your application, any of these could be used.
 
Oh gawd, doom munger!

But only a little advice. Relays seem to be very prone to corrosion internally, I remember cursing the one I had to keep opening on the vetus engine we used to have. Problem is, once you have to start cleaning the contacts it is a slippery slope, or not so as the case may be. The engine rooms on our boat are fairly dry places and the relays were still showing signs of rust.

Auto relays are not sealed units, just plastic or tin hoods to stop you damaging the coils. I think some silicon seal smeared generously to the hood joint might be good advice. Also care in location is very important, you tend to have your power panel in a dry place, but the relay would be most sensible close to the equipment if you are planning it to keep the cable run short.

Have a good look at specs, some relays are off/on but some are on/off, meaning they use current to hold the switch off, though they are rare.. Most auto relays tend to be double pole so you can just move the cables over. Be aware also, some relays take a daft amount of power when running, auto fuses tend not to have to worry about power consumption, like modern car stereos. The cheap ones do unfortunately tend to use more power. I think this slides into insignificance if you are pulling 8amp when it is in use, but still something to look out for.

I was considering one on my eber and radar as the cable run to the main panel and back is around 26 ft! In the end I opted for KISS and just fitted a mini panel to the port battery, ooh and the fridge too.

Be interested to see how you get on and if the corrosion thing is just something I experienced on poor quality vetus relays.

I know nigel calder <sic> was talking about the digital panel he had fitted, this meant he had relays all over the boat, but as you suggest, almost bell wire as the main cable runs to the panel, it is an interesting solution. I don't know why, but I have this voice saying noooo, even though they are the most reliable electrics in the car; so based on nothing of substance really..
 
Yes, agreed, but he had been given advice to go to a car scrappy...

Are not the solid state units quite pricey?

I know very little about them..
 
How about a delay relay since this is for a pump. If it's the typical pull switch by the sink, then just a normal one as discussed above, however, if there's a float switch or other level sensor involved, try one normally used for delaying the interior lights of a car, to stop excessive re-cycling. (it will allow the pump to run on for a few seconds after the sensor opens, to clear extra water, so the pump doesn't restart due to water falling back down the outlet pipe)
 
[ QUOTE ]
I Fully accept that I might be talking nonsense.

[/ QUOTE ]

Hi Robih!

I think you are trying to be too clever!!!!

Having been on your boat I can't quite understand your need for an extra device (relay) that could fail - especially if its an automotive product - when all you need is a breaker switch which has a built in fuse - fitted somewhere near the macerator pump - easy - it doesn't need to be remote!. But why are you using an existing 10 amp cable to feed the pump which must be designated and fused for something else? Why don't you use your main switch panel which has a heavy current supply to it already which then needs a 10 amp cable from there to the macerator?

Your relay idea is the start of a distributive supply setup where you have two very heavy duty cables running the length of the boat, and solid state relays which tee off to each load - all switched digitally by just one more cable. This could be a cheaper way to re-wire a whole boat but I don't think you are saving anything just for one load.
 
Do not go down the solid state / bus system route, unless you are really keen. It all runs from a micro, if it hic-ups, you have to take a labtop to reprogramme the whole system, in the meantime nothing works.
The main reason for a relay would be to allow remote switching where it was not convinient to run a power cable to a switch, and then back to the device. If you can fit a straight switch in the feed that is the most reliable.
If you need to fit a relay buy a relay and a waterproof juction box, B&Q were selling 3 for a £1, fit relay in box and it will give you many years use.

Brian
 
Hi Matt,

Me being too clever - ha! Now you know you are joking!

I'm proposing this solution because I don't want to run heavy cables up to the switch panel and back to the appliance because this would add about 3m to the cable run and the route for the cable is through some tight conduits. So I was trying to be clever (yes you were right!) and this relay idea popped in to my head. In years gone by I would have just run the 12AWG to the switch panel and back out again!

are relays so wholly unreliable then? this is for a poo tank so not mega critical if it fails - not like an autopilot for example (which interestingly is professionally fitted and uses a relay because the supply to the a/p is 25A).

yours confused..............

rob
 
Correctly spec'd and sealed they are almost 100 % reliable.
Do not buy a cheap one, or one from a scrap yard ( could be any contact rating ) but buy a decent 40 / 50 amp relay from your local LSUK or similar outlet, and put it in a water proof box ( B&Q) and you will have no problem. Cost about £5 - £7.
I have used many 1000's of relays for charging systems over the last 30 odd years with no problem.

Brian
 
Matt,

ps - I'm not using an existing 10A supply cable; I'm running a new one in from a new distribution panel which has been fitted.

rob
 
The cost of the extra 3m of 12 AWG cable is probably not that much more than sourcing the right relay - and the relay mounting and the lighter relay cable to do it your clever way.

And why do you want a remote switch anyway - why not just next to the pump. Oh and by the way the switch should be a push and hold button not a switch - so you can't leave it on by mistake when the holding tank is empty.
 
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