Author Topic: Relay Basics  (Read 1624 times)

Relay Basics
« on: 08 August, 2008, 10:44:27 pm »
I've got a 12V relay (Goodsky MI-SS-112L) that will switch a mains circuit, and I understand the basic principles.  But, never having used one before, I do have a few questions:

This does not label the five pins, and I haven't been able to Google it to produce a diagram, so I've drawn my own.  Can anyone confirm whether I've got the pins right?

I'd like to find a socket for it, as it seems much easier to solder to a socket and allow the relay to be replaced in future if necessary, but what socket should I get for this 5-pin relay?

I've seen descriptions of the use of a protection diode across the coil of a relay; my circuit will involve two relays: the first will latch and switch a 12V supply to a second relay, which in turn will switch a mains supply.  The first relay already has a protection diode fitted, and the circuit has been put together elsewhere, but I have to connect the second relay to switch the mains.  Does this relay also need a protection diode?

You can tell I'm a newbie, can't you?




rae

Re: Relay Basics
« Reply #1 on: 08 August, 2008, 10:53:49 pm »
Whichever relay has the switching done by something delicate needs the diode. So if you have an IC switching the first, then the diode goes on that.  The second is switched by the first, and a relay doesn't care about voltage spikes.   Not sure why you have two relays?

Re: Relay Basics
« Reply #2 on: 08 August, 2008, 11:10:33 pm »
Thanks, rae.

Here's an article about the application.  The first relay needs to latch in reponse to the input from two float switches, and send 12V to the second to switch a pump.

Thanks for such a quick reply.


tonycollinet

  • No Longer a western province of Númenor
Re: Relay Basics
« Reply #3 on: 09 August, 2008, 09:35:50 am »
Whichever relay has the switching done by something delicate needs the diode. So if you have an IC switching the first, then the diode goes on that.  The second is switched by the first, and a relay doesn't care about voltage spikes.   Not sure why you have two relays?

Relays do care about voltage spikes - if you don't fit suppression of some sort, you are likely to get arcing which will pit - and then eventually wear out - the contacts.

You can either fit a diode (if you don't care about switching time) or a varistor intended for 230V applications (if the first relay is also mains rated and you want fastest switching times.)

rae

Re: Relay Basics
« Reply #4 on: 09 August, 2008, 10:28:47 am »
Quote
Relays do care about voltage spikes - if you don't fit suppression of some sort, you are likely to get arcing which will pit - and then eventually wear out - the contacts. 

We're not switching 40A here.....the first relay will be pushing sufficient current to drive a second relay, so 200 mA or thereabouts.   

Diodes are reasonably new in relays.   My Series III has a relay that has switched 40A regularly since 1980....not a diode in sight.  Ditto the old Alfa.   New automotive relays all have diodes to protect more sensitive elements of the circuit.

Lots of Good Sky relay pictures and diagrams here:


   Rapid Electronics

Re: Relay Basics
« Reply #5 on: 09 August, 2008, 11:12:03 am »
Thanks for the help.

And thanks for the link, rae - I've already found the relay I've got in my hand on there.

But I don't see any links to pin diagrams.  I'm 90% sure I've got them sorted in my mind, but I would be reassured by having it confirmed.

Edit: Doh! Just seen the links to .pdf technical information - thanks!

Re: Relay Basics
« Reply #6 on: 09 August, 2008, 11:44:44 am »
I just put your part number in Yahoo and came up with http://home.comet.bg/shop/index.html  put your part number in there, open description and then datasheet for all you need.

tonycollinet

  • No Longer a western province of Númenor
Re: Relay Basics
« Reply #7 on: 10 August, 2008, 08:06:50 am »
Quote
Relays do care about voltage spikes - if you don't fit suppression of some sort, you are likely to get arcing which will pit - and then eventually wear out - the contacts. 

We're not switching 40A here.....the first relay will be pushing sufficient current to drive a second relay, so 200 mA or thereabouts.   

Diodes are reasonably new in relays.   My Series III has a relay that has switched 40A regularly since 1980....not a diode in sight.  Ditto the old Alfa.   New automotive relays all have diodes to protect more sensitive elements of the circuit.

Lots of Good Sky relay pictures and diagrams here:


   Rapid Electronics


Depends on the relay - automotive relays are designed for heavy duty siwtching, and can cope with some arcing - plus the very high currents actually cut through some of the resulting crap.

Smaller relays on the other hand do not cope well with arcing. Even though it is only 200ma, it is switching a coil of another relay. This is an inductor - and it is not possible to cutoff the current in an inductor using a mechanical switch witout significant voltage spiking which may well arc. We use this type of relay in our products and have instances where they are used for switching external contactor coils (for example) where arcing damage has occourred.

Re: Relay Basics
« Reply #8 on: 10 August, 2008, 03:08:59 pm »
Depends on the relay - automotive relays are designed for heavy duty siwtching, and can cope with some arcing - plus the very high currents actually cut through some of the resulting crap.

Smaller relays on the other hand do not cope well with arcing. Even though it is only 200ma, it is switching a coil of another relay. This is an inductor - and it is not possible to cutoff the current in an inductor using a mechanical switch witout significant voltage spiking which may well arc. We use this type of relay in our products and have instances where they are used for switching external contactor coils (for example) where arcing damage has occourred.

Thanks, Tony.  If this is so, what rating of protective diode ought I to consider fixing across this second relay's coil?  What are the relative costs between the diode and the relay, which is relatively cheap to replace anyway? 

tonycollinet

  • No Longer a western province of Númenor
Re: Relay Basics
« Reply #9 on: 10 August, 2008, 09:28:08 pm »
Almost any diode will do.

eg 1n4148 will do the job (it only has to carry the same current as the relay coil, while that current decays).

Connect the diode accross the coil, with the cathode (black bar) pointing at the positive supply from the driving relay.