20A switching relay

I'd just go with a 30A power relay to do the actual switching.  Don't make a simple job complicated.

Fair enough I figured I was over complicating things. Digi-Key best to find a relay or is there somewhere else I should look? I’ve skimmed Digi-Key before but I can’t remember if I was looking at 20-25A relays or if I was looking closer to 30A

Check out allelectronics too. 12v 40 amp relays for $2.55. Tough to beat.

Pete

I like Pete choice, the price is right.

That is tough to beat I’ll check that out.

Hi Pete:

Thanks for that suggestion. Is this the item you were referring to?

https://www.allelectronics.com...omotive-relay/1.html

If yes, please let me ask a few complete beginner's questions:

1. This is a 12V DC relay.  What do you use as your power source?  A DC transformer under the table?  Something else?
   
   
2. I'm interested in using relays for an old-fashioned insulated rail-operated block system.  What relays would be best for that application?  Do those relays need to be AC relays since they will be activated by the wheel's of the train passing over the insulated track sections?  Or would the 12V DC relay being discussed here be able to be used?

Thanks in advance for your help.  I'm not an EE by any stretch.

Steven J. Serenska

The relay in your link is the one I was referring to. It is a DC relay though you would only have to add a diode in series with the coil if you wanted to power it with track power. This could actually work to your advantage. You could use one of the inexpensive buck boost convertors to power the relay. That way the relay would work both in Command mode with 20v AC on the track or much less like 7-8 volts AC in conventional.

If this was not being powered from track power you could use any source of DC like a cheap HO power pack or old desktop computer power supply.

Not sure how many relays you need. These relays can be had at any auto parts store for around ten bucks. Lots of cars and motorcycles use them. Just make sure they are rated for at least 30 amps. If you only need one it may actually be cheaper as you would save shipping. If you need a dozen then allelectronics is the place.

Pete

@Norton posted:

Check out allelectronics too. 12v 40 amp relays for $2.55. Tough to beat.

If you do eBay, 12V 40A automotive relays WITH socket/harness are about $2 each in a 5-pack...free shipping from U.S. sellers.   Search eBay for "5 pack 12V automotive relay" and narrow to U.S. sellers.  For example,

Your initial premise, that a typical switch would handle less than a relay, is flawed. Unless there is really compelling reason to use a relay, such as changing from AC to DC or changing voltages, just use a more robust switch. Use the "KISS" principle. Fewer things to break.  There are lots of different switches available that will handle 20 or more Amps. The automotive after market and the boating market use them.

@stan2004 posted:

If you do eBay, 12V 40A automotive relays WITH socket/harness are about $2 each in a 5-pack...free shipping from U.S. sellers.   Search eBay for "5 pack 12V automotive relay" and narrow to U.S. sellers.  For example,

Only issue that I’d have with that is I do about those is the ones I saw were there only 18awg wire. At a possible 20A available during a short don’t feel too great about that in my case

@Arthur P. Bloom posted:

Your initial premise, that a typical switch would handle less than a relay, is flawed. Unless there is really compelling reason to use a relay, such as changing from AC to DC or changing voltages, just use a more robust switch. Use the "KISS" principle. Fewer things to break.  There are lots of different switches available that will handle 20 or more Amps. The automotive after market and the boating market use them.

While that could be the power district on this layout are up to 30’ away I’d prefer to do any high current switching as close to the track as possible

@Serenska posted:

... If yes, please let me ask a few complete beginner's questions:

1. This is a 12V DC relay.  What do you use as your power source?  A DC transformer under the table?  Something else?
   
   
2. I'm interested in using relays for an old-fashioned insulated rail-operated block system.  What relays would be best for that application?  Do those relays need to be AC relays since they will be activated by the wheel's of the train passing over the insulated track sections?  Or would the 12V DC relay being discussed here be able to be used?..

The automotive relays under discussion are indeed "DC relays".  That is they are triggered by a DC voltage...such as 12V DC in an automotive environment.  So if you "only" have AC track voltage or AC accessory voltage then to drive the relay coil you must convert the AC to DC.  In general, this means enlisting diodes to convert AC to DC and usually other components to convert the AC voltage to a suitable (e.g., 12V) level to reliably activate the relay coil without burning it out.  A suitable source for 12V DC would be an HO-transformer set appropriately...or a 12V DC-output wall-wart which you might have lying around in the garage or attic!

For insulated-rail block control, the "trick" is how to wire up the DC relays into the AC track system.

So let's say you're using a typical automotive relay as being discussed.  The relay turns on when you apply 12V DC to its relay coil which is on terminals 86 and 85.  For now we'll skip the history lesson of why it's terminals 86 and 85.  Above diagram shows how you might do this.  The "magic" is to connect the DC common from the 12V DC supply to the AC common of your track transformer.   In dozens of OGR threads over the years, this seems to be one of those mind-bending, scratch-your-head, take-it-on-faith, concepts but it is what it is!

So each 12V DC relay coil directly receives 12V DC+ (shown in red).  When a metal wheel straddles the outer rails in a trigger section, it provides the DC common to the other side of the relay coil and that relay is now ON.

So once the relay is turned ON, the next question is how do you hook up the relay outputs (the other 3 wires of a typical 5-terminal automotive relay).  That's a separate discussion depending on what exactly you mean by block control (some OGR threads call this ATC or automatic train control).  Again, I'm focusing on your questions on driving the relays themselves using the insulated rail method.

To peel another layer of the onion.  If you do spend the time reviewing OGR discussions on block control using relays, you will find many use DC relay that has a slightly different wiring configuration.

There's a widely available and inexpensive relay "system" as illustrated above.  These are 12V DC relays but come in a modular configuration with 1,2,4,8,16, etc. relays on a module.  Above shows a 30 Amp version but there is a less expensive 10 Amp version.  Your cost-per-relay can be as little as $1 with the 10 Amp version.

Back to the difference.  With these relay modules, you don't directly drive the relay coils when the metal wheels straddle the outer-rails of a trigger section.  Instead there are amplifiers on the module so you can turn on a relay with a tiny fraction of the current that it takes to directly drive a relay coil.  It gets a little techno-nerdy as to why this is a good thing.  But cutting to the chase (and this is well covered in previous OGR threads), you can more easily eliminate the flicker/chatter effect where a relay rapidly clicks on/off as a consist enters or leaves the insulated trigger section.  That is, lightweight or slightly dirty wheel will makes intermittent contact and the relay(s) rapidly click on/off.  This is a your-mileage-may-vary but can be annoying if your block control includes signaling indicators.  This method has additional benefits when you need more complex "logic" such as timing delays and the like commonly found in block control applications.

I will stop now since these issues are irrelevant to the original thread topic.

@stan2004 posted:

If you do eBay, 12V 40A automotive relays WITH socket/harness are about $2 each in a 5-pack...free shipping from U.S. sellers.   Search eBay for "5 pack 12V automotive relay" and narrow to U.S. sellers.  For example,

Thanks for the heads up. They show up under "automotive relay 12v 40a" from a few US dealers. Right now, they're going for around $10-12 per set. Basically, they're SPDT fog/driving light relays.

Wow...talk about over-complicating what is basically a simple project/question.

If all you want to do is switch track power on and off, all you need are THESE. 20-amp toggle switches, six for $10.

Done.

@zhubl posted:

My club has been building a new layout and we are planning to have a yard are possibly powered by 3 different power sources. Each of the main line 360w Legacy PM’s and a 360w PM for the yard. This is mainly to accommodate our small number of conventional trains so that going from the yard and onto any “main” is a smooth transition and no need to actually change transformers. In this case a TIU is in passive mode and with the 360w PM there is a available 20A on the track. Now I doubt in any given section will pull 20A but If someone gets a string of incandescent passenger cars and 4-6 Pul-Mor motors that is a power hungers trains with a 3.5% grade. With 20A I don’t feel a standard toggle switch is best, so now most likely a relay is the best option but got to thinking if a solid state relay was used would that be better? Or am I now making things more complicated. My thought is then there’s now contacts moving and possibly arcing inside the relay. I would think a TRIAC or FET could do it but I am not a electrical engineer. Then even if this could work would it be more reliable than a standard relay this is a layout that will get at minimum weekly use. Just looking for thoughts and opinions and if I’m just making this more complicated than it needs to be

Thanks

I am confused by your initial post.  Are you saying you want to be able to connect each (electrical) block of track to one of three power sources?  If so, then it might make sense for each block to use a rotary switch (that typically has a low amperage capacity) such as this one:

https://www.mouser.com/Product...t%252BcWo9NvXA%3D%3D

that drives relays that switch the actual heavy current - potentially 20 amperes in your case.  The rotary switch would select which of the three power sources you want to connect to the track and the relay would take care of actually switching the heavy current.

If you want to keep it simple, then just use the SPST switches that Rich referenced with three switches per electrical track block. Your operators just have to be careful to turn on only one of the three switches per block.

The rotary switch is not a bad idea for the 3 power source issue. I would most definitely not count on our operators to be disciplined think something through. This is a very odd situation all around personally I would prefer to not even mess with blocks in this under the layout yard area. But it has been insisted by other members to help accommodate conventional engines, it makes since just feels like this makes more complicated than it needs to (yes I see the irony after I was trying to make it even more so with a SS relay)

I think the only reason to use a relay would be if, for some reason, you don't want to run heavy-gauge wire carrying the full track current to and from your control panel.

Otherwise, as other posters above have noted, a 20A rated switch should be fine.

@PGentieu posted:

...

If you want to keep it simple, then just use the SPST switches that Rich referenced with three switches per electrical track block. Your operators just have to be careful to turn on only one of the three switches per block.

@zhubl posted:

The rotary switch is not a bad idea for the 3 power source issue. I would most definitely not count on our operators to be disciplined think something through.

So aren't you saying you do worry about shorting the outputs of (up to) three transformers?

@Professor Chaos posted:

...if, for some reason, you don't want to run heavy-gauge wire carrying the full track current to and from your control panel.

You mentioned a TIU might be in play.  The MTH high-frequency DCS control signal rides on track power.  There is evidence that willy-nilly wiring geometry can degrade signaling performance.  So this could be another reason to running low-current wires from a control-panel rotary switch to 3 high-current relays out at the yard.  Taking it to the extreme, the ultimate in reducing the wire gauge of the control wires is to have no wire at all...in other words, wireless. There have been several OGR discussions about wireless relay control using low-cost transmitter and receiver modules.  For ~$10 you can get a 3-channel (3-button) remote fob and the receiver electronics that can drive the high-current relays.

Obviously this adds even more complexity to a simple problem...but this is a discussion forum to exchange ideas.  Remember when copper was so expensive that thieves stripped homes of wiring?  Even at today's copper prices heavy-gauge wiring is not cheap.  I figure if you do the math there are circumstances where using $2 relays and low-cost multi-conductor phone/ribbon control cable would be more economical  (not saying more simple) than running bundles of heavy-gauge wiring to-and-from a distant control panel.  Your mileage may vary! 

@Professor Chaos posted:

I think the only reason to use a relay would be if, for some reason, you don't want to run heavy-gauge wire carrying the full track current to and from your control panel.

Otherwise, as other posters above have noted, a 20A rated switch should be fine.

I believe the issue is finding a 3-way switch (one input to one of three outputs) rated at 20A.

Since operating at 18V AC (max) you'd probably be OK with a 3-way fan switch which are widely available.  Here's one on eBay for $2 free shipping from Asia - 13 Amps @120V AC and it includes the knob!

Or, if nervous about the ratings, pair this with 3 of the 12V high-current relays that are ~$2 each plus a 12V DC wall-wart that you probably already have in a junk box.

I'd be nervous about having 3 separate SPST switches with a skull-and-crossbones warning saying to only turn on one and only one toggle switch at a time.