Running PRR signals - how to wire

The thing with these signals is they use the center rail to control the red and green signals. the yellow is on a timer circuit after the red has been triggered and is switched back to green. The timer circuit turns on the yellow and after a certain time switches to green. it was designed for a IR sensor. I use a automotive 12vdc SPDT relay due to I'm a automotive technician and have a large collection of relays. I use a 4amp bridge rectifier to power the relay. with the relay off, the circuit is switched to turn on the green. When the train runs through the isolated track, this supplies voltage connection to the relay and switches the contacts to the red signal. after the train passes, the relay switches back to green circuit but the yellow lights come on with the timer circuit and then green. It's very simple. wire one from transformer hot to the AC terminal of the rectifier and the other coming from the isolated track to the other AC terminal of the rectifier. DC+ and DC- of the rectifier goes to the coil of the relay(doesn't matter where + or - is wired on the relay coil as it is not specific how it is wired. the coil will work either way).

 

The only thing with this circuit is you will get flickering from dirty wheels or traction tires on locos. Also run the signals on aux power hookup at 12 volts. If you wire it to track power, the voltage could be too low to operate the relay.

Relays can be purchased from auto parts store and bridge rectifier can be purchased from Radio Shack. If you need a diagram I can draw one up. Very simple.  

I use a 22uh choke (DCS compatibility), diode, capacitor, and a 12VDC relay.  That gives me chatter-free contacts to drive signals from the insulated rails.

 

GRJ, is the Hennings lighting board available a la carte?  I can't tell from the web-site.

 

That is, I agree that a handful of low-cost components can do the job but like the 12V LED strip wiring there are lots of guys that don't want to fuss with soldering, wiring, sourcing components, etc.

 

So.  If you replace the 12V DC LED strip with a 12V DC relay (plus a diode across the coil), couldn't that lighting board (IIRC 45mA output?) implement this? 

Stan, it is, but that part of the site is a little confusing.

 

LED Constant Current Lighting Regulator, 2Pcs

 

 Thanks JOHN FOR THE INFO ON THE LED REGULATORS. WILL SURELY TRY A FEW. TOM

If you have any questions, just let me know.

 

Well, I had to see if it would work...

 

So here we have the Hennings LED lighting board connected to an isolated-rail simulator (blue masking tape).  Track voltage from Z4000.  LED board set to maximum output current (about 45 mA) driving an eBay 12V relay module - the one's that are just over $1 (free-shipping from Asia).

 

Worked fine over a 10-20V AC range.  Here it is in action. The relay module has a red LED which turns on bright when the relay trips.  Note the LED brightness decays rather than suddenly turning on-off which is from the capacitor performing its anti-chatter.

 

Looks like a winner.

 

Here are 3 other off-the-shelf isolated-rail-trigger relay modules that have been discussed in the last month or so on OGR.  If anyone else has photos or diagrams to add of other off-the-shelf relay modules, it might be useful to consolidate tack that on here (for the record). 

 

Azatrax MRAPR and Lionel 153E shown below in photo from another thread:

 

https://www.azatrax.com/track-power-relay.html

 

The 153E seems to be discontinued or very hard to find.  The Azatrax does not appear to have a capacitor on it but I don't have one so don't know for sure.

 

 

Train Electrics MTR-12T

 

http://www.trainelectrics.com/relays.html

 

 

If with roll-your-own, here's a write-up by Dale H about what's going on under-the-hood at the component level which has photos of his implementation:

 

http://www.jcstudiosinc.com/Re...Rail-Block-Detection

 

And here's GRJ's schematic I copied from this thread where he posted it.  He might have a photo of it somewhere but I can't remember.

 

I don't know that I posted a photo of these.  I built them out of salvaged relays, and put them onto small wooden platforms.  They get screwed under the layout and powered from track power, hence the choke for DCS and the resistor to limit the inrush current.  They work very well on 18V, and I never have to go back and tinker with them, they're pretty bulletproof.  It was a pretty crude assembly, I super-glue the relay upside down by the case.  I also had a barrier strip for the connections so there'd be no soldering under the tables when we were installing them.

 

 

I use a bridge rectifier on a DC relay. Then add a 500uf or a 1000uf

on the DC side. The 1Kuf gives me about a 1 second delay after the

train passes. I like the effect. This configuration also prevents

relay chatter.

This is a kind of "roll your own thread" on isolated rail triggering.

My lowly diode and 220uf capacitor has never chattered, at least none of us have ever noticed any of them doing it.  I figure the signals will add any delay if they so choose.

 

I looked through eBay and couldn't find the relays shown above, anyone know if these are still around?

http://www.ebay.com/itm/12V-1-...;hash=item20e865f2c8

 

 

Thank you, Stan!  I couldn't find them after 30 minutes of searching - I just ordered two.

Well I just got these in the mail today and already burned up one - can someone please tell me how to hook these relays up?

 

Question - is the relay linked in Stan's post above a direct replacement for the one sold by Scott's Odds n Ends (Train Electrics)?

 

 

Thanks

What are you trying to do?  Drive signals?  Are you using isolated-rail block detection with the accessory voltage from a transformer?

 

Out of curiosity what burned up?

 

No.  The Train Electrics relay accepts AC or DC input; the eBay relay is a DC device.  The Train Electric relay has 15 Amp contacts; the eBay is 10 Amps (albeit I don't think you need 15 Amps for anything to do with signaling).  But then the Train Electrics relay is $17.50 while the eBay relays are less than $2 w/ free shipping.  Part of the story in this thread is how you might save some $ if you are willing to fiddle with some components to allow the DC-only eBay relay to operate on AC.  As shown earlier this AC-to-DC conversion can be done by $1 or so of assorted components like diodes, capacitors, resistors, etc. but soldering and fussing with small parts is not everyone's cup of tea.  I was showing how GRJ's LED lighting board could also perform this AC-to-DC conversion.  Frankly I wouldn't expect anyone to do this and was more a case of idle curiosity that I tried it.

 

Great info...thanks for providing everyone.

-Greg

Thanks for the clarification Stan.

I've used a rectifier in the past, along with a terminal strip, to convert. I assume that I could do the same here with the eBay unit?

 

In principle, yes.  That is, by using a rectifier at the input to the eBay DC relay you can activate with AC.  But as GRJ shows in his circuit, for isolated-rail control, you might want to add a resistor (to limit current in-rush), a capacitor (to demote relay "chatter" or add some holding time delay), and an inductor (to make it compatible with DCS if using track voltage to drive the relay).

 

In addition to the attractive price (less than $2 shipped), a nice thing about the eBay relay modules are those screw-terminals.  It can be a hassle soldering to bare relay terminals.

 

But the real "win" with the eBay DC relay modules occurs IF you are willing to run you signaling circuits on DC voltage (say, 12V DC from a AC-adapter or wall-wart).  Then all kinds of benefits kick in.  This approach has been discussed in a variety of threads but I can elaborate specific to this application if there's interest.

 

I have 2 questions,  I have a bunch of 12VDC relays purchased years ago...   I also picked up an assortment of full wave bridge rectifiers at Radio Shack 90 percent off closing sale...

 

question 1 is:   I bought two different bridge rectifiers as they were only 15 cents each, 1st one is  4 amps,  50 volts,   the other is 25 amps,  50 volts.   I am assuming these will both work the same in a signal relay circuit....  is there any downside to buying the higher current rated bridge rectifiers ? 

 

question 2 is about limiting the "in rush" current discussed above,  on the schematic above in this thread there is a "22" to the left of the resistor,  is the recommended value  22 ohms ?  Is there an acceptable range of resistor which will provide the in rush current limit function ?  Also I am assuming  1/2 watt resistor is desirable in this application ?  

 

thanks

q1. both work the same. the larger device may be inconvenient because of its size.

q2. 22 is 22 ohms.  1/2 Watt is fine.   Other common values around 22 like 18, 27, 33 will work too.  If you go too small (say, 10 ohms or less) it defeats the purpose of installing a limiter in the first place.  If you go too high (like 47 or more) it may limit the voltage available to activate the coil.  See below.

 

GRJ's schematic shows a single-diode to convert the AC to DC.  This is call half-wave conversion since the diode only transfers one half of the AC waveform.  If you use a bridge-rectifier or full-wave conversion you transfer both halves of the AC waveform.  The math gets nerdy and depends on capacitor value, relay coil resistance, blah, blah, blah.  The bottom line is using a bridge-rectifier may apply too much voltage (power) to your 12V DC relays and overheat them.

 

Some off-the-shelf relay modules as shown earlier include electronics to limit the power to the relay.  Others simply specify the range of allowable track voltage thereby limiting the power in the relay.  If you want an "engineered" design, provide a link to the relay datasheet and range of track voltage over which you operate.

Stan,  thanks very much the explanation of the 1/2 wave conversion makes complete sense now...   

 

I really wasn't thinking about pulling my 12 VDC signal relay power directly off the track... I was kind of thinking about providing the proper 12VDC voltage off a spare post of one of my 4 PW  ZW's and running it through a full wave bridge rectifier... If I do this, and I have all my common posts tied together, I should be able to close a block relay anywhere on the layout once the train enters that block's insulated rail ...  Does that seem like a wise way to go, or am I dreaming ?    

 

I don't run DCS,  I do run TMCC and Legacy control on the layout.... Seems like I should still install the 220 uF capacitor on the relay power bus to stop chattering, and I can ensure that I am not putting anything over 12 VDC out to the relays on that bus line.....   

In concept that's a wise way to go but if I understand how you're planning to do it, you will see the dreaded magic smoke and it won't be because you're dreaming!

 

The bridge rectifier converts the AC voltage from a train transformer to DC.  So far so good.  But the "-" output from the bridge rectifier is not equivalent to the common post of the AC signal it came from.  And connecting the "-" output of the bridge to the outer-rail common (to implement isolated-rail block detection) will not turn out well.

 

The DC voltage to needs to come from an isolated power source.  It could be a 12V DC output wall-wart.  It could be 12 V DC from a bridge rectifier fed by a spare AC output train transformer whose common is NOT physically connected to your main AC transformer common(s).

 

Just to be clear, the anti-chatter capacitor needs to be on the relay coil side of the block detection circuit...one capacitor per relay.  The 22 ohm (or whatever) resistor is also one per capacitor/relay on the relay side of the block detection circuit.  In other words, chatter from intermittent/dirty wheel connections causes the relay coil to receive a choppy voltage.  By put the capacitor across the relay coil, it smooths out the voltage keeping the relay closed.

 

I'm interpreting "relay power bus" as the DC voltage (nominally 12V for your application) created one time so to speak.  You create this relay DC voltage once...rather than converting AC to DC for each relay.

 

FWIW, having the relay receive a bit more voltage, within reason, is not really an issue for most relays.  In intermittent duty, getting 15-16 volts on the coil shouldn't be an issue.

 

For 18V track voltage, I've found half-wave rectification and a decent filter cap (I use 220uf for small relays) has done the job for us.  I do add the small value resistor to limit inrush current and arcing at the rails, and the 22uf choke for DCS compatibility.  Yes, I power the relay from the rails, it's way more convenient for a modular setup than trying to run separate power.

Stan thanks very much for the detailed explanation and the warning...  I went back to Radio Shack and emptied the supply of bridge rectifiers at $0.15 each, it was a bargain, so now I have a bridge rectifier for each block signal relay.   I will just run a 12VAC buss line off the hot accessory terminal and then trigger the bridge rectifier and the 12VDC relay with an individual common coming off the insulated rail for each signal block.   thanks for taking the time and helping me out. 

Do you have a meter handy (or get one on clearance at Radio Shack!)?  If you don't have a datasheet for your relays, measure the coil resistance of the relay.  If figure you still want the anti-chatter feature (capacitor) and if you know the coil resistance of the relay you can choose a single resistor value that limits the inrush current as well as reduces power requirements to drive the relay.  Even if the 12V DC relay can handle more than 12V DC from full-wave bridge rectified output with capacitor, there's no point in dissipating excessive power (heat) if you don't have to.

This project has been an ongoing headache for me!

 

So I now have everything "working" except I cannot take my 18V AC ground trigger rail to DC - the relay chatters when a train is on the isolated section.  I tried a diode and a capacitor, but it didn't make any difference. 

Can you post diagrams, hand-drawn sketches, photos, etc. of what you are trying?  This thread has discussed a wide-variety of options.  I'm sure we can get to the bottom of it.

 

Edit: Oh yes, do you have a multimeter?

Dave Hikel originally posted this diagram for an MTH signal.  You can (I have 2) use the MTH PRR signal bridge.  You will need a DZ-1008A relay for each signal head.  My signal bridges use 2 heads.

 

 

 

George

How about using a Z- Stuff 1075?

Here's what I have so far: a relay for each of the two loops and a board that takes the incoming 14vAC to 12vDC for powering the relay.  Now I need to turn the 18vAC ground from the trigger rail to DC so the relay doesn't chatter.

As far as Z-stuff, I don't want to have four sensors on the layout or spend $40 dollars in relays.  I have a very tight hobby budget; I have less than $10 in this now and am so close to having it right.

Hard to see from your pic, but it looks like the 14VAC to 12VDC converter is the hand-built board on the right...using, say, a 7812 type voltage regulator?  And is there a bridge-rectifier or just a single-diode for the AC-to-DC conversion portion of the circuit?

 

Is the 14VAC electrically isolated from your traction (track) 18VAC supply.  This is VERY important as was discussed earlier in this thread.

 

So if you're feeding the isolated outer-rail trigger to the trigger terminal of the relay module, the module must be configured for "low-level" triggering (vs. hi-level).  Again, hard to see from the pic but I think you're using the module that has this configuration capability using the push-on jumper plugs.  To provide anti-chatter, the capacitor goes between the module's VCC input (+) and the trigger input (-).  Something like 100uF (25V) will be more than adequate.

 

And of course the reason for the electrically isolated 14V AC source is to allow you to tie the "-" of the derived 12V DC supply to the outer-rail.

I have a couple of the MTH PRR signal bridges.  I just think they look so cool...

I brought a bunch of AC 12v relays from Mouser and the relay sockets, that puts you at about $17 per relay.  Early on I used 4amp rectifiers to convert the AC to DC but it was such a pain to solder up everything when wiring up the relays.  So I went with the sockets and changed over the rest of the signaling relays to 12v AC.  All you hear are the relays closing, a click and that is it.  I put them in over 5 years ago.  The contacts on the relays are rated for over 1 million uses....

 

Marty

Hey Stan,

 

the rectifier board was a DIY kit from eBay and has a bridge rectifier.  On the test bench this board works great with the meter. I use a Z-4000 for power; I used the 14vAC accessory posts for the rectifier board and track power is also from the Z-4000.

All the "black" posts on the Z-4000 are tied together.  The black post of course goes to the outer-rail.  If you run the Z-4000 14V AC accessory output thru the bridge-rectified 12V regulator module, the "-" output of that module is NOT in common with the "black" common Z-4000 posts.  The relay module is powered by the 12V DC and, if configured for "low-level" triggering, wants to see the "-" voltage to trigger it.  But the isolated-rail is NOT "-" when a wheel-axle connects it to the outer-rail!

 

The following thread is kind of difficult to follow but explains what's going on.

 

https://ogrforum.com/t...12#39441155848604212

 

So as configured in your picture, it won't (can't) work.  If you're sick-and-tired of this and just want a get-out-of-jail card that's one thing - use an isolated power source so let you tie the "-" of the 12V DC supply to the outer-rail.  I'd have to think about it but you might be able to use some external discrete components (diodes, capacitor, etc.) to "hack" a trigger to the relay module as-is using the "black" post outer-rail signal rather than the "-" of the 12V signal.