I need a "simple"way to detect polarity of a reverse loop for "DC"


T.C.

T.C.

Active Member
I don't know if I can say "DC" anymore and not get rocks thrown at me? lol
Anyway here goes, my grandson and I have cleaned the track on my model RR that hasn't been run for over 30 years.
It is powered by a "DC" power supply that my father had built 50 years ago, it's a very large transformer with a rectifier that is made of large metal plates that are 8" to 10" square. He used it to power a commercial slot car track back in the 60's, I think this is just one of many because the track was 1/24 scale with 8 lanes and had a 100' backstretch .
I built the layout when my son was born about 35 years ago, then "life" got in the way of working on it so it sat until last week when my grandson said "paw-paw can we play trains?"
That's all it took, we got it all running except for the reverse loop polarity detector that turned on "go"-"no go" signals that I had built many years ago.
I found the plans in a mag and built it with radio shack parts, I surprised myself because it actually worked, it didn't switch automatic but the led's did tell me weather the polarity was OK to enter or leave the loop . (now I cant find the plans, like to rebuild it) quadcomparator comes to mind when I think about it ?

Sorry to be so long winded but with everything plug and play nowadays I figured it may need some explanation?
I guess I should ask if any one knows of a simple circuit that can read the polarity of the loop and main and give me a red or green ?
I am really looking for something that I can install from above as It's getting hard to get under the layout, or should I say it's getting harder to get out from under the layout? lol
Thanks for any help
Terry
 
I am not at all familiar with your set up, but a small multi-meter can detect polarity. You can pick them up for $10-$20.

Good job Paw-Paw!
 
Get a railroad signal light with red and green bulbs or LED's (with LED's you'll need resistors to drop the track voltage so you don't burn them out). Simply connect one lead from the red lamp to one side of the insulated gap between the main track and the other to the other side of the gap on the reverse loop. If the red light comes on, the polarity doesn't match. Connect one lead from the green light to the same rail on the main track that you did the red wire, but connect the other lead from the green lamp to the other rail on the reverse loop side of the gap (you need gaps in both rails between the main and reverse loop). When the polarities are correct, the green light will come on. Hope this helps.
 
Thanks for the reply Louis
I guess I didn't explain it very well, what I want to be able to do is have a signal that shows if the reverse loop section has the correct polarity to enter or exit without causing a short.
That is what the electronic component I made years ago did, it compered the plus and minus of the main line to the plus and minus of the reverse loop section, then gave either a red or green lite on a track side signal to let me know if it was OK to enter or exit .
I wish I could find the schematic, I would rebuild the one I have.
I guess it would be like an auto reverser, but I know nothing about those so I'm not sure about that ?
Make and sense?
Thanks Terry
 
Thanks Trailrider
I will certainly try your method it sounds like the answer for my problem, actually I created my problem by burning up what I had, it was all up and running 35 years ago.
I actually have two reverse loops in my track and it's all running now, it's just hard to tell weather I can enter a loop w/o some kind of indicator light.
If I may ask a few questions ?
Would it work ok to make the bulbs or LED's a constant brightness and did you notice any loss of power in you engines when the bulb/led's were on ?
Thanks Terry
 
Terry It was me who misunderstood you, forgive me.

I'm glad you found a solution.

LEDs draw an insignificant amount of power and should not affect your locomotive.
 
T.C. said:
Thanks Trailrider
I will certainly try your method it sounds like the answer for my problem, actually I created my problem by burning up what I had, it was all up and running 35 years ago.
I actually have two reverse loops in my track and it's all running now, it's just hard to tell weather I can enter a loop w/o some kind of indicator light.
If I may ask a few questions ?
Would it work ok to make the bulbs or LED's a constant brightness and did you notice any loss of power in you engines when the bulb/led's were on ?
Thanks Terry
Click to expand...
Neither should draw any detectable power from you locomotives. NJ International has some great signals, assembled and ready to go. Be sure to check whether these are incandescent bulbs or whether they are LED's and whether they require resistors or have them built in. The only drawback to this method is if the entrance to the reverse loop is where you can't see the signal, in which case you might have to use longer wires and put the signal in a more visible spot. I use this method for showing how turnouts are thrown, as well. Of course if you use this with Atlas Snap-Trak or Customline, you may need to use a relay with contacts. For turnouts that have electrofrogs, and can cut off power to the oncoming track when the points are thrown against it, the signals can have the light wires connected to the frog (common) and the stock rails.

Oh, yes, for reverse loop polarity, you can set a signal at both "ends" of the reverse loop. Obviously, when the one signal shows green (matching polarity) at one approach, it will show red for the other.
 
Most LEDs will only illuminate when correct polarity is applied. You can get a red and a green LED and connect the hot lead of one to one rail, and the hot lead of the other one to the other rail. You can install them on a signal, or simply put them on a trim piece where they are visible.
If you only want one LED, some are availavle that change color according to polarity. Just don't forget to use resistors, most LEDs run off roughly 4v.
 
Thank you all for the help, you have given me some good ideas, I think I like the LED idea better just because they may not be as variable in there brightness as the bulbs . Also I have signals that I made 35 years ago with LED's already in them, I just need to add the proper resistors and wire them to the track
I have two loop sections, one I can see both ends and one has a hidden end. All I would have to do is read the color of the hidden section at the end I can see and if it is red then it would mean the loop was good to enter from the hidden end. Sound rite?
Thanks Terry
 
yep, LED's with resistor at the break point of the reverse loop track will light up if polarity is wrong, heck, a simple lamp will do that too. you would need 2 LEd's because of their polarity detection, the reverse loop if you switchh direction you may not get the LED to light, why you need 2 in reverse polarity wiring.
I have a reverse loop on my layout and I plan this exact thing using DCC, only need 1 LED because of the AC nature of the DCC, real simple.
 
THe rectifiers with metal plates are likely old selenium rectifiers that were used from the 1930s to 1960s before good silicon diodes replaced them for mere pennies.

They are rather dangerous and can emit bad fumes when they fail - usually with flames. Make a mental note to find a more modern power pack if the renewed hobby keeps up good momentum.

here's a youtube video some guy made of a small one burning up in a old tube radio set.

https://www.youtube.com/watch?v=zU8C5p7CQ_c
 
Thanks for the tip, that's what they are .
Any suggestions as to a good power supply ?
I'm not interested a new system as I have everything wired for DC, I had made throttle boxes made with MRC rheostats and small Radio shack boxes with coil cord like used with a CB radio so I could move around while I run trains.
It's all prey much old school stuff but it works really well for what I was/am doing.
I guess I would need a filtered DC power supply ? But witch one ?
Terry
 
Probably any of the MRC DC throttles would work, depending on the power rating. I've had good luck with that brand over the years.
 
T.C. said:
Any suggestions as to a good power supply ?
Click to expand...
If you are only going to use it to power your current throttles and not eight lanes of slot car tracks :D ... I frequent the local thrift stores and look for 12-14VDC wall warts, or lap top computer supplies that have a amp rating of 2 or more.

I guess I would need a filtered DC power supply ? But witch one ?
Click to expand...
um, don't know. I ran DC on non-filtered power for years and years. All the ones I have purchased for my DCC systems are AC. You would have to add your own rectification and filters to any of those.
 
Thanks again for the replies:
I like Iron Horseman's suggestion as I have two control panels with AMP and Volt meters along with block control, turnout switches and sound system.
I really have nowhere to set a power pack that I would have to tend to all the time.
I made two controllers, one is a rheostat type and the other is a transistor type, both work but as most of my engines have older open frame Pitman motors in them the rheostat seems to work better.
Two things I'm worried about though
# 1 the fire thing although it doesn't smell or get hot it is switched and has a circuit braker .
# 2 the fact it's not filtered DC, I'm confused about weather that's a good thing or not? I'm thinking if I want to run some can motors, will they may be trashed because it's not clean DC?
On the other hand I know for a fact that what I have is better than any China POS that I can buy a Wally World, I guess the computer power supply may be an option if need be?
I see a lot of plans for building power supply's .................... but witch one is the best ? There are so many it's mind boggling and then that's another big project taking up time I could use building structures or scenery.
Terry
 
T.C. said:
Two things I'm worried about though
# 2 the fact it's not filtered DC, I'm confused about weather that's a good thing or not? I'm thinking if I want to run some can motors, will they may be trashed because it's not clean DC?
Click to expand...
Unless they are also coreless I don't think so. Even then I don't think the frequency is high enough to cause any damage. I've been running can motors since 1983 or so on even old MRC gold case supplies. Not noticed any issues. If you are really really worried about it you can just add some capacitors of various sizes in parallel to the power output to filter the DC.
 
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So in a nut shell, If I like the system I have and the way it operates there is no reason to change anything ?
Or has it come to light that the old selenium rectifiers are prone to catching on fire or are a hazard ? Is there an expiration date on them ?
I see on the news now and then where a new type power converter or charger has caught fire and burnt down a house, so I guess you take chances with most anything you plug into the wall .
Thanks again for the replies
Terry
 
T.C. said:
So in a nut shell, If I like the system I have and the way it operates there is no reason to change anything ?
Or has it come to light that the old selenium rectifiers are prone to catching on fire or are a hazard ? Is there an expiration date on them ?
I see on the news now and then where a new type power converter or charger has caught fire and burnt down a house, so I guess you take chances with most anything you plug into the wall .
Thanks again for the replies
Terry
Click to expand...

I'm a little late to the party here but I thought I would add my .02. There is no need to use a vintage selenium rectifier, especially if there is a chance of starting a fire. Any small rectifier, say something from Radio Shack or eBay will perform nicely. May of them are rated at 25, 50 or even 100 amps and they are very safe and provide excellent DC output. This is the type that I'm talking about,

Good luck!

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Thank for the reply, I am considering taking you guys advise and changing out the rectifier.
I guess the wiring is straight forward AC in goes to AC, DC out goes to track control system.

One question I have is looking at the part pictured above there is a hole in the center, does this need to be mounted to a heat sink?
Thanks Terry
 
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