the patience to answer.
I have several older locos that are not "DCC ready" (in any sense of the phrase) that I want to convert to DCC. I've found instructions on the web for most of them, and I think I can follow those.
But I don't understand why the motor has to be isolated from the frame in DCC but not in DC. It seems like the frame would have electrical contact with both rails, thus shorting out a DC motor. Or if the frame only has electrical contact with one rail, why does it need to be isolated for DCC?
Obviously I'm way off base, but I really would like to understand the principles involved. Thanks.
- Harry
In many older engines, the frame is part of the electrical circuit. However, it is not in contact with both rails, only one. If you take a look at an old Athearn engine, for example, you'll see that the truck frame is asymetric, that is, the left and right sides are different. One side has a plastic insulator that separates the side frame from the center. The other side is directly connected to the center. Power can flow up through the wheels, the side frames and the center of the truck, and eventually to the body and the motor of the locomotive. On the other side, there is a wire which connects the side frame to the motor, thus completing the circuit.
As David said, the decoder has to go between the rail power and the motor. So, that connection between the motor and the frame has to be broken. That's what's meant by "isolating" the motor.
It takes an iron man to play with a toy iron horse.
Harry,
It should also be explained that the DCC signal on the rails is not an electrical signal that is totally friendly to a DC motor. The decoder not only regulates the speed and direction but also "converts" the power to what the motor expects. Because of this fact it is not possible to use a "common ground" arrangement as was used on older DC models.
As you have by now figured out, those older DC locos only had one of the two motor leads "grounded" to the loco frame.
Today, even in DC, it is prefered to have the motor isolated from the bulk of the metal parts of the locomotive. Having large parts of the superstructure of a brass or die cast loco as part of the circuit is an invitation to short circuits.
Sheldon
One minor addition to the above.
It isn't necessary to isolate the entire motor from the frame - only the brushes. In most older open-frame motors this only involves breaking one solder joint (brush connection soldered to motor frame) or slipping insulation (from stripped wire) over the uninsulated end of the wishbone brush tension spring.
I, personally, have yet to encounter a motor where both brushes can't be isolated from the motor frame. There may be such, in which case it would be necessary to isolate the motor frame from the locomotive frame - if the manufacturer hasn't done so already.
Chuck (Modeling Central Japan in September, 1964)
Thanks for the replies. I've learned things from all of them, but I'm still not sure I "get it."
Please tell me if this is basically correct:
1. For simplicity, let's say the decoder has only four wires: 2 which receive the input from the rails, and 2 which output the signal to the motor.
2. If the motor (or brushes) are not isolated from the frame, then one of the decoder output wires will receive an input signal from one of the rails, which is definitely at cross purposes with the design.
I'm sure there's more to it, but does the above accurately describe why the decoder will get fried?
You got it right.
David Starr www.newsnorthwoods.blogspot.com
HarryHotspur ...then one of the decoder output wires will receive an input signal from one of the rails...
I bring this point up to emphasis the isolation of the motor from the rails. If the motor is isolated from the frame, you don't want to just cut a couple of wires and insert the decoder in the circuit and assume you are done. Always double check to make sure that both motor brushes are isolated from both rails before installing a decoder.
Thanks guys! Now that I can see the forest, maybe I can start learning about some of the trees.
CSX Robert I bring this point up to emphasis the isolation of the motor from the rails. If the motor is isolated from the frame, you don't want to just cut a couple of wires and insert the decoder in the circuit and assume you are done. Always double check to make sure that both motor brushes are isolated from both rails before installing a decoder.
Okay, but how do I do that?
HarryHotspurCSX Robert I bring this point up to emphasis the isolation of the motor from the rails. If the motor is isolated from the frame, you don't want to just cut a couple of wires and insert the decoder in the circuit and assume you are done. Always double check to make sure that both motor brushes are isolated from both rails before installing a decoder. Okay, but how do I do that?
One thing to understand is in DC you're controlling the DC volts and amps being sent to the track, which goes from the track to the motor of any engine on that section of track.
In DCC the track has a constant 14V AC on it. The pickups from the engine connect to the decoder, and the decoder connects to the motor. The signal from your DCC throttle tells the decoder in that particular engine how much power to draw from the 14V AC and convert to DC and feed to the motor to make the engine move.
If the motor isn't grounded, when you put it on the track with DCC it's going to be getting the DC power from the decoder and the full 14V AC from the track...for a few seconds until the decoder starts smoking!!
For a slightly more technical explanation of why this causes the magic smoke to leave the decoder at high speed, look up a circuit called an H-Bridge. If this one works, great:
In a decoder, the motor is in the middle just like this, and the lines labeled +2.2-9 VDC and GND at the top and bottom are the track connections. The 4 resistors, 2 on each side, connect to the cpu in the decoder and are what controls the transistors to allow power to flow to the motor.
Now, say your motor is not isolated properly, and the side of the motor labeled + is connected to the same track pickup as the top input. You giveit some juice, and transistor Q1 turns on - oops! full track voltage with no resistance to reduce the current is switched through that transistor. Smoke generator time.
--Randy
edit: fixed the img tags. doh!
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
I recently bought a multi meter, but it's still in the box, so I haven't learned how to use it yet. But I will soon.
CSX RobertHarryHotspurCSX Robert I bring this point up to emphasis the isolation of the motor from the rails. If the motor is isolated from the frame, you don't want to just cut a couple of wires and insert the decoder in the circuit and assume you are done. Always double check to make sure that both motor brushes are isolated from both rails before installing a decoder. Okay, but how do I do that? Two ways: Sometimes a visual inspection is enough. If you can clearly see that the brushes are in no way connected to the rails, you should be good to go. If a visual inspection is not enough, use a multi-meter to check for continuity between each brush and each rail.
That makes sense, but walk me through this if you will. Right now I'm looking at a Mehano 4 wheel diesel industrial loco (a Plymouth I think) which I bought maybe 15 years ago for about $10.00. It must be the simplest engine ever made, and I'm sure some of you are familiar with it.
Anyway, it has a plastic frame with brass pickups on each side which rub against the inside of the 4 wheels. Only the rear wheels are powered, and the motor sits vertically above the rear wheels. The drive shaft has a gear which meshes with a gear on the rear axle.
I would guess the motor is called a can motor, although it is not exactly round. Two opposite sides are flat, the entire motor is enclosed in a metal shell, and the end piece on the top is made of what looks like nylon.
One wire runs from each brass pickup to a small metal terminal protruding from each side of the nylon top piece. This is probably a lot of irrelevant info, but I just don't have the experience to know, so here are my questions (finally):
1. Given the above description, it appears that cutting the wires from the pickups to the motor terminals, and then installing a decoder in the middle would work. I can't see any way the motor could make electrical contact with the rails except through the wires.
2. But to make sure (after I learn how to use my multimeter), would I connect one lead from the meter to the right side rail and the other lead to the right side terminal, and then repeat the process on the left side, before installing the decoder?
3. I haven't mentioned brushes because I can't see anything that looks like a brush, so do I need to worry about them in this instance? If I do have another engine where brushes are visible, what would I connect the multimeter leads to?
Finally, thanks again to all who replied. This thread has been more helpful than all the articles I've read about installing decoders.
First, before you cut any wires on the motor make a note of which wire connects to the right and which wire connects to the left rail. You'll need this info later.
Now cut or desolder the wires from the motor terminals. Get your multimeter out and set it to Ohms or Continuity ( depends on the meter-some have both, others have ohms only)
Now test the meter by touching its two probes. The meter should register infinity or 0. That means the meter is working.
Make sure the loco is not sitting on a powered track . Touch one probe to the wheels that have pickup and the other probe to one of the now disconnected motor tabs. You should get no reading. Now move the one probe to the other motor tab and there should again be no reading. Repeat these tests on both motor tabes and all wheels and metal frame areas. if they all have no reading, you have the motor brushes isolated.
You should test the lights also to make sure there is no connecion between them and the rails or frame.
Now it's time to install the decoder . Cut the gray and orange wires to the desired length and connect each one to the motor tabs. The Orange wire should be connected to the tab that originally was connected to the right rail. Gray goes to the tab that was connected to the left rail.
Red wire connects to the right rail and black to the left rail. You can splice to the same wires that were originally connected to the motor or replace them tom their source. Choose the method that makes the neatest job. Insulate all connections with shrink tube, insulation paint, tape, etc
Now test the installation again with your meter and make sure there is still no reading between the motor tabs and the rails.
Connect the light(s) using the blue, white and yellow wires. Blue wir goes to the anode (positive) if the light is an led. Either side if its a bulb.
White wire goes to the cathode (negative) of the front LED or the other connection if a bulb. Yellow wire goes to the rear light in the same manner. If there are two lights, you will need to splice an extra piece of blue wire to also connect it to the rear light. If there is anly one light (cab diesel) both the white and yellow wires can be connected to the same terminal on the light. That's your choice. Light would function in both directions.
Now it' s time to test your work. Place the loco on the programming track and try reading its address or changing its address if your system doesn't have the CV read feature. If this step is successful, you are safe to place it on the full powered track and test run.
Martin Myers
That is very, very helpful. Exactly what I needed to know. Thanks a lot, Martin.