Decoder installation problems

[pdlethbridge]

After my 0-6-0 T install went up in smoke, I took a careful look at the innards of the engine. I carefully cut the circuit board out of the loco and had several wires coming in from different directions. There were 2 sets of green and red wires, the front set was curled under the circuit board and were for the front light. There is a tube just in front of the cylinder saddle that directs the light from the under floor bulb up to the front light. I guess if the lights get out of position slightly you'll not get a light. The other 2 wires are long and go to the floor under the cab. Again there is a tube to direct the light up and out. Attached to each motor brush are 2 resistors??? and I left them on for later use. The last 2 wires are red and black and come up from the wheel wipers. The motor is a tiny thing but works great. With the circuit board out, I'll try another install, sanes the circuit board.

[Jim Banner]

Here is what a HD123 decoder looks like with its skin peeled off.



The four diodes form a bridge rectifier that convert the ac picked up by the wheels to dc to operate the motor, lights, and the decoder itself.  The two diodes on the left are connected from the left wheels (black wire) to the blue decoder wire (+ voltage for the lights) and to the decoder "ground."  The diodes on the right connect from the right wheels (red wire) to the blue wire and ground.  One of those left hand diodes blows if the blue wire accidentally connects to the right wheels.  And one of the right hand diodes blows if the blue wire accidentally connects to the left wheels.  When I say "connects to the right wheels" that includes connecting to anything that is connected to the right wheels.  Thus my concern about the headlight and rear light being isolated from the frame and from anything touching the frame.

The motor drivers each contain two transistors and some over current protection circuitry.  I believe the over current protection monitors the chip's temperature and shuts it down if the chip overheats.

The f0 light driver contains two solid state on/off switches to turn on the headlight and the rear light.  If the headlight or rear light draws too much current, this is the chip that pops.

The CPU or Microcontroller is the brains of the unit.  This is where the CV values are stored.  It also contains the firmware that makes the decoder do what it does.

The capacitor stores energy and allows the CPU to continue operating even during momentary power losses (think dirty track.)

A reminder to the uninitiated - the device shown in the photo is a "decoder."  It is NOT a "chip."  It contains half a dozen or more chips.  Each chip is a sliver of silicon (not silicone) with transistors, resistors etc. etched into it.  Each chip is encased in an epoxy package complete with external connections.  These are known as Integrated Circuits or ICs.  The decoder shown also contains diodes, resistors and a capacitor.  All these parts are soldered on to a double sided printed circuit board (PCB.)  The whole mess together makes a decoder.   

[pdlethbridge]

Great explanation! I've noticed a couple of things during my installs. Any resistors go on the blue wire, not the white or yellow. Whats with that? I have mine on the white and yellow wire. I tried to reset the factory settings on one decoder but could not do it by following their instructions of setting Cv 8 to 08.

[pdlethbridge]

I want to try something. I have a decoder wired to the turntable. red and black take power to the decoder and the gray and orange go to the turntable motor. Now I still have a few wires left, blue, white and yellow. Could I use them to power the lights in the roundhouse? I have several 1.5 v bulbs wired in parallel in the roundhouse but all are connected to the same 2 wires for power. How much resistance do I need per bulb on this set up?

[Jim Banner]

On a simple installation where either the headlight is on or the rear light is on, the two lamps can share a resistor as long as the lamps are identical.  The shared resistor in this case has to be in series with the lead they share - the blue one.  They can share a resistor in this case because they do not use it at the same time.  But on an installation where the lamps are switched independently, you have to use a separate resistor for each lamp because the lamps might both/all be on at the same time.  In general, you also need separate resistors if the lamps have different ratings.

Putting the resistor(s) in series with the blue lead helps protect the diodes in case of a short.  For example, a 36 ohm resistor would limit the current to 1/3 amp if the down stream side of it was accidentally touching a frame that was connected to the wheels.  This would not pop a diode.  If the same resistor was in series with either the white or the yellow, it would still limit the current through the light driver to 1/3 amp in case of an accidental short but this is over 3 times the current limit of the driver.

In the case of a decoder with many function outputs, loss of one output is not a disaster.  You can just use another output and remap it to the control button of the faulty one.  However, loss of a diode adversely affects all the function outputs and the motor output.

The resistance per lamp depends on both the voltage rating and the current rating of the lamps.  It is equal to the amount of voltage you need to get rid of divided by the current the lamp draws.  For example, if your lamps were rated at 1.5 volts, 15 milliamps, and you were using an E-Z Command system, you would need to get rid of

Voltage drop = 17 - 1.5 = 15.5 volts.

Dividing this voltage drop by the current, the resistance required would be

Resistance = 15.5 / .015 =  1033 ohms.

A stock value of 1100 or 1200 ohms would be just fine.  However, if your lamp was rated 1.5 volts, 50 milliamps, and you were using a Digitrax Zephyr as your command satation the required resistance would be

Voltage drop = 12.0 - 1.5 = 10.5 volts

Resistance = 10.5 / .050 =  210 ohms.

You would probably choose a 220 ohm resistor.  Resistors also have power ratings, measured in watts.  This is equal to the voltage across the resistor multiplied by the current through it.  In the first case, the power dissipated by the resistor would be

power = 15.5 x .015 = .233 watts

You would choose a 1/2 watt resistor to make sure it ran cool.  In the second case, the power dissipated by the resistor would be

power = 10.5 x .050 = .525 watts.

You would want a one watt resistor in this case.

To recap

Resistance = voltage drop DIVIDED by the current.  Choose next larger standard resistance value.

Power = voltage drop MULTIPLIED by the current.  Use a resistor rated for double this power, or even more, to keep it from melting plastic.

[pdlethbridge]

Ah nuts, My brain just cramped I appreciate the info, but it really went over my head. I have 4 grain of rice 1.5v bulbs in the roundhouse. They are wired in parallel so if I loose one bulb I won't loose them all. I use the NCE power cab which has a 13.5VDC regulated, what ever that means, The decoder is a digitrax gh123.

An e-bay site that sells LEDs suggested that dividing your source voltage by .02 would give the resistor needed.

[pdlethbridge]

While researching the use of bulbs and resitors on the net I found this info at tony's trains that might help someone besides myself.
http://www.tonystrains.com/technews/install-lamps-decoders.htm

[taz-of-boyds]

Pdlethbridge,

The LED info will not help with the 1.5 v grain of rice light bulbs.  But you do have half of the info you need on the bulbs, that is the voltage (1.5v)  You also need the current of the bulbs.  Jim discusses current below, that is milliamps (or amps/1000), you will note in the text he says 15 milliamps, but in the equation it ends up as .015 because 15 milliamps = .015 amps.

So, you also need to know the current of the bulb when operated at 1.5 volts, then you can just follow Jim's equations, or we can show you with your example if you can find the current rating of your grain of rice bulbs.

Once you discover the current rating of one bulb there is one more thing, you will want to give each bulb its own resistor, because if you use one resistor for all four bulbs, and a bulb burns out, the other bulbs will likely go soon since they will be taking too much current.

Have fun carefully!
Charles

[pdlethbridge]

I was going to try method 3. I all ready have the parts ordered. It shows only 1 resistor for all the bulbs. http://www.tonystrains.com/technews/install-lamps-decoders.htm

[taz-of-boyds]

Pdlethbridge,

You found some good stuff.  What I was warning you about is this, which is a simple circuit with one resistor:

"1.5 Volt Lamps - Wiring two 1.5 Volt lamps in parallel is not a good idea. When one lamp burns out, the other lamp immediately becomes bright and burns out shortly after."

The material you reference goes on to explain some more complex circuits that will allow bulbs to be wired in parallel.  Following the directions from your reference should work just fine.

Charles

[pdlethbridge]

I'll be trying it out today, just got the parts. I feel its better to ask than buy new light bulbs every day.

[pdlethbridge]

I got the parts and checked everything. The lights were working before the install and the decoder was working as well but when I installed the circuit for the bulbs, nothing. The turntable still works but the lights don't. I have the NCE power cab that has the light switch right on the controller so I can turn lights on and off if I need to.

[taz-of-boyds]

Do the lights operate while the turn table is moving?  Could you provide the exact details of the way you have the lights wired in the circuit to the controller?
Charles

[pdlethbridge]

I followed the step 3 instructions to the letter. While the turntable was moving I tried the lights and the turntable slwed a touch and when I shut them of the turntable sped up a bit. I used the blue and white wires off the decoder. My thoughts are that its working, but the circuit doesn't provide enough current to light the bulbs.

[Jim Banner]

How much current do those lights draw?  If you don't have a meter to measure it, seriously consider buying one.  You can get a digital voltmeter (DVM) that measures resistance, voltage and current for about $15, which is less than the price of the cheapest decoder.

DVMs are pretty near bullet proof.  A typical analogue VOM (Volt-Ohm-Milliameter) will go up in smoke if you have it on the wrong range when you go to make a measurement but with most DVMs the worst you can do is blow a cheap, replaceable fuse inside the meter.  And that only happens when it is set for measuring currents and try to measure a voltage.

If you (or anyone else) does not know how to use one, let me know and I will post some kind of primer.