Bachman DCC Decoder Installation

[buzzard975]

Jeff and Jim - thanks for the latest posts.  Very informative.  I believe I understand everything you're both saying.  In one of my earlier posts I mentioned I was just going to switch the contacts on the top and bottom of the motor - so I got that covered.  Understood also on that metal strap across the top - it would dump the current straight to the motor which is the decoder's job.   I know that decoders need a good consistent current so I was already planning on powering the decoder straight off the trucks.  I've already had one truck completely apart (with the plastic removed and everything) so melting them won't be an issue.  I figure I might use a dremel to scratch up a small area to better accept the solder.  So should I run wires from the left side of both trucks together, then to one of the power inputs on the decoder, and do the same on the right?  That way each input will have 2 sources?

Thanks a lot for the tip on the wheels!   I've spent hours "googling" this stuff and never saw that mentioned.   That's the great thing about these boards - you get answers straight from people who know what they are doing.  So both of those companies make replacements for these Athearns with the proper gear and everything?

Jim - good advice on the bulb - I was already thinking about an alternative.  I will look much better when stationary and not emitting light out the bottom.   Thanks for the specs on the replacement bulb - I wouldn't have had a clue.  I know LEDs are popular too, and I think you just have to put a resistor in series with it?

Again, I really appreciate you guys taking the time to help.

[jward]

on the pickups from the trucks. YES!! most locomotives on the market to-day pick up power off both trucks. dcc is alot more sensitive to disruptions of track power, whatever the cause. the power pickups on both trucks was always a good idea, with dcc it is a must!!! while in theory it is possible to run dcc off one truck, in practice one truck pickup will cause you problems eventually.

as for the replacement wheels, they are just that-wheels. athearn wheels have stub axles that the axle gears press fit on. you reuse the athearn axles and simply replace the wheels. it is very simple, twist off the wheels from the axles, twist the new ones back on. the only tool you need is an nmra guage, something you probably already have, and should get if you don't have one.

please note: some older proto2000 and stewart, and walthers diesels use athearn copycat drives, and athearn parts can be used on these engines.

[Jim Banner]

Yes, an LED is also a good choice for headlight.  A 5 mm warm white looks good in these locomotives, and yes, you have to use a resistor.  1000 ohms, 1/4 watt is a good place to start.  The resistor can be in series with either LED lead.  If you look closely at the rim around the LED, it has a flat side.  The lead next to the flat connects to the white (negative) decoder lead.  Your LED may also have one lead longer than the other.  This lead connects to the decoder blue (positive) lead.  The resistor can be connected between one of the LED leads and its decoder wire or it can be cut into the wire closer to the decoder if you prefer.  Use shrink tubing to insulate the joints.  I often slip a piece of shrink tubing right over the resistor, insulating both ends and protecting the resistor, all at once.  If the tubing is clear, so much the better - you can see and read the resistor value.  But coloured shrink tubing will work too - enough of the shape of the resistor shows through for you to tell where the resistor is.

[Stephen D. Richards]

Jim,  when using the 12v light, do you have or have you noticed any excess heat?  I usually do this very option in my Atherns but on a very recent venture, I installed a 12v light in a Kato body switch engine and the area around the light buld  got hot and softened to a great degree!  I have repaired by cutting it off and gluing on a new one then installed an LED.  These were brand new bulbs from Micro Mark and were supposed to work directly off the decoder without any further modifications.    Stephen

[buzzard975]

Oh wow, I had no idea the wheels came off the axles.   Ok, I think I have enough info to be dangerous now!    Thanks to everybody for the informative posts.  I've learned a lot from them!

[Jim Banner]

Jim,  when using the 12v light, do you have or have you noticed any excess heat?

Using the original Athearn lamp in a half wave circuit (current return to one of the rails instead of the blue wire) means power is applied only on alternate half cycles of the DCC waveform.  This is turn means the light is operating at only half power so it does not get overly hot and lasts for a long time.

The 12 volt 25 milliamp bulbs are low power and do not get very warm at 12 volts.  If you multiply volts times amps to get watts, they produce only 300 milliwatts or .3 watts of heat.  Compare this to a grain-of-wheat bulb operating at 12 volts, 80 mA which is almost a full watt.  Surface temperature of a bulb does not depend on wattage alone.  It also depends on the surface area that is dissipating the heat.  A  T-1-3/4 bulb has about 5 times the surface area of a GOW bulb.  If the heat dissipation per unit area were constant, the surface of the GOW bulb would be about 15 times hotter than the T-1-3/4 bulb (3 times the power divided by .2 times the area.)  Things become a lot more complicated when part of the bulb is touching or embedded in plastic, but it is still easy to see why one bulb can be perfectly safe while another can destroy a locomotive shell or a passenger car body.

For comparison, a white LED typically operates at about 3 volts.  If we use a 1000 ohm resistor, and the output of the decoder is 12 volts, the voltage across the resistor is 9 volts (12 - 3) and so the current through it is 9 milliamps (9volts/1000 ohms.)  The power dissipated by the LED is 27 milliwatts (3 volts x 9 milliamps) which will cause virtually no temperature rise.  The resistor, on the other hand, will dissipate 81 milliwatts (9 volts x 9 milliamps) which is still only about a quarter of what the low power bulb produced.  No melted plastic here.  But let's suppose someone wants the maximum light out of his LED so he uses a 180 ohm resistor.  The voltages across the LED and the resistor stay the same, but now the LED is producing 150 milliwatts (no problem) while the resistor is producing 450 milliwatts (enough to melt lots of plastic.)  Suppose to make matters worse the installer chooses to use a 1/4 watt resistor.  At half a watt, this little resistor will almost glow in the dark.  It will certainly get hot enough to not only melt plastic but also cause it to smoke (not so good.)  Some times an owner wants his locomotive headlight to be as bright as possible.  The trick then is to spread the resistor heat out, such as using ten 18 ohm 1/4 watt resistors or five 36 ohm 1/2 watt resistors in series.  If space is really tight, a pair of 91 ohm 1/4 watt resistors epoxied to the frame can be used.  The epoxy helps conduct the heat from the resistors to the frame.

I apologize if this is more than you really wanted to know about light and heat, but I though you might be interested.

[Stephen D. Richards]

Jim, just what I was looking for.  Sorry I got you started but thanks for the info.  I was having some problems with the different bulbs and I now understand why.  I generally use a smaller resister with the LED's and vary them according to how much light output I like looking at with each locomotive.  I have used some 1/2 watt but gennerally stick with 1/4 watt.  Without really understanding why, they just seemed to work the best.  These bulbs were a pretty good deal, I thought, so I bought a bag of them.  (40).  Now I know why they were such a good deal!  lol  Thanks again for the explanation.   Stephen

[buzzard975]

Well I got my decoder installed, but I dont have the replacement wheels yet.  I ran it to see what happens and it will run most of the way around the track but will occasionally stall out and my controller flashes.  I am noticing lots of sparks that appear to be just inside the wheels around the axle.  I suppose this is a result of the wheels?   I am going to go ahead and order replacements.
On the bright side, Ive discovered how smooth these old Athearns are, even in DC mode.  Either way they are nice trains.   This is a bit of a tangent, but on these Athearn PA1s, some have metal trucks, some have plastic.  Two of mine have metal, one has plastic.  Is there any consensus one which is better, or is it just a matter of personal preference?  The metal ones are a bit heavier, but I think the ones are quieter.  As I look for new ones for my collection I am seeing the same road names in both configurations, and Im trying to decide which I should buy. 
Thanks.