Well, I guess it depends. Does it alternate polarity? Then I suppose it is AC - what would constitute 'true AC'? Only sine waves? Sawtooths and square waves don't count? Granted it's generated from a DC source.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
@Stix:
...just to verify that I don't have things all screwed up in my head... the voltage on the rails is a bipolar DC waveform...not true AC, right?
Dave Loman
My site: The Rusty Spike
"It's a penny for your thoughts, but you have to put your 2 cents in.... hey, someone's making a penny!"
Ya it's more likely a problem of amps rather than volts. Many decoders have a max rating of 1 amp, so an engine whose motor is drawing more than that would be the issue.
Remember how DCC works - the rails have a constant 14V AC on them. The decoder converts the AC to DC, and the instructions you send the decoder from your throttle tells the decoder how much power to draw from the track and convert to DC and send to the motor, usually with a maximum of 12V DC. The CV's tell the motor how much power to draw at each step. If CV5 is 250, when you turn the throttle all the way up the decoder is going to be delivering just about all the power it can (12VDC). If you change CV5 to 125, when you turn the throttle all the way up, the decoder will only provide about half the available power to the motor (about 6 VDC).
Anyway, it sounds like you can either install a decoder made to handle more amps, or install a new motor in the troublesome engine.
Well, the highest rated ones are in the neighborhood of 4-8 amps and are far too large to fit in an HO locomotive. It's usually a size issue. Some smaller HO locos can;t fit an 'HO' decoder so you end up having to use one designated 'N' scale. Decoders don;t have a scale, breakign them down into scales is more to get an idea of physical size and in some cases current rating. And the opposite question - why not use the smallest one - well, the smaller they are the more expensive they are.
As for the high starting voltage. Again, No. It did not burn up the decoder because the motor needed too high a starting voltage,. I burned up the decoder because there was something wrong with the motor which made it need too high a starting voltage, which you proved when you replaced it and put another decoder in and it now runs fine. Actually I shouldn't have said too high a voltage in the previous sentence either - if your DCC system puts 14 volts on the track then the most it can send to the motor is 14 volts (and actually a bit less because of losses in the motor drive circuitry). Some systems can be set to deliver 20 volts or more to the track (for runnign G scale) and that indeed could be too high a voltage. But if that is exceeding the decoder's maximum voltage, it would damage the decoder even with no motor connected to it. And setting the start voltage CV doesn't reduce anything - it just puts that much more voltage to the motor at speed step 1 so you don't have to crank the knob halfway to get the loco to move.
Again - the problem was that the motor drew too much current, that's what melted the decoder. The high starting voltage was because it needed that much total enrgy just to get the motor to move. Most modern HO loco motors should draw 1 amp or less if they are in working condition. The better ones should be much less. But never assume, grab the meter and a DC power pack and check. There have been 'ringers', one run of the Proto E units for example, that sucked up as much as 2.5 amps - that due to the motors being made that way by mistake. Old open-frame motors, and Athearn motors from before the gold-sided ones are all probably well over 1 amp.
cudakenFew weeks ago I saw a posting about a Athearn RTR SD-50 that took 50 to 60% percent throttle to start moving. I jumped in waring the owner I had the same engine, same decoder and same problem. The decoder ended up melting. More learned folks here said it was the CV's where not right. Owner corrected the CV's and problem went away.
Claymore, mine SD 50 did melt the decoder. After it ate the decoder, I changed the motor and PC board, reinstalled a new DH123 decoder and it runs great. But I did not do it in steps, in other words was it the first decoder with a to high of starting voltage, bad motor or the PC board? I am going to guess it was the motor. I will fine out later this week when it goes into my FP-45 that I took the motor out of.
cudakenToday, I took one DH123 out of my PK 2000 BL 2 and installed a different DH 123 (reason will be in a different question I will be posting). Same engine with the same type of decoder now takes 2 times the power to start moving and has half the speed with only one car in tow.
Armchair, I have 4 Proto 2000 BL 2's. This one was on the RIP track listed as having a bad motor. Guess it had sat for 2 years. I found it was missing a brush spring. Reason I took it a part to see what I was getting into as far as the conversion. If it did not run, well I could not break it. Ran it on a test stand on DC after I got it running, lubed and ran some more. Installed the decoder still seemed a little stiff, lubed and ran it some more. Finally got to the point it started at around 20% power with a drag. Changed the decoder and that is what started this posting.
On the MM meter, for $5.00 what the heck.
Now the dumb question, I know there are different ratings for decoders. Why not just buy the highest rated decoder and be done with it?
I want to thank all of you great folks for trying to teach me the way of DCC. Electrical work is far from being my strong point. Funny thing is I can degree in a cam shaft, set valve lash, jet a carb, build a motor and paint a car. Comes to DCC, all thumbs. I have had more train engines go bad in 3 years than real car engiens in 30 years.
Cuda Ken
I hate Rust
I posted in one of the other "Ken's burning up locos again" about using a multimeter to check the current draw on DC WITHOUT a decoder. If you don;t have one, there's a $5 one at Harbor Freight that works just fine and has a 10 amp 'high amp' range which, while not super accurate, it plenty accurate for deciding on which decoder to use. No excuse not to have a digital meter when they are only $5. And highly useful for other things. The $5 one doesn;t have the 'beep' continuity check, they have one for a couple bucks more that does.
cudakenDave, there is a reason I want to talk with you. How do you test the motor when it is not running?
You need to hook the motor leads to a DC power supply. Make sure the amp-meter is in series with the power (obviously). Then turn on the power up to 12 volts. Stall the motor with your finger, or press down on the chassis till the motor stops moving.
The current going across the motor is the stall current. WARNING: Make sure you DCC decoder is well unhooked from the DC supply.
Don - Specializing in layout DC->DCC conversions
Modeling C&O transition era and steel industries There's Nothing Like Big Steam!
cudaken , is this the same BL2 You changed the gears in on one of my other posts ? If it is maybe like Dave suggested ,the gears are dry &/or misaligned ? What kind of system are You running that You can't set cv's ? I know you've had some bad luck by reading some of Your other posts.
davidmbedardCSX RobertcudakenHow do you test the motor when it is not running? The only way to test the current draw of a motor is with it running. No. Stall current is tested while the motor is NOT moving but full voltage is given to it...
CSX RobertcudakenHow do you test the motor when it is not running? The only way to test the current draw of a motor is with it running.
cudakenHow do you test the motor when it is not running?
No. Stall current is tested while the motor is NOT moving but full voltage is given to it...
@cudaken:
Did *your* decoder end up melting, or was it the person's who made the post you mentioned? *confused*
Regardless, basic motor therory states that a motor's starting current will be significantly higher than running current.... SO, having to 'rev' your throttle to 50% and then throttle down to the desired, say, 25% sounds perfectly normal to me.
If the motor is attached to any kind of mechanical load, then that could easily make starting current even higher. If the gears are a bit dirty, misaligned, not lubed properly, or if the universal joints connecting the motor to the gear box(es) are dirty, misaligned, etc, then this all added mechanical load, which adds to the potiential peak starting current.
If that model's gearboxes, drivetrain and motor are all designed such that a higher mechanical load is 'normal operation' and the decoder is undersized(amperage wise), then its possible that repeated 'high current' draws on motor start fried the decoder.
Perhaps a good cleaning/overhaul and a new decoder with a higher amp rating would be the trick?
(Its rather odd, but I am in the process of overhauling an old Athearn SouthPac GP40-2 for the reason of smoothing the starting action/lowering starting amp draw... small world eh?)
My answer still stands. If it takes half throttle to start that's not going to cause the decoder or the motor to melt.
However if it takes half throttle to start that could indicate a problem, not just that the motor needs a lot of volts to start - P2Ks usually don't need a huge adjustment to start volts to get them to move at a low throttle setting.
cudakenToday, I took one DH123 out of my PK 2000 BL 2 and installed a different DH 123 (reason will be in a different question I will be posting). Same engine with the same type of decoder now takes 2 times the power to start moving and has half the speed with only one car in tow. For what ever reason, same type of new decoder needs more power to start. I only thing I can think of is starting voltage CV is not the same, right?
Rinker, read above again please.
Dave, there is a reason I want to talk with you. How do you test the motor when it is not running?
Ken
Answer to the question: No. However, the fact that it needs to be cranked way up to start moving probably means there is a problem with the motor or drivetrain wich is binding it up and making the motor draw more than the normal amount of current.
Few weeks ago I saw a posting about a Athearn RTR SD-50 that took 50 to 60% percent throttle to start moving. I jumped in waring the owner I had the same engine, same decoder and same problem. The decoder ended up melting. More learned folks here said it was the CV's where not right. Owner corrected the CV's and problem went away.
Today, I took one DH123 out of my PK 2000 BL 2 and installed a different DH 123 (reason will be in a different question I will be posting). Same engine with the same type of decoder now takes 2 times the power to start moving and has half the speed with only one car in tow. For what ever reason, same type of new decoder needs more power to start. I only thing I can think of is starting voltage CV is not the same, right?
For now I cannot read or change CV's, so the way they come is the way they are run.
Meat of the question, with my layout and engines eating decoders. Could a higher starting voltage CV be the root of the problem? Most of the engines that cooked it decoder need say 50 to 60 percent to get moving. Could it be that simple?
One old Athearn BB that ate two PC boards, problem was where the wires where solder to the trucks and has been fixed.