Railpower 1300 testing

[nkalanaga]

On the DC locos with headlights on both ends, the two LEDs should protect each other, even if they have separate resistors.  And if the DCC decoders can supply motor power for even a few milliseconds that should eliminate most momentary power losses, which is what causes the flickering of the rear headlights on DC.

Thank you!

[peteski]

On the DC locos with headlights on both ends, the two LEDs should protect each other, even if they have separate resistors.  And if the DCC decoders can supply motor power for even a few milliseconds that should eliminate most momentary power losses, which is what causes the flickering of the rear headlights on DC.

Thank you!

Unfortunately if each LED has separate resistor, it will not protect the other one from a high voltage spike.  It is because of that 2nd resistor.  Usually in that type of circuit there is a small capacitor in parallel with each LED.  Also, those milliseconds of stored energy is just for the decoder's electronics, not for the motor. If that capacitor was not there, the decoder would reboot with even the shortest loss of power. During those short power drops the motor's inertia keeps the loco running.  Only a very large capacitance add-on  keep-alive circuit will keep the decoder powered and the motor running over longer power dropouts.

[peteski]

Looks like we have a new example of a decoder damaged by a DC throttle.  This time it is a MRC 4-4-0 loco and a MRC Tech 4 260 throttle.  Interesting . . .  But it could have been just a weak component that burned up - we will never really know for sure.

See: https://www.therailwire.net/forum/index.php?topic=44738.0

[CBQ Fan]

Once I cut out the board I can drop it into an envelope and mail it to anyone who would want to inspect it.

[mmagliaro]

Once I cut out the board I can drop it into an envelope and mail it to anyone who would want to inspect it.

Actually, I'd be more interested in seeing an oscilloscope trace of the output of a Tech 4 260.   It could be a weak component in the decoder board or just a fluke.  But it could also be another power pack that has an output that's on the high side.

EDIT
This fellow scope an MRC 220, which I believe would be the same as the 260 for the voltage pulses
http://www.sumidacrossing.org/LayoutElectricity/ModelTrainPower/PowerPackTesting/
The good news is that the half-wave pulses fade out as the throttle is turned up, so you don't have peaks jumping above the top voltage level.  The bad news is that it really does put out about 19 VDC under load, straight DC.  That's awfully high.
I think it's well under the guidelines for a decoder, however, so I wouldn't worry about that.  I'd be more worried about running straight DC engines at high throttle settings on this thing.

EDIT

The MRC Tech 4 260 manual says the output on the track terminals is 23 VDC (as opposed to the 1300/1370, which say 15 VDC).   We know from scoping the 1300 that the *RMS* output is about 15, and the peaks are about 25.  So now I have to wonder if the 260's  23 volt rating is RMS.   I would tend to bet that it is, because a customer putting a volt meter on it would expect to see what's in the instruction manual.

Does anybody have a 260 AND an oscilloscope who can look at it?  (Or want to send me the 260 for a few days to scope it?)

[alhoop]

There are also components like diodes and a transistor in series with the transformer's secondary winding (coil) which also seem like they would prevent any inductive kickback voltage.  And on the DCC-equipped locomotive side of the equation, we also have rectifier diodes and couple of transistors in series with the motor's coils to minimize any inductive kickback from the motor's coils.  I'm also slightly rusty on my AC theoretical knowledge, but I don't believe that momentary loss of contact between the loco wheels and the track would generate any inductive-based spikes.

I think it could:
http://www.x2y.com/publications/dcmotors/feb14-05.pdf
You can do further checks with an AM radio while running an engine close to the radio.
AL

[peteski]

I think it could:
http://www.x2y.com/publications/dcmotors/feb14-05.pdf
You can do further checks with an AM radio while running an engine close to the radio.
AL

Yes, with a DC loco (where the motor is wired directly to the track), its motor windings are coils, so yes, when the energized coil (one of the motor poles) is abruptly disconnected from its power source (track), there will be kickback voltage generated.  I'm not disputing that.  You will also hear lots of nose on that AM radio from the motor brushes crossing the commutator segments.

Maybe I was not clear enough in my post. We are discussing a locomotive with a decoder installed.  In those, the motor (and its poles) is isolated from the track by the motor driving transistors, and then further by the bridge rectifier).  Those components block the  motor's inductive kickback voltage from reaching the track when the loco is lifted.  Try that with AM radio.  The only RFI noise you will hear is the radio frequency noise generated by the motor's brushes when running.  If you were to attach an oscilloscope to monitor the track voltage and you lifted a DCC loco of the track, there will be no inductive spike.

[Doug G.]

My MRC Tech IV 260 measures 16.9 VAC RMS at terminals, no load and about 20 VDC at the track with no load, full output. I don't have an oscilloscope.

I don't believe the maximum output is really a problem with DC locos because the loco is going way faster at that setting than one would ever run it and current draw is way more important than voltage as far as heat. The current draw doesn't change much over the output range.

Doug

[mmagliaro]

My MRC Tech IV 260 measures 16.9 VAC RMS at terminals, no load and about 20 VDC at the track with no load, full output. I don't have an oscilloscope.

I don't believe the maximum output is really a problem with DC locos because the loco is going way faster at that setting than one would ever run it and current draw is way more important than voltage as far as heat. The current draw doesn't change much over the output range.

Doug

Doug, can you measure it with a simple load on it?  Something like a grain-of-wheat lamp connected across the rails, or a lighted passenger car (so you can turn it all the way up and not worry about an engine zooming off into outer space).  I'd be curious to see if that 20 VDC  is just "floating" because there is no load present.

I think the speed issue is something that power pack makers count on, but they shouldn't.  Why output a voltage so high that nobody will ever use it, especially when motors are going to get dangerously hot if you ever crank it up.

While it's true that current is usually the "motor killer", remember that the total power consumed by the motor is  V x I, so as the voltage rises, even if the current stays constant, the motor is going to make more mechanical power and it is going to generate more heat. 

I also don't agree that the current is constant.  True, nobody is going to run an engine at 20 volts, normally, because it will be going way too fast.  But if an engine is heavily loaded down or going up a hill, it definitely draws more current.  And if the user decides to push up the throttle because the engine is slowing down, now you've got higher voltage + higher current + more heat.  Maybe they won't push it to 20, but maybe 15?  16? 

I have never understood what it is with power pack makers and these voltages.  What happened to the "12 volt standard"?  Why can't they just build a power pack that maxes out at 12 volts, RMS, under load, and ALSO maxes out at 12 volts RMS under very light load?  It's not that hard.  It wasn't even hard in 1975.

[alhoop]

Here is a pertinent quote from another forum.

"The only system in the past known to put too much voltage on the track was the Atlas Master/Lenz Compact when used with  the recommended power supply. A string of back to back diodes was used to reduce the voltage, which you could build yourself. MRC sold a premade one, so perhaps one of their older systems also tended to be too high a voltage to the track - probably that honking 8 amp booster they made."

 I think Jason is correct, MRC should warn their customers about
the problems Rapido is having with the MRC 1300 and include that diode string with all 1300s along
with the warning.

PS- Question for Max: what was your line voltage for the scope tests - 110v,120v or 130v ?
Al

[mmagliaro]

I don't know what the line voltage was when I scoped the 1300, but assuming it's not changing much, right now it is 121 VAC.

Where did you find that other info on high voltage?  I would like to read it.  (You can send me a PM if it is somehow not allowed to post links to other forums).

[Doug G.]

Today:

MRC Tech IV 260:

Full output (DC to track) - 20.7 VDC - No load

Full output - 19.5 VDC - Atlas/Rivarossi illuminated passenger car load (approx. 50 mA)

Full output - 18.5 VDC - MRC RSD-15 locomotive load (approx. 200 mA).

Full output - 18.5 VDC -Same as above but with a Kato SD45 loco.

Full output - 16.5 VDC - Kato SD45 full slip (I briefly held it in place so the wheels spun)

I believe the reason most power pack makers don't make them so the voltage is a constant 12 volts at full output is cost. The pack would have to have a fully regulated supply for that which, of course, means a bridge and associated circuitry. More money to build.

If you watch a current meter connected to the pack output with a locomotive on the rails, you will see that the current draw does, indeed, increase very little as the speed is increased. The draw doesn't really go a lot higher until you really load it down, like when MR did the full-slip test (do they still do those electrical tests in a review)?

[peteski]

If you watch a current meter connected to the pack output with a locomotive on the rails, you will see that the current draw does, indeed, increase very little as the speed is increased. The draw doesn't really go a lot higher until you really load it down, like when MR did the full-slip test (do they still do those electrical tests in a review)?

That is a perfectly normal behavior for unloaded (or lightly loaded) permanent-magnet DC motor. The (reverse polarity) BEMF generated by the rotating armature actually reduces the current consumed by the motor.

[mmagliaro]

Well, the cost for me to cap the low-end Railpower 1300 at 12 volts was exactly the price of a single zener diode, which costs me 50 cents if I buy just a few at retail pricing, and would cost a manufacturer pennies.

As for load, yes, an engine by itself won't increase current draw at any speed.  Like I said, under load (as in pulling a string of cars) and up a hill, if certainly will.   The coreless motors I put in most of my engines will typically draw only 50 mA until the engine climbs a hill with a string of cars.  Then it will jump to over 100 mA. 

[peteski]

Well, the cost for me to cap the low-end Railpower 1300 at 12 volts was exactly the price of a single zener diode, which costs me 50 cents if I buy just a few at retail pricing, and would cost a manufacturer pennies.

It would be interesting be able to ask the designer of these throttles and ask what were his criteria for coming up with the circuit he designed.  But that is not something that will happen.