Many years ago, I was told that if you use the pulse switch on a power pack it could melt a motor. I use two MRC TECH II RAILMASTER 2400s. They haven’t seemed to have damaged anything. Many years ago I found a new MRC Throttlepack 500 still in the package. I had to have it for nostalgia’s sake. Back in the day, when I was just a pup, that was THE powerpack to have. Now, it also has a pulse switch. Even if the TECH II’s pulse is safe, this critter is from another era. Can anyone fill me in on the history etc. of pulse power? My friend has a TECH4 and I can’t see enough of a differernce to consider replacing the TECH IIs. Anyone else have any thoughts? Thanks!
I have 3 Tech II 2400’s and haven’t damaged anything in 25 years. I just leave the pulse switch in the on position almost all the time too. If one of them dies I will try to get another one on the bay. I don’t know about that other pack, but hook up a digital voltmeter and check it out. I bet it’s almost the same as the Tech IIs.
I have a old MRC powerpack with that option.I only use pulse on old open frame motors ,like Athearns or cheap locomotives that need a boost to get them started.I read that newer canmotors or better built motors don’t need it and the pulse might overheat them.
The pulse power feature normally is only prsent at low voltages, read slow speeds and disapears into regular DC above a certain level. You can hear it pulsing at very low voltages on some of the motors, but how often do you run at scale 5 miles per hour for a very long distance??
Pulse power was an answer to get slower speed out of the trains.
Then there was mostly only the larger pittman style motors, no can motors yet.
Pulse power just cuts the AC wave in half in the powerpak then delivers half wave DC to the rails, same top voltage, but half the frequency. The we got into more pure DC throttles and pulse power was problematic, till electronics did better controlling and in lower speeds spiked some pulse power in.
It may not be very practical with can motors today.
The other problem in using a Throttlepack 500 is that the pack is not capable of controlling low current and low voltage to needed to run a modern model smoothly at low speeds.
“The other problem in using a Throttlepack 500 is that the pack is not capable of controlling low current and low voltage needed to run a modern model smoothly at low speeds.” Not even in the pulse mode? I noticed that in the regular mode slow speed opperation is not possible. In the pulse mode it seemed to be fairly decent at slow speeds. Since I have two TECH IIs, I don’t need to use the Throttlepack. I just have it for sentimental reasons.
I’ve been using pulse for years I have an MRC 7000 and have had no problems,I use it for switching most of the time[:D][^]
JIM
The problem with the Throttlepack and other rheostat (resistance) based control systems is that the newer can motors don’t draw enough current for the rheostat to work properly. The fomula is voltage = resistance times current. The old open frame motors typically used .3 to .5 amps. Using .4 amps as an example, and say you wanted 4 volts at the track to run your train slowly. The rheostat needs to drop 8 volts (12-8=4) with a current of .4 amps gives 20 ohms to be added in series with the motor. Typically rheostats had a maximum of 50 ohms (at the slowest speed position).
If the new can motor only needs .1 amp at slow speed, then to get 3 volts at the track (typical starting voltage), the rheostat needs to drop 9 volts which at .1 amp requires 90 ohms. Since it doesn’t have 90 ohms, the minute you move the control off “stop” you are giving 7 volts to the track, dropping 5 in the rheostat (5=.1 times 50). Hence, the lack of slow speed control.
As an earlier poster pointed out, the pulse power switch takes out every other pulse - the old power packs put out a rectified 60 Hertz AC, not pure DC. The effective voltage is reduced (not in half because of windings in the motor, but reasonably close) so that the rheostat only has to drop approximately 4 volts to run the can motor at 3 volts. This only takes 40 ohms (4 volts = .1 amp times 40 ohms) so that you do have reasonable control of the can motor with pulse power on.
The extra heat from pulse power comes from running higher instaneous voltages and currents because the power is compressed into a shorter time span. Our motors are generally unable to cool much between the power spikes so the result is somewhat higher temps running on pulse power. The newer can motors are much less tolerant of this extra heat than the older open frame motors. Also, the can motors, coupled with a decent gear system, can generally run at very slow speeds without the assistance of pulse power.
Newer power packs (like Tech I
I used MRC pulse power back when most motors were open-frame. It did allow for slower speed control, but as I remember, it cut out when the motor reached about a scale 20 MPH, so it was hard to damage a motor. I had one with cans, when they first came out, and had no adverse effects. I now have an MRC control 20 with what is called ‘Nudge’ for slow speed control with can motors, and find that it operates well with the open-frame motors I haven’t yet replaced on my older brass. Actually, what I’m getting around to, is the ‘nudge’ switch on a modern power pack the same idea as the ‘pulse’ on an older one? I do know that the ‘nudge’ on my power pack will help my BLI’s with sound kick in at a somewhat lower voltage rating, and the person I contacted at MRC said that leaving the ‘nudge’ on won’t do any damage to the BLI’s, in fact it’s reccommended for we DC operators. Anyone have any ideas?
Tom [?]
You wrote: I now have an MRC control 20 with what is called ‘Nudge’ for slow speed control with can motors, and find that it operates well with the open-frame motors I haven’t yet replaced on my older brass. Actually, what I’m getting around to, is the ‘nudge’ switch on a modern power pack the same idea as the ‘pulse’ on an older one? I do know that the ‘nudge’ on my power pack will help my BLI’s with sound kick in at a somewhat lower voltage rating, and the person I contacted at MRC said that leaving the ‘nudge’ on won’t do any damage to the BLI’s, in fact it’s reccommended for we DC operators. Anyone have any ideas?
Looking at the manual on the MRC web site, the Controlmaster 20 is specifically designed to delivered filtered DC in normal operating mode - the normal 120 Hertz pulses are smoothed as much as possible, generally with capacitors. A voltage regulation scheme is used to directly control voltage to the track. The momentum and brake settings further modify the voltage settings and the rate of change of the voltage settings. The nudge switch probably introduces (manual isn’t real clear and I don’t have my own to put on an oscilliscope) some mild form of pulse power. I say mild becasue the manual goes to great lengths to stress the unit’s compatibility with even the most finicky can motors.
Which leads into a discussion of smoothly starting a stopped locomotive. Friction and current draw of an electric motor peak just BEFORE the motor starts to turn. As soon as the motor actually begins to turn, the current draw and friction decrease. With a rheostat control, the locomotive tends to jump because the sudden decrease in current draw reduces the voltage drop in the rheostat, and increases the voltage on the rails. The concept of the pulses is to start the motor turning, and then drop the power. The cycle of starting and stopping the motor is repeated (theoretically) with each pulse.
Another hindrance to smooth starting is the “cogging” effect of the poles of t
I agree and disagree…
As far as how the DCC signal is transmitted, I’d disagree with your description, but that isn’t really relevant here.
Unless I am mistaken, there is no spec on how the decoder powers the motor. I think that is one of the things that can give different decoders advantages in different situations. I think that in general you are correct, the decoder has the rectified DC available, and can then regulate the voltage, or use pulses, or some combination to actually control the motor. I think that DCC does has some advantages that can lead to better control that DC. One is that there can be a regulated supply right next to the motor, which, in theory, can help minimize the difference in pickup through the rails. How much advantage is taken of this, I don’t know, so it might be a non-issue. The other advantage, that is used, is that since the decoder is controlling only one motor it can utilize back EMF to close the control loop to a degree, which seems to me should help low speed control as well as help in
I am not familiar with that specific power pack but I do not think a nudge is the same as the old pulse power.
To summarize… In general the older MRC power packs “pulse power” were 1/2 wave DC while the newer ones have the pulses added electronically. Comparing the two is an apples to oranges deal. So if one has a new electronic Tech II (1400 or greater I believe) the pulses will not harm anything. They are automatically decreased and turned off as the throttle is advanced. If the pack is an older one (Like the ThrottlePacks in the golden cases) the rough 1/2 wave power can overheat and burnout a motor. This is more an issue for can motors than open frames. My opinion is continual use of a “pulse power” on an open frame will eventually weaken the permanent magnets in the motor.
After I learned what pulse power was, I modified my MRC Ampack<sp?> by putting a SPST on one of the rectifiers. The first time I used it, just hearing the difference in the sound of the motor told me it could not be good for them.
Hi All: In my opinion, the decoders control the motors by PULSE WIDTH MOCULATION. I used to work in Maintenance on Numerical Controlled Machines, way back when we used TUBES to make DC, to control motors, and Relay Logic. As the machines got newer, the Electronics got more complicated, and simpler at the same time. The First machine we got that had complete Solid State control of motors (Transistor & Scr’s) the motor speed controls were PWM. At slow speed the motors would WHINE, like a dc loco on address “0” only Louder, because these were 3 to 5 HP motors. These motors were driving the table, on Milling machines, or boreing machines. As we got newer machines, the controls were changed so that they didn’t use the 60 cycle line frequency, and you couldn’t hear the whine anymore because it was above the normal hearing range, like the newer Silent decoders.
This is just my own opinion, from what I have seen in industry, and what I have learned about DCC.
SAM
I suspect they use PWM as well, though they wouldn’t have to, and they could vary voltage as well. Or even both. Like I said, I don’t think there is a requirement here, and the actual motor control could be different from controller to controller. (Feel free to correct if I am mistaken.)
SIMPLY PUT:
Early throttles added AC to the DC to ‘nudge’ balky drives. The MRC 500 was designed when popular engines (such as Athearn B.B.) required 6 volts to overcome friction, and 1 amp to run. These throttles produced noticible ‘buzzing’ and the added AC could de-magnetize the motors over time.
This was replaced by Sawtooth waves, then bumpy 1/2 wave rectification with Tech IIs.
Most of todays throttles are Solid State, and gone is the slide switch labeled ‘Pulse Power’. The low end 1/2 wave rectification is internally diminished as the voltage increases - at least on the better (read: more expensive) throttles. The switch may remain for those with old engines.
Depending on WHEN your particlar throttle was designed, will determine HOW your particular throttle operates … OR,
If you attach an Oscillascope, you can SEE the actual wave form. THIS is what it looks like n_n_n_n_n wheras ‘sawtooth’ is more like NNNN. (DC is ___________).
Re: Pulse Width Modulation. (PCM) is the ‘COMMAND’ portion of DCC. The motors and lights use DC passed on by the DA decoder.
Don
Is that Square Wave the signal on the track or is it on the Motor Leads AFTER the Decoder?
I believe that in general decoders DO supply a PWM signal to drive the motor. This is based on info from various websites, and diagrams that I have seen. Also, the references to the ‘silent drive’ feature that many decoders have, which use a higher pulse frequency to drive the motor.
So, the signal to the motor is pulsed DC, with the ratio between on and off determining how fast the motor goes.
The signal on the track is sort of a square wave, both rails are going from low to high i opposite phase, the length of each pair of transitions determines whether a ‘1’ or a ‘0’ is being sent.
Sam: I belive the “square” waveforms of PCM are imbedded into DC via the rails and rectified by the decoder before reaching the motor.
Perhaps that’s why metal wheels traversing a reversing loop gap create a temporary short circuit - regardless of direction.
‘Pulse Code Modulation’ is a DIGITAL property. ‘Pulse Power’ is 60 cycle pulses added to DC and ANALOG in nature.
FWright
wrt to pwm
Isn’t that exactly what DCC does ???
Villy