Yet another thread brutalized by misinformation from one resident expert and sent to the gallows much too soon. Sorry to have to cut this one short, John, but it's best to end it on a less positive note before it gets even worse...
Tom
https://tstage9.wixsite.com/nyc-modeling
Time...It marches on...without ever turning around to see if anyone is even keeping in step.
Were is Steven Otte when we need him?Mel My Model Railroad http://melvineperry.blogspot.com/ Bakersfield, California Turned 84 in July, aging is definitely not for wimps.
Ok, I'm out. This thread has exceeded my willingness to type any further rebuttals ....
Mark.
¡ uʍop ǝpısdn sı ǝɹnʇɐuƃıs ʎɯ 'dlǝɥ
So much unnecessary confusion in a thread about multimeters.
PWM is not FM; it doesn't use any sort of a sine wave, far less AC reversal. It chops a reference DC voltage into on-off pulses, full voltage alternating with zero voltage.
Most of the motors in model railroad equipment are voltage-controlled, meaning their speed control is a function of input voltage. PCM yields a percentage of reference voltage as the average of high-frequency 'on' and 'off' times; it does not send some digital code to a BLDC motor controller or whatever. The 'flicker' comes when the frequency of PWM is relatively low and hence the 'off' time is a large proportion of the duty cycle; you won't see any such flicker from higher frequencies.
Part of the DCC 'decoder' is a PWM synthesizer, for motor control. It has nothing to do with the DCC waveform or logic-pulse structure other than seeing a rectified DC input and being controlled by speed-step control data interpreted by the decoder.
DCC modulation is not a 'sine wave' -- it is a square wave, in the same way as PWM. That it is not a perfect square wave (physics limits it) is of comparative unimportance, as the decoder responds to pulse and gap durations far longer than the transition, and is not affected by the 'ringing' overshoot, etc.
The DCC voltage swings twice the reference voltage (28V rail-to-rail for 14V nominal) but only 14V is ever seen by connected equipment; the periodic reversal keeps DC charge transfer from accumulating (much as modem coding did in the dark ages before broadband Internet).
Is there a multimeter-related question still unanswered, or new multimeter-related questions to be asked? That is what needs to happen to bring this thread down from the space academy.
Mark R.BEMF would not exist if the decoder was supplying a linear variable DC voltage to the motor.
BEMF is determined by motor RPM reagardless of what causes that motion.
LastspikemikeDCC has its zero line at the "bottom" of the wave form (in effect, the actual electronics are not so simple I'm led to believe). DCC is always positive, har har. You should see a digital effect in a PWM wave form in the displayed frequency of the power pulses. DCC should look similar albeit much more obviously digital looking. The PWM creates a power signal by altering the frequency and therefore total duration of the pulses. DCC uses a similar signal to transmit a lot more information and does not vary the power delivered by any material amount. Except if used to drive a DC motor directly. Then DCC mimics PWM and address zero, in effect, is a decoder in the DCC control unit rather than each individual locomotive. When address zero is broadcasting all DC motors directly connected to the rails will move in the same direction at roughly the same speed. All decoder equipped locomotives will operate as they usually do.
not sure the above says anything coherent ...
in addition to a DC component, DCC zero stretching includes an AC component that results in heat without any motion. PWM does not include an AC component resulting in useless heat.
greg - Philadelphia & Reading / Reading
Brent
"All of the world's problems are the result of the difference between how we think and how the world works."
I just try to help with what I know.
-Kevin
Living the dream.
LastspikemikeI understand how PWM works also. PWM and DCC are similar ideas. DCC allows transmission of a lot more information concurrently with the power delivery. PWM creates the effect of reduced voltage but it is delivered at full voltage. If you examine the wave form required to achieve that you should see a similarity with a DCC "wave" form. I use the term "wave" loosely of course. DC isn't really a wave but then neither is DCC. The frequency variation in PWM controls speed. Polarity controls direction, as usual for DC. The pulses of full voltage interrupted by periods of no voltage are equivalent to a digital speed control signal. DCC has its zero line at the "bottom" of the wave form (in effect, the actual electronics are not so simple I'm led to believe). DCC is always positive, har har. You should see a digital effect in a PWM wave form in the displayed frequency of the power pulses. DCC should look similar albeit much more obviously digital looking. The PWM creates a power signal by altering the frequency and therefore total duration of the pulses. DCC uses a similar signal to transmit a lot more information and does not vary the power delivered by any material amount. Except if used to drive a DC motor directly. Then DCC mimics PWM and address zero, in effect, is a decoder in the DCC control unit rather than each individual locomotive. When address zero is broadcasting all DC motors directly connected to the rails will move in the same direction at roughly the same speed. All decoder equipped locomotives will operate as they usually do. The two systems are quite different but exploit similar ideas. The decoder delivers DC power to the motor and the lights, of course. The internal rectifier takes the DCC power and converts (rectifies) it into DC power for the motor and all the other functions. The decoder part interprets the digital signal also carried with the power. The signal is used to, among other things, vary the average voltage delivered to the motor as well as its polarity. Quite clever in fact. Those lights are dimmer because the decoder has built in resistors of correct to step down the voltage leaving the diode bridge. Dimming those lights further, for variable lighting, would require additional electronics to further vary the delivered voltage. LED don't dim in the same way as incandescent as you will discover when selecting household lighting for dimmer controlled circuits.
PWM and DCC are similar ideas. DCC allows transmission of a lot more information concurrently with the power delivery.
PWM creates the effect of reduced voltage but it is delivered at full voltage. If you examine the wave form required to achieve that you should see a similarity with a DCC "wave" form. I use the term "wave" loosely of course. DC isn't really a wave but then neither is DCC. The frequency variation in PWM controls speed. Polarity controls direction, as usual for DC. The pulses of full voltage interrupted by periods of no voltage are equivalent to a digital speed control signal.
DCC has its zero line at the "bottom" of the wave form (in effect, the actual electronics are not so simple I'm led to believe). DCC is always positive, har har. You should see a digital effect in a PWM wave form in the displayed frequency of the power pulses. DCC should look similar albeit much more obviously digital looking. The PWM creates a power signal by altering the frequency and therefore total duration of the pulses. DCC uses a similar signal to transmit a lot more information and does not vary the power delivered by any material amount. Except if used to drive a DC motor directly. Then DCC mimics PWM and address zero, in effect, is a decoder in the DCC control unit rather than each individual locomotive. When address zero is broadcasting all DC motors directly connected to the rails will move in the same direction at roughly the same speed. All decoder equipped locomotives will operate as they usually do.
The two systems are quite different but exploit similar ideas.
The decoder delivers DC power to the motor and the lights, of course. The internal rectifier takes the DCC power and converts (rectifies) it into DC power for the motor and all the other functions. The decoder part interprets the digital signal also carried with the power. The signal is used to, among other things, vary the average voltage delivered to the motor as well as its polarity. Quite clever in fact.
Those lights are dimmer because the decoder has built in resistors of correct to step down the voltage leaving the diode bridge. Dimming those lights further, for variable lighting, would require additional electronics to further vary the delivered voltage. LED don't dim in the same way as incandescent as you will discover when selecting household lighting for dimmer controlled circuits.
Your copy and paste rebuttal is good until the last two paragraphs, where you appear to freelance ....
The motor and the function outputs do not use DC. That statement alone shows you aren't grasping PWM completely. BEMF would not exist if the decoder was supplying a linear variable DC voltage to the motor. Same for the function outputs, they are receiving PWM voltage. If you dim an LED way down using the CV adjustment, you can clearly see the pulsing. At full brightness, the pulsing is still there, but it is so fast, the human eye cannot detect it .... well, 99% of people can't.
The voltage supplied is either positive or negative in relation to the zero base as you can see in the DCC sine wave, not true DC voltage. The frequency of this wave is high enough that most humans cannot hear it. Some older decoders (like the old Atlas 340 and 341) used a lower frequency that would produce a very annoying buzzing sound. Again, DC voltage would not do that.
I haven't been bothered by anybody's contributions to this thread. My last post was meant to be self depricating. The problem with online forums is the tone of remarks isn't always apparent.
It is normal for threads to digress away from the subject of the OP. I have no problem with that either. Even if the information being presented is beyond my level of understanding, it might be useful to others.
John-NYBWAs I said in the OP, I know just enough about electronics to get by (barely). You guys are up in the stratosphere now.
I tried to help, and I posted helpful information.
Fortunately Spike showed up and pointed out I was wrong and then SPIKED your thread with nonsense, so you didn't get bogged down with helpful information from someone who spent years training technicians on proper usage of a digital multi-meter.
You are better off for his participation. I mean, I can't even get facts right.
I earned a living using a multi-meter, but what would I know?
Sorry, but this is what the forums become with Spike input. Helping the OP takes a backseat to insulting helpful people, blowing hot air around, chest thumping, and general bully tactics. And of course, lots of incorrect information presented as fact just to start arguments.
LastspikemikeDCC digital signal still has to be a wave, with a rise and fall.
LastspikemikePWM creates the effect of reduced voltage but it is delivered at full voltage. If you examine the wave form required to achieve that you should see a similarity with a DCC "wave" form. I use the term "wave" loosely of course. DC isn't really a wave but then neither is DCC.
So, is DCC a wave or not? (It is.)
Lastspikemike DCC is always positive, har har.
Each rail is always positive (or zero) in reference to ground, but the voltage across the rails alternates.
LastspikemikeYou don't see the similarity. PWM sends a speed signal to the DC motor combined with the power to drive it. DCC does the same thing. The difference is only a power level signal is contained in the PWM "signal". I referred to the manner in which DCC current returns to the booster as a pseudo ground because that's what it is. That's why boosters that are connected together require a third wire. I may not understand the technical aspects of DCC but a I "know how it works".
I do see the similarities, but they also have major differences, the biggest being that the DCC voltage is constanly reversing and PWM is not.
Again, there is no "pseudo ground". The ground connected between boosters is just that, ground, the system ground, there is nothing "pseudo" about it.
John-NYBW As I said in the OP, I know just enough about electronics to get by (barely). You guys are up in the stratosphere now.
As I said in the OP, I know just enough about electronics to get by (barely). You guys are up in the stratosphere now.
Rich
Alton Junction
betamaxWith PWM, it is the On/Off sequence that determines direction: In one direction the motor turns CW, reverse the sequence and the motor turns CCW.
the period that it is on determines the effective average voltage
reversing the polarity affects the direction, not the sequence
With motors, it is all about the direction of current flow (Fleming's Law).
With PWM, it is the On/Off sequence that determines direction: In one direction the motor turns CW, reverse the sequence and the motor turns CCW.
The same thing works with Zero Stretching: Under normal circumstances the On/Off sequence on the rails won't develop much torque in a motor (or analog meter). By holding one rail On for a longer period, the current flows long enough to produce torque in the motor. The longer the period, the more torque and the motor spins faster. Direction is determined by which rail is held On for the longer period.
LastspikemikeI read that some time ago. The net result is delivery of the mostly one way current the DC motor requires. That seems indistinguishable for practical purposes from a real DC current. After all that's how PWM delivers the correct power at a full twelve volts DC. I don't see the square wave DCC power delivery as different from the sine wave delivery of DC in this situation. After all the DCC square wave is actually a sum of a large number of different sine waves. DCC digital signal still has to be a wave, with a rise and fall. If you wish to explain where I'm mistaken that would be fine by me. Just referring me to the source you have doesn't do that, or at least I don't see where it does.
PWM motor control switches between 0 and +V, unless the motor is running in reverse where it switches from 0 to -V, but under normal circumstances it never constantly switches between +V and -V. DCC pulses constantly switch between +V and -V and are never at 0V, except for the moment it crosses from one to the other - definitely not the same as PWM.
LastspikemikeIt seems to me that you measure voltage from each rail to a pseudo ground rather than across the rails as you would would DC power.
It's not a "pseudo ground", it's the system ground.
LastspikemikeYour analog multimeter really can't measure anything between the rails because the signal your multi meter is getting isn't a voltage signal per se.
I don't quite know what you mean by this but there certainly is a voltage between the rails that can be read by an analog multimeter. It won't likely be accurate because the meter is most likely measring the average votage and is calibrated for a sine wave but is trying to measure a square wave (RMS (root mean square) is what you really want and a RMS meter will be correct).
Measuring DC from each rail to ground and adding them together will give you an accurate voltage because the meter won't be callibrated for a sine wave, but that's not real easy to do around the layout unless you run a ground a round the layout. Measuring between the rails will give you a higher reading, but it's consistantly higher and can still be used to test for voltage drop.
John-NYBWInteresting. I was testing a second hand DC loco I picked up using address 0000. I wanted to see if it was working prior to installing a decoder. I can only do that with the main LH100 tethered throttle because my wireless throttles don't allow me to enter address 0000. I don't think I had left it on address 0000 when I was testing the track power but it's a possibility.
Generally speaking, when using DCC and running a locomotive on a throttle, when you select another loco, if you don't stop the first one before releasing it it keeps going. In other words, if you did not make sure address 0 was set to stopped, even if it's no longer selected on the throttle the command station may be doing the 'zero stretching' to run a DC loco. That was probably the source of your different readings.
Lastspikemike I read that some time ago. The net result is delivery of the mostly one way current the DC motor requires. That seems indistinguishable for practical purposes from a real DC current. After all that's how PWM delivers the correct power at a full twelve volts DC. I don't see the square wave DCC power delivery as different from the sine wave delivery of DC in this situation. After all the DCC square wave is actually a sum of a large number of different sine waves. DCC digital signal still has to be a wave, with a rise and fall. If you wish to explain where I'm mistaken that would be fine by me. Just referring me to the source you have doesn't do that, or at least I don't see where it does.
I read that some time ago. The net result is delivery of the mostly one way current the DC motor requires. That seems indistinguishable for practical purposes from a real DC current. After all that's how PWM delivers the correct power at a full twelve volts DC. I don't see the square wave DCC power delivery as different from the sine wave delivery of DC in this situation. After all the DCC square wave is actually a sum of a large number of different sine waves. DCC digital signal still has to be a wave, with a rise and fall.
If you wish to explain where I'm mistaken that would be fine by me. Just referring me to the source you have doesn't do that, or at least I don't see where it does.
Maybe check this out ? ....
https://dccwiki.com/Pulse_Width_Modulation
DC voltage is not used to control the motor. For that matter, there is no Straight DC voltage outputs on a decoder. Even the function outputs utilize PWM operation as well. Dimming a light on a decoder doesn't reduce the DC voltage (there isn't any) it reduces the pulse width, creating less on time so the light "appears" to be dimmer.
Lastspikemike John-NYBW Lastspikemike Is this a Digitrax DCC system? No, Lenz I just wondered if your analog multi meter is reading voltage delivered for address 0000 which would be a biased PWM simulated DC power. If these DCC systems with DC power available at address 0 then maybe a multi meter set to measure DC might pick up and display that voltage which would be much higher when measured one way than when measured the other.
John-NYBW Lastspikemike Is this a Digitrax DCC system? No, Lenz
Lastspikemike Is this a Digitrax DCC system?
Is this a Digitrax DCC system?
No, Lenz
I just wondered if your analog multi meter is reading voltage delivered for address 0000 which would be a biased PWM simulated DC power. If these DCC systems with DC power available at address 0 then maybe a multi meter set to measure DC might pick up and display that voltage which would be much higher when measured one way than when measured the other.
Interesting. I was testing a second hand DC loco I picked up using address 0000. I wanted to see if it was working prior to installing a decoder. I can only do that with the main LH100 tethered throttle because my wireless throttles don't allow me to enter address 0000. I don't think I had left it on address 0000 when I was testing the track power but it's a possibility.
This morning I have been working out some wiring bugs for my Atlas switch control boxes and other accessories. I have the various panels daisy chained together using a Miniatronics 12V DC power supply. Some of the remote panels were not working. In testing the voltage at various points, I discovered that polarity mattered on those when I checked them with my Multi-Meter, so what you are saying makes sense.
Lastspikemike I just wondered if your analog multi meter is reading voltage delivered for address 0000 which would be a biased PWM simulated DC power. If these DCC systems with DC power available at address 0 then maybe a multi meter set to measure DC might pick up and display that voltage which would be much higher when measured one way than when measured the other.
Nope .... not even close. Investigate zero-stretching ....
https://dccwiki.com/Zero_Stretching
gregc John-NYBW If either of those were the case, I would think it would show up no matter which way the probes were set. what were the measurements? AC/DC?
John-NYBW If either of those were the case, I would think it would show up no matter which way the probes were set.
what were the measurements? AC/DC?
i do mean the actual voltages being measured as well as whether AC or DC?
DC may be different. can't see how AC measurements can be?
I have 3 ways to check my DCC track voltage.
1. Analog Voltmeter: For this purpose, I use a Radio Shack multimeter dialed to AC voltage. My best guess from the position of the dial is approximately 12.5 volts. I get the same result with the same dial position no matter which way I place the two probes on the rails.
2. Digital Voltmeter: For this purpose, I use a Gardner-Bender multimeter dialed to AC voltage. The readout shows 12.4 volts with the probes placed on the rails one way and -12.4 volts with the probes placed on the rails the opposite way.
3. RRampMeter: The measurement shows 13.5 volts which I believe to be the most accurate measure of DCC voltage. If you operate in DCC, you should own a RRampMeter.
John-NYBWIf either of those were the case, I would think it would show up no matter which way the probes were set.
The lower end Analog meters can introduce meter loading just to make it interesting. The real low end ones can be 1k-Ohm/Volt, where a DMM will often be in the range of 1M-Ohm/volt. Your typical Fluke DMM is 1MOhm, a few are 10Mohm/V.
Even the pressure on the probes can make a difference between a good connection or not when taking reading. Especially important when measuring resistance. With analog meters a fresh battery can help, they usually don't have an indication for that on the meter face when measuring resistance.
Change the battery.
I deleted my previous, factual, and helpful replies.
As usual, Mike has pointed out that I was wrong about everything, sorry guys.
Happy Thanksgiving everyone. Enjoy you newly SPIKED thread.
I am out.