Observation of a model railway PWM signal - Page 1
Forum section: electronics / beginners

author: Deboscope
author_rank: Contributor
timestamp: November 17, 2019, 09:53:26 am
content: To observe the PWM signal from a model railway, what would be the proper way to connect the probe to the live rails? In particular I am concerned about the earth part of my probe.Voltage is about 22 V AC. It is powered though a control station and a step down transformer connected tot he mains.

author: German_EE
author_rank: Super Contributor
timestamp: November 17, 2019, 10:52:52 am
content: Most model railways that I have seen operate on DC, not AC.Check the output terminals of your train controller using a multimeter set to the ohms range. If you see a low resistance between the AC power ground and one output terminal then connect this terminal to ground on your oscilloscope. The other terminal should be connected to the probe tip.By setting the input stage gain on your oscilloscope you should be able to see the PWM waveform.How Not to Blow up Your Oscilloscope(where Dave looks about 20)

author: Zero999
author_rank: Super Contributor
timestamp: November 17, 2019, 08:41:22 pm
content: Some model railways use an AC signal to both power and transmit data to the locomotive. It's rectified in the locomotive to power the motor and the changes in duty cycle or frequency are used to tell it the speed and direction of travel.I've never worked with such a system, so wouldn't know how to 'scope it. I would start by checking which conductor is connected to mains ground and connect the 'scope ground pin to that. If neither conductor is earthed, then I suppose it doesn't matter which side the 'scope ground pin is connected to.

author: Deboscope
author_rank: Contributor
timestamp: November 18, 2019, 08:12:54 am
content: i had a look and no, the power supply to the transformer has no earth connection. The system is definitely afloat.

author: MarkF
author_rank: Super Contributor
timestamp: November 18, 2019, 02:37:19 pm
content: The old Lionel trains ('O' gauge three rail) were AC. The two rail was DC.The smaller gauges were DC or PWM or sometime rectified AC.There were several schemes to enhance slow speeds and improve start-up.Most of the new model railroad sets are DCC controlled or DCC ready.The DCC waveform is pulse width encoded messages for control and it is rectified in the locomotive for power.Seehttps://dccwiki.com/DCC_Powerhttps://sites.google.com/site/markgurries/home/technical-discussions/boosters/nmra-track-voltages

author: German_EE
author_rank: Super Contributor
timestamp: November 18, 2019, 04:03:50 pm
content: It seems that model trains have advanced in technology since my 12 V DC powered set from fifty years ago. Rectified AC drive and a variable resistor to control the speed,

author: rfeecs
author_rank: Frequent Contributor
timestamp: November 18, 2019, 06:07:23 pm
content: This is an interesting page from a guy who claims to be an expert on model train controllers, with pictures of oscilloscope waveforms:https://www.scottpages.net/ReviewOfControllers.htmlInteresting you can see the back EMF from the motor. So you will get a different picture with the train on the track vs empty track.