Electronics > Beginners
Observation of a model railway PWM signal
Deboscope:
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.
German_EE:
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)
Zero999:
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.
Deboscope:
i had a look and no, the power supply to the transformer has no earth connection. The system is definitely afloat.
MarkF:
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.
See
https://dccwiki.com/DCC_Power
https://sites.google.com/site/markgurries/home/technical-discussions/boosters/nmra-track-voltages
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