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Problem with Chattering and Oscillating Servo Point Motors

He is a question posted by Barrie:

“I have a problem with the servo motors that control the turnouts on our N scale club layout chattering or oscillating when an old type DC loco is run on the layout. This doesn’t happen if new type locos are run or the system is run as DCC (Our N scale Layout has been wired to operate as DC or DCC). The club N scale layout has a HO/OO layout directly below and when they run some of their old DC locos it causes their turnouts and ours to chatter. I think it may be RFI generated by the loco motor and the minute sparking that occurs at the loco wheels while running. We have used about 70 model aircraft type servo motors to control all the turnouts on both layouts. These turnouts are wired back to electronic control boards around the layout that can each handle 8 servos.

Each servo is then operated by single pole single throw switch on the main schematic panel which is wired back through the electronic control board. The N & HO/OO turnout systems are separate and each is powered by a 5v & 12v computer type power supply.  The turnout power system is also electrically separate from the track power supply.  Any suggestions how we can overcome this problem would be appreciated.”

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6 Responses to Problem with Chattering and Oscillating Servo Point Motors

  • Jay Rosssays:

    This is a tough one, fortunately this sounds like the same turnout control system that I use, if you are using the Tam Valley Depot Singlet & servos. These servo control circuits can be run on a DCC or DC buss, I use a DCC buss. I use momentary push buttons to control each turnout, you mentioned a switch, is it a momentary switch? My servos also chatter if the servos endpoints aren’t correctly set. If they aren’t set, the servos will always be trying to drive the switch points harder to the rail, thus pulling more current, and over working the servo. If you are using the Octopus controllers, I got this info from Tam Valley’s web site: Power with 7.5-24V filtered DC (do not use AC and please do not use old “powerpacks” – they are not filtered). Octopus III has a built-in switching power supply for high efficiency and greater range. You should budget 1 Amp per board…Be that your power source is filtered DC, not AC or unfiltered power packs.

    • Barrie Morgansays:

      Hello Jay,
      Thanks for your reply, you’ve given me a few things to think about. The servo control units are not Tam Valley but home made using a picaxe 20M2 chip which controls 8 servos per unit. It also controls the endpoints of the servo travel which we have set so the servos don’t over tension the turnout throw limit. As mentioned there are 5 control units on our layout fed from a power supply out of a desk top computer, I’m not sure if it is filtered so I’ll check that. The servo switches are not momentary which means that there is power to the servos all the time.
      We have cured but not solved the problem by putting a push momentary switch to the incoming supply to the control board. We then set the turnout switches (single pole single throw) to the desired direction then push the button to power up the unit which changes the turnouts to the desired position then shuts down again when the push button is released. The servos will chatter if the RFI is present while the push switch is held down but stop when released.
      I’ll report back when I’ve checked if the power supply is filtered.

      • Jay Rosssays:

        Using a PC power supply should good as long as it isn’t being loaded to it’s max rating. Sometimes a PC power supply can have bad filter caps etc, and will have some ripple on the output. That happened to me on my desktop PC, and it damaged not one, but two mother boards before I realized the power supply was bad. You can always check your operating voltages in DC and AC. For example, check your 5vdc, then put your meter on AC, and you should have less than 100mv AC, if it’s more that that, then it’s not filtered enough. The momentary push button sounds like a good idea, less constant load on the supply.

        • Barrie Morgansays:

          Hello Jay,
          I know a guy who has an oscilloscope, that may confirm the purity of the out put.

  • Frank Bsays:

    Old DC locos with open frame motors do generate electrical “noise” (RFI) that can interfere with electronic systems. Even way back then, they were normally fitted with a little suppression circuit of a capacitor and inductor to prevent interference with radios and TVs.. If this circuit is missing, the RFI is likely to cause problems for DCC systems.

    • Barrie Morgansays:

      Hello Frank,
      Thanks for your comment.
      The locos that cause problems do have the suppression capacitor but I guess it is inadequate.
      Being what we think is an RFI problem we put capacitors in various places on the track supply but to no effect. If you have any ideas how to eliminate the RFI I’d be interested to trial them. Our most popular idea at the moment is to ban any loco that causes this problem from the club layout.

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