Peak back EMK issue on Eureka clocks

Discussion in 'Electric Horology' started by praezis, Feb 5, 2018.

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  1. praezis

    praezis Registered User

    Feb 11, 2008
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    Already earlier I suspected that the missing of any devices that can limit the back EMF on Eurekas may be the reason of coil damages. The old and brittle wire lacquer has to withstand voltage peaks of 150 volts and possibly more!
    It is very astonishing that none of the experts ever adressed this issue or took any measure during restoring.

    I made measurements on my coil that show a drastic reduction of the peak voltage with a simple resistor, with neglectable prolongation of the current flow. The table also shows the longer current flow when a diode is used (often proposed, but not advisable).

    On my clock I added a smd sized 330 Ohms resistor on a tiny board that fits nicely on the impulse roller of the Eureka balance.


    Eureka data.jpg Eureka1.jpg
  2. James McDermaid

    James McDermaid Registered User
    NAWCC Member

    Apr 29, 2011
    I have never measured the back EMF (inductive kick) on my Eureka but In a previous post I did wonder if this could lead to pitting of the balls or possibly arc through the coil wingdings.

    As small as that resistor is, it can still change the balance of the balance wheel although so could a bad solder job on the contact pin.

    Maybe a diode would be better.

    My Eureka runs fine as made original but this did cause a little thought.

    They certainly understood the principals as I have cars that use spark coils made before 1910..

    I want to get my Eureka to run good on 1.5 volts which will reduce the back voltage. It runs fine on a little more that 2 volts. As I use a Horologix regulated #6 it gets to eat the spark.

    A capacitor would cause the coil to ring and this could make the spark worse.

    Hi-Tech aside, if you look at the contacts the spark is almost not visible as seen in the dark.


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