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Thread: diodes on solenoids PA18

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    cubnut93's Avatar
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    diodes on solenoids PA18

    I was looking at a drawing for the later model PA-18 and noticed that they showed a diode connected to the master contactor and one for the landing/taxi lights but none for the starter solenoid. Anyone out there that can tell me how to hook up a diode for that starter solenoid??? See drawing below.

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    mike mcs repair's Avatar
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    the starter one goes between ground and the little lug/start switch.... not sure of direction, the ACS key switch shows on its paperwork... it's the only one I put on usually

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    nanook's Avatar
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    Thanks for the info supercub24a, back when I was studying electronics in college( mid 70's ) we called those blocking diodes....cheap insurance.

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    cubnut93's Avatar
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    Thanks for this link--it is very good--however, the drawings that they show are for solenoids that are grounded by a switch in order to complete the curcuit. If you look at my original drawing (I attached it again here) you can see that the "starter solenoid" is case grounded and the curcuit is completed by closing the starter switch which in turn sends 12volts to the solenoid. I am sure that the placement of the diode in this case would be different. Does anyone have a drawing of that type of solenoid/diode??

    I have also attached another drawing that was sent to me by a friend--it shows a case grounded starter solenoid and the placement of the diode for this type of solenoid. However, looking at this drawing I am having trouble understanding how it works once the starter switch is opened! When I look at the "Master contactor" I see that any surge of current that happens when the contactor is switched off goes back to the battery--but when looking at the "starter solenoid" drawing that my friend sent me--it looks like that surge just goes back into the solenoid coil--what good is that???



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    mike mcs repair's Avatar
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    Quote Originally Posted by cubnut93 View Post
    ..looking at this drawing I am having trouble understanding how it works once the starter switch is opened! ...
    diode routes the reverse surge away from the switch contacts.... preventing the little spark that would have jumped/pitted the contacts in switch when you turned off switch from happening....

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    fobjob's Avatar
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    Another(non-electronic) name for them (in this application) is: "snubbers".... in semiconductors, the little arrow in the symbol points in the direction of 'conventional current flow', which is positive to negative(when the diode is conducting)....because, when Ben Franklin invented polarity nomenclature , he didn't know about electrons....he had to set a standard, and he did...

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