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Land Rover DIY Potato Condenser



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This is an old story that has cropped up Land Rover forums where someone measured the capacitance across a similar potato & was unable to detect any meaningful capacitance.


Well I have just selected a similar type & size of potato using test prods that are about half an inch long probably similar to the length of the screws used & measured 0.09 mfd. So this is slightly under half the normal 0.2 mfd used in most distributors.


Before penetration I measured the surface resistance from one end of the potato to the other & this was about 100 megohms. After penetration with the same prods the resistance fell to 1.2 megohms but this was with a multimeter at very low voltage. The role of the capacitor is to absorb the Back EMF from the primary which is in the order of 300 volts. The effect of this is to reduce sparking at the points to enable a more abrupt collapse of the magnetic field in the coil. This leads to an improved HT output than if no capacitor is used.


I tested the resistance at 250 volts & found that the resistance fell to 0.01 megohms. So at 250 volts a current of 25ma will flow, clearly a useful path for the Back EMF. Apply a rough calculation for 12 volts only 0.01ma would flow & have negligible effect.


So it looks as if the potato might be performing two roles. Acting as a crude VDR (Voltage Dependant Resistor) clipping the Back EMF & also providing about half the required capacitance.

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I've been doing a bit more experimentation today :shocked:




I repeated the measurements a bit later & found using the same holes, although the resistance remained the same, the capacitance had reduced a bit. Cleaning the prods & inserting them in new areas nearby still produced these new readings. All I can think that the cold potato was now a bit warmer now it had been indoors for a few hours.


Anyway I found moving the prod insertion points closer increased the capacitance to such a point that the prods one cm apart had nearly double the capacitance of the prods at the furthest point. So a better result could have been achieved if the AA man had put the screws nearby to each other.


The curious thing is that wherever I inserted the prods be it, near to each other, on opposite sides across the width or the length the resistance remained constant.


Next I tried a carrot & this behaved in the same way with capacitance being greatest with the prods nearby. But the resistance remained the same here on both measurements as 1.1 megohms.






I measured the capacitance as 0.08 mfd with the prods close by then chopped the carrot & it remained at 0.08 mfd




So as far as vegetables go, size or even length does not matter :-D


Looks like I'll be eating perforated vegetables tonight :shocked:

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The versatile spud! On an entirely unrelated subject, early gold miners used to extract fine gold from the concentrate in their gold pans using mercury (artisanal miners still do) and when the mercury started getting lumpy, they knew it was saturated with gold as an amalgam. They would then place the lumpy Mercury inside a raw spud and roast it on a fire. The Mercury would mostly vapourise off, leaving gold (mostly) in a nugget in the potato. Alas, some miners then liked to eat the baked spud, Mercury residue and all. "Gold fever" was not a myth!

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