# Long supply line and capacitor to prevent voltage drop

I have a 30 feet line with DC supply 12 V. A motor on the end draws about 0.5 Amps initially (100 ms). It's logic shuts down because of the line's voltage drop caused by inrush current. A capacitor of 1000 uF prevents the shutdown.

However, if I calculate the size of the capacitor that prevents a drop of more than 1 V, a much larger (x50) value comes out (0.5 A * 0.1 s / 1 V = 0.05 Farad).

I figure this is because the supply isn't disconnected but continues to supply power, so the capacitor does not have to provide the current alone.

How can I improve my model so the calculation is closer to the real thing?

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You need to treat the line as an in series inductor.

This may be the limit of lumped element mode analysis(treating the line as a simple inductor is an example of this), but it will at minimum give you a good starting point.

The line has two values, an inductance and a capacitance, if you place the supply entering an inductor, then a capacitor from the line in parallel with your capacitor it should work quite well.

There are many tools for calculating your inductance, I quickly turned up this one.

Since you are putting a large capacitance at the end it will probably be so large in comparison to the capacitance of the line that the line capacitance is negligible.

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