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# How much should the capacitors capacitance be for my race track, car project? Answered

Hi im doing a project for my brother for christmas.
Its a Drag race track thing, I have it all planned. Buuuuuuut for the cars I am using a capacitor, and a motor rated for around 6volts. WHAT should the capacitor's capacitance be?
A high capacitance or a low one?

I have a charging spot on the track to charge the capacitor, and then the energy stored in the capacitor powers the motor for long enough time to make it to the end.(Which is only 3 feet).

Note:I found a small car that does just that but the motor doesnt work and the capacitor doesnt show the capacitance on it and I used my multimeter but it said DisC.

I will post a picture of it later if it even helps. :(

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## Discussions

To start the calculation, you need to know the motor current. Like Lemonie says, you'll be horrified how little you can store in practical capacitors.

Steve

It's possible to figure out how large a capacitor you need.  You need to know the power you will have to supply to the motor for 1-2 sec.  That will give you the current, which can be converted into charge.  From the charge, you can figure the size of the capacitor.  As you see, I have not made the calculation because I don't have to time to relearn it all.

Why don't you start by getting as high capacity capacitor you can find and charge it with a 9 v battery?  Then attach it to the motor, and see what happens.  If the motor spins like crazy for a moment, put a little resistance in series with the motor and capacitor and see if it will turn it for a second or two.  You can increase capacitance storage by wiring them in parallel.

Oh yes, camera stores often give away disposable cameras.  They have big caps inside.  Discharge the caps with a screwdriver first to prevent a shock when getting them out of the camera.

Why are you trying to charge it at one point? If it is part of the game, why not do it digitally?

A metal contact on the track could be used to tell a microchip when the car is charging, which can then count up, and even display the power on a LED bar chart. It could then count down with time so that you have to get around the track quick enough or your power runs out. The main power could come from the track such as a scalextrics, and the microchip have a small lithium battery to keep it on.
If the power runs out, the chip could pulse the motors to give reduced speed so the car will make it home, but much slower.

To expand, you could add more levels to the game, like the clock counting faster, challenging you to go faster. You could have a second microchip on the trackside with a 7-segment output displaying lap times.

As Lemonie point out, you wont get much charge in a capacitor to run the motor.
What about using a scalextrics track. That way you can power the motor on the cars from the tracks.

People usually over estimate the amount of charge that a capacitor will hold. You'll not get enough in anything of a reasonable size.

L