I have a circuit as such that will ultimately be used for a remote controlled race car:

Remote Control Race Car Circuit

I have a 5V power regulator (SMPS) to take 4 AA batteries to 5V. With this, I will power a Raspberry Pi, a DC brush motor, and a Servomotor. The construction of this circuit will be pretty modular, as the regulator, the Pi, the H Bridge circuit are all their own self contained components, and of course plus two motors - I will primarily be connecting leads together.

The race car will ultimately weigh 2 - 3 pounds, no more than 5.

My questions are:

  1. If I can count on my power regulator to have a filter capacitor to keep the voltage ripple to a minimum, do I need a bulk capacitor additionally for a 5V circuit?
  2. Do I gain advantage in having one bulk capacitor adjacent to each motor? Or would one be sufficient?
  3. Granted that this is a 5V circuit, how do I choose the capacitance for my bulk capacitor(s)?
  • \$\begingroup\$ 1. Build the circuit without bulk capacitors, and connect an oscilloscope where the bulk cap would go, and see what happens while operating. Then install a bulk cap and re-test. Add more, test again. 2. Possibly, depends on results of #1. 3. Pick ones at least two or three times the supply voltage. The lower the ESR or equivalent-series-resistance, the better. \$\endgroup\$
    – rdtsc
    Jan 13, 2016 at 2:47
  • \$\begingroup\$ As suggested by @rdtsc you have to do this experimentally because you are missing multiple parameters like, is there decoupling capacitor in the H-bridge circuit, how much voltage ripple does the R-Pi tolerate, what is the impedance from the regulator to the H-Bridge, what is the switching frequency of the H-Bridge, etc. \$\endgroup\$
    – jpcgt
    Jan 13, 2016 at 18:46

1 Answer 1


You need at least to know how much supply ripple your different components will tolerate. This is your starting point.

Then I would draw an equivalent circuit of your power distribution network and the main load (the motor as an inductor in series with a resistor and a switch)


simulate this circuit – Schematic created using CircuitLab

You will need to approximate/measure the L and R values for the cables going to the H-Bridge and other parts. Do a transient simulation where you open the switch after the motor has reached maximum current. You should see a large oscillation at the voltage input to the R-Pi. Adjust C2 and C3 to reduce it to acceptable levels.

If you cannot measure the inductances, try approximating them using an online calculator.


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