Below is the circuit diagram of an L293D motor driver IC driving 2 12V DC motors.
What I don't understand is the use of the capacitors marked 104 in parallel with the motors.
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Generally a 0.01~0.1uF capacitor is wired across brushed DC motors to reduce radio frequency EMI caused by arcing between the brushes and commutator. Sometimes two capacitors are wired in series, with the center connection going to the case to 'ground' it at RF frequencies.
For best effect the capacitor(s) should be placed on or inside the motor. In this case a capacitor has been included on the driver board. This makes it less effective at higher frequencies because the wires from the board to the motor will still be able to radiate EMI. Still it's better than nothing, and may prevent misoperation due to interference from an unsuppressed motor getting into the driver and input wiring.
DC motors use brushes on the commutators. These spark due to inductance of the coils as they switch from segment to segment.
Figure 1. Communtators and brushes. Image source: eReplacmentParts.
The capacitor shunts (or "absorbs") the high frequency spikes from the commutation and prevents damage to the driver chips.
A further factor not clear from the schematic is that snubber diodes should be used to prevent inductive kick-back from the motor's inductance causing damage to the driver's output transistors. While the diodes protect the driver, the capacitors take the "edges" off the current spikes and help reduce EMI, etc.
Figure 2. The L293 datasheet shows that snubber diodes should be connected across the motor unless the D version is used.
The left side of the schematic shows a H-bridge connected motor and the right side shows alternate configurations for single-direction motors - one connected to GND and the other to VCC.
Your question is probably only answerable through context; you're right, instead of C2 and C6, everyone would expect flyback diodes, forming a ground path for voltage spikes that happen upon switching off an output.
Maybe these capacitors are supposed to fulfill the same role? Generally, you'd avoid having capacitance here – it's hard enough on the output drivers to reverse the output voltage, no need to burn an extra bit of energy from a capacitor to heat them up!
Maybe these capacitors were necessary for EMI reasons, as they absorb high-frequency noise, e.g. from a mechanically commutated DC motor.
Now, honestly, seeing that
this clearly has not been designed by someone with a lot of experience or attention to detail.
Therefore, the answer to your question might simply be that they don't fulfill any useful role here, but have been erroneously included and shouldn't be there.
I've tried something of this sort, place a large electrolytic capacitor say like 100 uF in parallel with the motor.
I'm not too sure if it is due to resonance etc, but that the motor would spin with more power and less switch contact sparking etc.
i've not analysed the model, though.
I think I've placed that capacitor once across the manual switch itself. That reduced sparking at the switch and increased power to the motor as well. I'd guess it is rather similar to the parallel case.