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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.

Schematic

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    \$\begingroup\$ Sometimes this is a kludge added to prevent the motor-spikes from resetting the processor. That includes PWM and motor on/off signals. Ideally place those caps on the motor terminals, right at the motor's case. (And, if your flyback diodes aren't 2mm away from the motor terminals, without those capacitors you may be creating a loop-antenna driven by few-amps MHz pulses.) \$\endgroup\$ – wbeaty Apr 6 at 0:39
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    \$\begingroup\$ Does this answer your question? Purpose of the diode and capacitor in this motor circuit \$\endgroup\$ – Dmitry Grigoryev Apr 6 at 15:12
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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.

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    \$\begingroup\$ This is the correct answer. Motor-sparking is broadband. The inductive spikes can get into supply rails, or even jump across space via inductive/capacitive coupling, and end up in your sensor channels or your audio outputs, or start resetting your cpu. (In old tape-players, there's always ALWAYS a 0.01uF cap soldered to the motor's terminals.) Sometimes the spark-plasma can involve nanosecond transients, and can even radiate GHz trash, especially if the motor-leads have a resonant loop size, or resonant length. \$\endgroup\$ – wbeaty Apr 5 at 21:29
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DC motors use brushes on the commutators. These spark due to inductance of the coils as they switch from segment to segment.

enter image description here

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.

enter image description here

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.

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  • \$\begingroup\$ But the D version of an L293 already contains the flyback diodes inside IC. \$\endgroup\$ – G36 Apr 4 at 12:02
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    \$\begingroup\$ OK. I didn't find that datasheet. OP should have supplied it. \$\endgroup\$ – Transistor Apr 4 at 12:08
  • \$\begingroup\$ But read the note below figure 2 that you have posted -> "Output diodes are internal in L293D" \$\endgroup\$ – G36 Apr 4 at 12:11
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    \$\begingroup\$ Ha! I missed that. I'll fix the answer. Thank you. \$\endgroup\$ – Transistor Apr 4 at 12:16
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    \$\begingroup\$ I was very confused about the way your answer suddenly jumps from talking about capacitors and high-frequency spikes to a diagram showing a bunch of flyback diodes (and no capacitors). Looking at the edit history, the version from before you added the diagram seems a lot clearer. Maybe you meant to add some kind of connecting paragraph before the diagram, but forgot to? \$\endgroup\$ – Ilmari Karonen Apr 4 at 21:28
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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

  1. this is a motor driver using the often abused 1986-designed L293D,
  2. the layout quality of the schematic is more than questionable,
  3. the schematic happily mixes capacitor values and SMD capacitor designators,

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.

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    \$\begingroup\$ Output flyback diodes are internal in L293D \$\endgroup\$ – G36 Apr 4 at 12:19
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    \$\begingroup\$ ah, true, I was missing the D \$\endgroup\$ – Marcus Müller Apr 4 at 12:32
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    \$\begingroup\$ @MarcusMüller: The capacitors are definitely there to reduce the commutator noise, so they're not useless. \$\endgroup\$ – Edin Fifić Apr 4 at 18:26
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In this circuit capacity it act as filt to filt the ripples of ac to allow DC only to move. And we are known that capacity is a device that can use to storage the charges in a circuit

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  • \$\begingroup\$ Welcome to EE.SE, Sandile, but this is incorrect. The motors are DC and they are fed from a DC source using PWM. The capacitors are too small to hold enough charge to power a motor. Read the answers with high votes to understand. \$\endgroup\$ – Transistor Apr 6 at 6:34
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In this circuit capacitys it act as filters to filter the ripples of ac to allow DC only to move to motors because this motors the allow 12v of DC only to operate. And we are known that capacity is a device that can use to storage the charges

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  • \$\begingroup\$ Same answer again? \$\endgroup\$ – Mike Apr 6 at 6:38

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