I am currently building my own control board for a high power brushed DC motor (24V 16A). I am using an Arduino to provide a PWM signal to an IRLB8743 MOSFET. The reason I am trying to build my own is because I have blown up so many cheap unbranded controllers, and I want to implement a safe-boot routine and slow-start functionality, along with a few other custom functions.

From disassembling wasted boards, I have noticed that large electrolytic capacitors (63V 470uF) are placed in parallel across the power supply. From my knowledge, small 6V motors have ceramic capacitors across the terminals for smoothing.


simulate this circuit – Schematic created using CircuitLab

Here is the schematic of the board I plan to create, but it is unfinished; I have salvaged two of these capacitors from other bards, but I can't decide where to put them. Should I copy the control boards, and place both caps at C1, or should I move one cap to C2 for some smoothing benefit? I don't really understand the purpose of either C1 or C2.

Another final mention is that I hope to swap out the motor in the future for an even bigger one, and maybe power it from 48V.

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    \$\begingroup\$ C1 is good, it provides some bypassing for the motor current. I think C2 will be harmful, as it will be charged and discharged every PWM cycle. \$\endgroup\$
    – Drew
    May 11 at 22:24

The MOSFET you chose will probably fail. Using a 30V device on a 24V system is asking for problems. Go to a minimum of a 60V MOSFET and use a UIS, avalanche rated one.

R4 is very weak for what you need to do. I would suggest something in the 10K range to swamp out any leakage in the Arduino.

If the MOSFET is properly rated, the flyback diode is not needed but make a space for it incase you have to sub a MOSFET.

Putting C2 in parallel with C1 will help. I do this a lot with electrolytics. I double up - each one is the needed size so when one fails you don't notice it.

I would suggest a couple of 220nF capacitors across the motor but that is application dependent. For more information Renesas publishes a nice application note: Unclamped Inductive Switching (UIS) Test and Rating Methodology Nov 9, 2015 so it has been around for a while. https://www.renesas.com/www/doc/application-note/an1968.pdf Vishay published another good one called: AN601 Unclamped Inductive Switching Rugged MOSFETs For Rugged Environments. https://www.vishay.com/docs/70572/70572.pdf

  • \$\begingroup\$ Thanks Gil. Does R3 and R4 constitute a UIS? As for the motor, I am planning on coding in an electronic lock-out if it isn't rotating, to try to inhibit voltage spikes. EDIT: What is a UIS. I ascertain that it is to stop ringing, and to provide something like 'critical damping' on a car suspension. Surely the slow-start function on the Ard will help with this too \$\endgroup\$ May 12 at 10:01
  • \$\begingroup\$ UIS (Unclamped Inductive Spike) also sometimes referred as Avalanche. It is the transient that comes back when an inductor is charged and the power is removed. When the power to the inductor is turned off it reverses polarity and the voltage rises until it is limited regardless of the supply voltage. That principle is the heart of the SMPS, boost and buck converters. Capacitors will adsorb it, so will light bulbs or any other type of load. \$\endgroup\$
    – Gil
    May 14 at 0:59

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