SERPENT I - DC motor controller

INTRODUCTION: The H-Bridge DC motor driver/controller (code-name: SERPENT I) is based on the powerful MOSFET IRF3710 declared on 57 amps of continuous current. The driver can be divided into two parts, the first dedicated to the control units, such as microcontrollers and development boards, and the second one, energy part, dedicated to the power MOSFET transistors. SERPENT I – DC motor controller/driver is announced by article publication titled “DC motor controller” at an InfoElektronika magazine from Niš – Serbia.

SERPENT I - H-Bridge IRF3710
IRF3710 H-Bridge
H-Bridge is composed of four MOSFET transistors IRF3710 declared at 57 amps. Power MOSFET transistors are driven by a gate driver LM393 which is actually integrated circuit with two comparators/operational amplifiers. As LM393 is a comparator, it means you need to provide a reference voltage point that is made in a single voltage divider. The voltage divider is implemented by R10 potentiometer with a nominal resistance of 50K ohms, which is regularly attached in series to the resistor R9 with 50K ohms resistance, providing Vref voltage used by LM393 gate driver. By doing so, power consumption through the same branch R9-R10 is reduced. Resistor R9 plays the role of additional protection if the slider of potentiometer R10 is positioned near the VDD power supply. Without R9 resistor, uncontrollable current may cause damage to the potentiometer and the rest of the electronics.
Rectifier diodes
If the positive input of the comparator takes a voltage level higher than the threshold voltage Vref, the output of the LM393 becomes approximately the same as the motor power supply voltage, providing current flow through the MOSFET transistors. At the same time, between the gate and source, there is a certain difference of potential, which must be within the limits VGS maximum. Voltage gate also depends on the PWM signal.

Diodes 1N4002 / 1N4004 are fly back diodes and play a role in protecting the MOSFET transistors from the peaks generated by the back electromotive force (BEMF). On the other hand, as H-Bridge works in switched mode (Pulse Width Modulation - PWM), fly back diodes allows the generated back electromotive force to “go back” to the power source (power supply), at the same time providing H-Bridge to work as a battery charger.

LSP1 and LSP2 are connections used for DC motor, the soldering points of the DC motor windings are on the bottom side of the H-Bridge PCB. No special connectors are intended for the DC motor connection with the H-Bridge. SERPENT I electrical schematic is presented on figure #1.

Figure #1. SERPENT I - DC motor driver electrical schematic

Motor power supply

+ Motor Power Supply branch is intended to supply DC motors with electrical energy.
There are two ways to bring power to the DC motor: via connector x2-1 (x2-3 GND) and via direct connection to the PCB by soldering the LSP3 point to the positive power supply pin directly (solder on the bottom side of the PCB). The motor power branch related to the DC motor, is presented on the both sides of the PCB, which means that certain components must be soldered on both sides, figure #2. MOSFET transistors IRF3710 (TO-220 package) are in the same line together with voltage regulator / stabilizer 7815 (TO-220 package). Because power MOSFET transistors as well as voltage regulator 7815, are much higher than other components, all other components like diodes, resistors, DIL sockets are soldered first. After all components are soldered, TO-220 heat sinks are installed on the PCB. Available PCB space regarding TO-220 heatsink, fits to the heatsink model HSINK TR-5. More details about heatsink at the following address: Additional cooling is regulated by a copper surface on the top and bottom of the SERPENT I PCB.

Figure #2. PCB - DC motor power supply branch

7815 - CMOS Logic power supply
7815 step-down voltage regulator

7815 IC is a step down linear voltage power supply with +15 volts and 1 amper at the output and dropout voltage of 2.5V maximum, which means that minimum power supply should be rated at 17,5V in order to provide 15V at the output. 7815 IC is intended to provide power supply to the H-Bridge CMOS logic gates used to driver gate driver LM393. Both sides of the 7518 IC, input and output, have a block and/or electrolytic capacitor used as additional voltage stabilization, low pass filter / baypass capacitors. JP1 is a jumper intended primarily for H-bridge circuit testing, and it is used to allow testing below 15V power supply sources. For example, to allow testing at 12V power supply voltage level. When the jumper JP1 is shorted, 7815 is used as power source regarding CMOS logic gates (4082, 40106). Otherwise, CMOS logic gates are powered through VDD connector X2-2 (x2-3 is GND). JC2P and JC3P are power supply pins related to CMOS logic gate ICs. Each logic gate, including LM393 gate driver has its own additional block capacitor in order to provide extra low-pass filtering (additional integrated circuit power supply stabilization).

CMOS Logic gates

CMOS Logic gates
There are two logic gates (40106 INVERTOR, 4082 AND) that are used to provide control signals to the gate driver LM393: Enabled, Direction and PWM. If the Enable signal is set to logical zero H-Bridge brakes, does not work. In this case the outputs of the 4082 logic gates are set to the logic zero and therefore the comparator outputs as well, controlling the IRF3710 MOSFET transistors as inactive – in that case, H-Bridge does not work. Otherwise, H-Bridge circuit is active. Direction signal determines the direction of the DC motor rotation. If the Direction signal is set to the high logical level, IC3A output has a present PWM signal. In that case the output of the IC3B is set to a logical zero, and it is done over the 40106 integrated circuit (more details at present schematic). PWM signal is constantly present at both 4082 logic gates at the same time. R7 and R8 are pull-up resistors at the output of the AND logic gate 4082. The signal from the 4082 outputs is forwarded to the LM393 positive inputs.

LM393 as IRF3710 Gate driver

Control logic and opto-coupled inputs

4N25 opto-couplers with protoboard
Control is performed via the control signals: Enable, Direction and PWM, throughout the microcontroller/processor or any other control unit/development board. Each control signal is opto-isolated by using opto-couplers 4N25 IC, consisted of the photo diode as transmitter and photo transistor as receiver. Example: if the photo - diode is hardwired in series with the resistor of 30 ohms in resistance, the control signal logic level should be 0 or 3.3 volts. Photo diode cathode is connected to the ground GND_OC which is an integral part of the microcontroller or any other control unit device. On the receiving side, the emitter of the phototransistor is hardwired to the DC motor power supply GND. By doing so, control logic devices and energy part of the H-Bridge are not hardwired directly, there is no physical connection between these two parts. On the other side, phototransistor collector is connected in series to the resistor of 10K ohms in resistance. The cutoff area between the collector and emitter presents high logical value/level (VDD voltage). In opposite, the saturation of the phototransistor is presented as a logical zero, approximately zero volts, but it is not a zero volts. 4N25 IC opto-coupler inverts the logic of a computer system. Example: if the Enable logic level is set at 3.3 volts (high logical level), between the collector and emitter logic zero is presented, because current flows throughout the phototransistor, that means that Enable control signal is inverted by 4N25 IC. SERPENT I do not provide any predefined connector except screw connectors. Instead, protoboard matrix with 4x8 dimension is provided as a generic solution in order to build right adapter connectors for target development board/platform/microcontroller. IRF 3710 DC motor driver is equipped with 32 soldering points, accessible from both sides. For each control signal (Enable, Direction, PWM) eight soldering points are available in one row, and eight additional points are available for opto-coupled ground (GND_OC).

H-Bridge DIY (do it yourself) assembling

All H-Bridge (SERPENTI) components are through-hole components and can be soldered by standard soldering irons. The component soldering should be completed in the following order. First, all lower components (lower to the PCB) such as resistors, diodes and DIP / DIL sockets are soldered. After that, higher components like IRF3710 and 7815 voltage regulators are soldered. During the SERPENT I assembling be aware that you are discharged in order to avoid electrostatic discharges generated by wearing synthetic clothes. After soldering, flux should be removed completely from the PCB. In order to clean PCB from soldering flux, 70% alcohol is good enough. At the following link, a couple of videos presents DIY (do it yourself) SERPENT I assembling.

SERPENT I - assembled blue-white edition

SERPENT I - assembled black-yellow edition

SERPENT I - blue-white, white-black and black-yellow editions:

SERPENT I - blue-white, white-black and black-yellow editions
Posted by Zilsel Invent on Sunday, December 27, 2015

zilsel-invent assumes no responsibility or liability for any errors or inaccuracies that may appear in the present document.
Specification and information contained in the present schematics are subject to change at any time without notice.



  1. Article describes only fundamental concepts/electronics around H-Bridge DC motor controller (code-name: SERPENT I). In the near future, some parts of the article will be explained with more details.


Post a Comment

Popular posts from this blog

Electrolytic capacitors and design rules