SERPENT II - Pit VIPER Rattle - DC motor controller

SERPENT 2, Codename Pit VIPER Rattle, is independent ecosystem DC motor driver/controller designed according to the IRF3710 power MOSFET transistor. IRF3710 is bases, but it is possible to use any IRF3710 replacement, which fits into the TO220 package including pin configuration. For example IRF540 is also suitable replacement instead of the IRF3710 power MOSFET. Of course, it is important to pay attention on maximum continuous DC current related to the power MOSFET, since the PCB (printed circuit board) is designed to handle IRF3710 57 amperes of continuous DC current, as it is declared under specification.

If we look closely to the electrical schematic, Pit VIPER is nothing else then four channel current amplification circuit, but it is intended for DC motor control. For details about current amplification follow this link: power switch as current amplifier. As any other DC motor driver designed by zilsel-invent, Pit VIPER is consisted of two parts: first part is related to the control devices such as microcontrollers, processors, SoC or any other prototyping / embedded system development board, while the second part is related to the linear voltage regulator, gate drivers and power MOSFETs (energy part). Physical connection does not exist between control device part and energy part, since opto-coupled device 4N25 (or replacement) is used in between.

SERPENT II - Pit VIPER Rattle - DC motor controller/driver (click on the schematic to zoom in).

DC motor power supply and voltage regulator 

Regarding DC motor / DC motor controller power supply there is a limitation which depends on the chosen linear voltage regulator. For instance, if LM7815C TI linear voltage regulator is used, packed into the TO220 IC package, that means that maximum DC voltage power supply is limited to the 35 VDC. Beside 7815 linear voltage regulator, it is also possible to use 7818 IC, packed into the TO220 IC package as well. Minimum power supply is rated to the 17.7 VDC since dropout voltage is set to 2 VDC. What does that mean? If 16VDC is applied to the 7815 input, that means that the output voltage is below 15 VDC, and that is not good, because target output voltage is 15 VDC (calculations: 16 VDC – dropout voltage < 15 VDC -> 16 VDC – 2 VDC = 14 VDC). Output current of the 7815/7818 linear voltage regulator, regarding available output current intensity is enough to drive opto-isolated receiver parts (opto-sensitive transistors) and LM393 comparators regarding their current consumption. If you do Pit VIPER assembling by yourself, information about maximum input voltage, dropout voltage, available output current etc. are provided under 7815/7818 IC specification. For example: 7815 voltage regulator provides about 1.2 amperes as output available current.

On both sides of the 7815/7818 IC electrolytic capacitors of 100uF (micro farads) are used to provide additional voltage stability. The output of the 7815 IC beside electrolytic capacitor provides additional ceramic capacitor of 100nF as additional voltage stabilization against voltage noise (low pass filters / baypass capacitors). If you do assembling by yourself, capacitor voltage should be above maximum DC power supply voltage. On the PCB, 7815/7818 voltage regulator is placed in the top right corner.

SERPENT II - Pit VIPER Rattle - DC motor controller/driver
Opto-coupled 4N25 device as input stage

The input of the 4N25 is dedicated to the control devices such as microcontrollers, processors and any other type of development boards like: Arduino UNO, Intel Galileo / Galileo Gen2, Raspberry Pi, MikroElektronika, Beagle Board etc. Input stage is open anode and open cathode, and for each pin three soldering points are dedicated on both PCB sides. Beside open anode and open cathode there are a plant of free soldering points available for building an adequate adapter necessary to interface Pit VIPER with Arduino UNO development board, or any other development board available on the market. 

If you do interfacing with the Arduino UNO, open cathode can be connected to Arduino UNO ground while open anode can be connected to resistor in series, where the second pin of the resistor is connected to the one of the Arduino UNO digital pins or directly to 3.3VDC or 5VDC depending on the configuration you need. Anyway, that is only an example, but you should know that input stage of opto-coupler 4N25 device belongs to the control devices and no physical connection should exist between two parts of Pit VIPER DC motor controller. By doing so, Pit VIPER provides safety against high DC currents from energy part. Note: if you do configuration in terms that both sides of the Pit VIPER has the same ground potential and the same positive voltage potential (+ battery/power supply pin), you are dealing with your own, so be aware that these two parts are physically disconnected/isolated (two isolated DC power supplies are request, first for the control devices like Arduino UNO and second for the Pit VIPER energy part - MOSFET transistors, voltage regulator, gate drivers etc).

The output side of the 4N25 opto-isolated device is opto-sensitive transistor, NPN type. The collector is connected with resistor in series and the second pin of the resistor is connected directly to the output of the 7815/7818 voltage regulator. The resistor value is set to 10K ohms. If you do assembling by yourself, stay at a 10K value to eliminate current consumption through photo transistor. Also, for LM393 comparator, used as a gate driver (comparator as a gate driver will be covered in separated article), only potential from the photo transistor collector pin is required, and that is the reason why current consumption regarding photo transistor should stay as lower as it is possible. 

Pay attention here that opto-coupled output provides inverse digital logic, but that doesn’t mean that you have to think in inverse way when you write software for DC motor control. If the DC current does not flow through the photo diode, photo sensitive transistor is switched OFF, which means that the collector potential is set to 15VDC (in case when 7815 IC voltage regulator is used) and that is a logic one. In that case, potential on the photo diode anode is set to logic zero or it is in floating state (opened, not connected to any of the external components). If the photo diode anode is connected to the resistor in series and current flows through resistor and diode, photo transistor is switched ON, which means that the potential between collector and emitter is very low, half value of one volt (for example), and that is a logic zero. 

Now, because the collector potential of the opto-coupler is connected directly to the inverting pin of the LM393 comparator while voltage reference is connected to the non-inverting pin of the LM393 comparator, the output of the LM393 is set to logic one or 15VDC (in case when 7818 IC voltage regulator is used, output of the LM393 will be set at 18VDC) when anode of the opto-coupler is set to the high potential signal, and that means that logic one in control software triggers current flow through the power MOSFET transistor. 

If control software generates logic one (5VDC from microcontroller pin) 4N25 photo diode conduct DC current. By doing so, the NPN photo transistor is switched ON providing logic zero between collector and emitter. Because logic zero between collector and emitter is lower than the reference voltage at the non-inverting pin, +15VDC voltage is presented at the LM393 output at the same time providing amplified current flow through power MOSFET transistor. 4N25 is not the only option regarding Opto-coupled device. It is possible to use any replacement, which fits into the pin configuration and internal structure.

LM393 comparator as gate driver

The power MOSFET gate driver is built around LM393 comparator instead of using special purpose integrated circuit designed to be used as gate driver. Comparator as a gate driver will be covered in separated article, anyway, to work properly voltage reference is necessary part and it is implemented by two voltage divider resistors R1 and R2 both 10K ohms. Voltage reference VREF_CMP is additionally stabilize with 100uF electrolytic capacitor (low pass filter / baypass capacitors, only low frequency signals could pass filter, high frequency signals like instant voltage noise are filtered since the capacitor is not possible to discharge/charge immediately throughout the resistor network, except if both capacitor pins are connected directly). The reference voltage is connected to the non-inverting input of the LM393 comparator, while collector of the 4N25 photo transistor is connected directly to the inverting input pin of the LM393 IC. The output of the LM393 is pulled-up with 2.2K ohm resistor since LM393 provides open-collector connection as an output pin. LM393 collector connected to the output pin (pulled up with 2.2K ohm resistor) is directly connected to the power MOSFET gate pin, providing at the same time gate driver. The potential difference between the collector of the LM393 comparator and source pin of the IRF3710 provides VGS voltage necessary to control current flow through power MOSFET (do not forget that N-Channel power MOSFET is voltage controlled current source or current amplifier).

SERPENT II - Pit VIPER Rattle - PCB Printed Circuit Board manufactured by Ei PCB Factory Niš Serbia

IRF3710 N Channel power MOSFET transistor 

IRF3710 was the basis for the Pit VIPER electrical and PCB design. It is a power MOSFET transistor with good switching characteristic able to handle 57 amperes of DC continuous current (pick current is much higher, but do not forget that pick current is possible to drive only with PWM (pulse width modulation) signals with certain duty cycles). IRF3710 is packed into the TO220 package with external TO220 heat sinks for each power MOSFET. Anyway, four heat sinks could be replaced with one heat sink since all power MOSFETs are placed on the same line. Power MOSFETs are configured as open drain while the source of the power MOSFETs is connected to the power supply ground. Each power MOSFET has its own rectifier diode 1N4004 in order to preserve power MOSFETs damaging against back electromotive force generated by DC motors. Rectifier diode 1N4004 could be replaced with any rectifier diode which fits into 1N4004 characteristics.

LED indicators

Pit VIPER provides two types of LED indicators. LED on power is first indicator. It is an LED diode which anode is connected to the 7815/7818 voltage regulator and cathode connected in series with resistor R15. Pit VIPER provides two power on LEDs, one is intended for 3mm package and second is intended for 5mm LE diode package. Resistor R15 value depends on chosen LE diode.

The second type of LE diode is power MOSFET signal indicator and it is configurable LED by using jumpers. If jumper JP4 is set, LED 4/LED 8 conducts current when the power MOSFET is switched ON and diode emits light at the same time indicating that power MOSFET is active. This is an excellent feature since it is not necessary to use DC motors at development time. LEDs are good enough to check does power MOSFETs are active at the right time. As for the LED on power, Pit VIPER supports 3mm and 5mm LED packages for active power MOSFET indication.


Pit VIPER is intended for DC motor control systems, for embedded robotics and Internet of Things (Elektor publication). It is possible to drive four DC motors in one direction at the same time with or without PWM signal modulations (speed controllers). Pit VIPER is also suitable controller to drive stepper DC motors, at the same time Pit VIPER is applicable for different types of stepper motor algorithms like: stepping or micro-stepping algorithms (here you will find stepping algorithm example regarding ULN2xxx DC motor driver, anyway the same approach is used for stepping algorithms related to the Pit VIPER DC motor controller/driver).

These are only technical details related to the Pit VIPER DC motor control for all the people who like to know something about Pit VIPER electronics. Regarding usability it is simple as it is, with a couple of resistors connected in series with photo diode open anode, interfacing between Pit VIPER and any kind of development board is very easy. Writing the control software is the meter of requirements, but only thing you should know is that the logic one generated by the control software does switch power MOSFET ON. For details follow SERPENT 2 - Pit VIPER Rattle - DC motor controller link.

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.


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