This instructable describes the electrical design of the BioMONSTAAAR's H/C control board.
Here are the Instructables for other parts of the bioreactor:
Heating and Cooling System
Raspberry Pi manager
This board is the interface between an Arduino on one side, and a peltier (for thermoelectric cooling/heating), a fan, and a water pump on the other side. It is part of a temperature control loop in which the Arduino measures the water temperature, and controls heating or cooling of the BioMONSTAAAR bioreactor. The peltier cools, or heats the water. The fan blows air over the peltier heat exchanger. The pump transports the water around the bioreactor tank. Mosfets turn ON/OFF the power to the peltier, the fan, and the pump. Relays are used to invert the power to the peltier element.
2x Perf board (general purpose printed circuit board)
2x Relay TE PCLH-202D1S DPDT 10A 12V coil
1x Diode 1N4148 DO-35
1x Voltage regulator 7812 12V 1A TO-220
2x Heat sink vertical 5 K/W TO-220 vertical PC mount
7x Screw terminal 5.08mm pitch
2x Insulator vinyl TO-220
2x Screw set for TO-220 heat sink (screws, nuts, washers, locking washers)
3x Mosfet IRLZ44PBF-ND N-channel TO-220
1x Mosfet IRL1404ZPBF-ND N-channel TO-220
4x Resistor 150 1/4W
4x Resistor 10k 1/4W
4x Resistor 4k7 1/4W
2x Resistor 2k2 1/4W
1x Capacitor 100u electrolytic
1x Capacitor 100n
6x LED red 5mm 2/package
6x PC board stand offs and mounting materials (screws, nuts, washers)
The schematic is designed in CadSoft Eagle and can be downloaded below.
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Step 1: Arduino Interface Connector
The BioMONSTAAAR H/C Control Board is designed to be controlled from a microcontroller with 0-5V digital outputs. The connector has screw terminals.
0V on PELTIER turns peltier OFF , and +5V turns peltier ON
0V on POLARITY is COOLING, and +5V is HEATING
0V on PUMP is OFF, and +5V is ON
0V on FAN is OFF, and +5V is ON
Step 2: Power Supply
The H/C Control Board is designed to operate from an external power supply, the Supernight 24V 14.6A 360W. The voltage on the power supply can be adjusted between 15V and 30V. Because the peltier (24V/15.5A) can be used at 50% to 80% of maximum power, set the voltage between 15V (63%), and 20V (80%). Also, because the power of the peltier exceeds the max of the power supply, the voltage must be maximal 20V to prevent over loading. The power supply screw terminal connector on the control board has two connections. Vin and GND. Warning: take into account the polarity of the wiring, because incorrect polarity will damage the circuit! An LED is designed to indicate the presence of voltage.
Step 3: Peltier Control
The H/C control board is designed to be used with Custom Thermoelectric - 19911-5L31-15CQ. 24V/15.5A. The Peltier is placed in between the fan heat sink and the water cooling block. The current through the peltier is linear with the voltage. By inversing the polarity you can choose between cooling and heating. It is not recommended to use PWM to control the average power, because when ON, the current is maximum, and the internal resistance may even heat up the cool site of the element. Because the peltier (24V/15.5A) can be used at 50% to 80% of maximum power set the supply voltage between 15V and 20V. Because the current of the peltier (24V/15.5A) exceeds the maximum current of the power supply (14.6A), the power supply must be set between 15V (9.7A) and 20V (13A). Never allow the peltier to heat up beyond Tmax (125 degrees Celsius). If you are unsure if the heat sink is large enough, use thermo couples mounted close to monitor the temperature.
A mosfet in TO-220 package is used to turn the peltier ON/OFF. It connects the negative side to GND. This MOSFET was selected because of extreme low Drain-Source resistance (6 mOhm at Vgs=4.5V). A heat sink of 5 Kelvin per Watt is selected to keep the temperature rise below 15 K. Two LEDs are used to indicate the polarity. A 10k pull down resistor is used on the gate of the mosfet to turn the mosfet OFF when the input is left unconnected. A 150 Ohm resistor in series with the gate limits the current when the Arduino is charging the gate capacitance of the mosfet. Double pole double through (DPDT) relays are used to inverse the polarity to the peltier element to either heat or cool. Two 10A relays are used in parallel to be able to handle 15.5A. The peltier has two screw terminal connections on the control board. When Peltier+ is positive, it cools the water.
Step 4: Water Pump
The circuit is designed to be used with ZKSJ DC40-2470 Brushless DC pump 24VDC/1.15A/26.4W. An N-channel mosfet in TO-220 package is used to turn the pump ON/OFF. The pump is supplied from the same voltage as the peltier element. The mosfet connects the negative side to GND. This MOSFET was selected because of low Drain-Source resistance (28 mOhm at Vgs=5.0V). The calculated temperature rise of the mosfet is less than 1 Kelvin, so it does not need a heat sink. A LED is used to indicate the pump is ON. A 10k pull down resistor is used on the gate of the mosfet to turn the mosfet OFF when the input is left unconnected. A 150 Ohm resistor in series with the gate limits the current when the Arduino is charging the gate capacitance of the mosfet. The pump has two screw terminal connections on the control board: Vin and Pump_GND. The red wire of the pump connects to Vin and the black wire connects to Pump_GND.
Step 5: 12V Power Supply
A 7812 voltage regulator in TO-220 package is selected to create 12V from the external power supply (14-20V). The 12V supplies the relays (2x 75mA), and the current for the fan (210mA). A heat sink of 5 degrees Kelvin per Watt is selected to keep temperature rise below 15 K. Because of the minimal 2V voltage drop of the 7812 a minimal voltage of 14V is required on the input to create a stable 12V on the output for the fan and the relay control. A LED is used to indicate that the 12V is active.
Step 6: Fan Control
The circuit is designed for Thermalright - Venomous XR–T 12VDC/210 mA. The fan is mounted on the heat sink. The peltier is mounted in between this heat sink and the water cooling block. The fan connector pin out is according to table. In our application the 12V and the control line are connected together run at maximum speed. The sense signal is left unconnected.
A mosfet in TO-220 package is used to turn the fan ON/OFF. It connects the negative side to GND. This MOSFET was selected because of low Drain-Source resistance (28 mOhm at Vgs=5.0V). The calculated temperature rise of the mosfet is less than 1 K, so it does not need a heat sink. A 10k pull down resistor is used on the gate of the mosfet to turn the mosfet OFF when the input is left unconnected. A 150 Ohm resistor in series with the gate limits the current when the Arduino is charging the gate capacitance of the mosfet. The fan has two screw terminal connections on the control board: +12V and FAN_GND. The yellow and blue wires of the fan are connected to +12V, and the black wire is connected to FAN_GND.
Step 7: Improvements and Next Steps
The current circuit is not designed to monitor the peltier temperature, or correct functioning of the cooling system. Failure of the cooling system may lead to overheating and dysfunction of the peltier. As a result the cultures in the bioreactor may behave unexpectedly. There is reasonable probability of failure, because of user faults or component failure. The fault may be detected by the Arduino that monitors the temperature in the bioreactor, but it may be too late for the culture. In a future control board a solution must be provided to monitor correct functioning of the cooling system. The proposed solution is to place a temperature sensor on the peltier and to design a circuit to disable power to the peltier. Another possible solution is to use a thermal switch (thermostat).
In the current the design the fan and pump are turned OFF when the control inputs are left unconnected. An accidentally disconnection of jumper wires between the Arduino and the control board may lead to a nonfunctional cooling system and overheating of the peltier. In stead of a 10k pull down, it is recommended to use a 10k pull up on control inputs fan and pump.