FeaturesReplacing C/AA batteries with HOWICO allows Hot Wire Foam Cutters to cut foams faster or cut other materials (e.g. polypropylene). Similarly, Handy Heat Sealers can seal speedy or weld thicker films.The size of HOWICO is compatible with one C battery or two AA batteries.Powered from USB PD power banks or chargers, no need for disposable batteriesOne-button heater power control and indicators with three LEDsThe initial state is the Lowest power mode (equivalent to C/AA batteries). Each time you click the button, the heater power mode steps up (up to the Highest power mode). When you press the button for at least 3 seconds, the power mode turns to Lowest.PIC16F1503 MCU controls the heater power in the programmed range.You can modify the program: power control range, button commands, indicators patterns, etc.You can also make larger hot wire foam cutters (up to 15W) with HOWICO.Hot Wire Foam Cutterswith one C-batteryReplacing with HOWICOHandy Heat Sealerswith two AA-batterieswith HOWICOLet's make HOWICOHow designed and works HOWICOForm factor45mm x 25mm board size makes HOWICO fit battery holders for one C or two AA batteries.Right-angle pin headers with additionally bending make mate with battery holder terminals.The gouged shape of the board allows the tip of the USB cable to fit into the battery holder.Electric power and its controlUsing the USB Type-C connector and suitable USB PD power banks or chargers, we can consume up to 5V 3A: 15W power. It is strong enough for C Battery-powered foam cutters and AA battery-powered sealers. (Our targets needs no more power although many USB PD power sources can supply more than 9V 3A: 27W)If we connect the 5V power supply to the heater of the cutter or sealer directly, the current and power will be too large. So we control the power with pulse width modulation(PWM).Determining the power control circuitWe use a field-effect transistor (FET) for the principal switching device of PWM. When the FET turns on, the current from the power supply flows through the heater and the FET. In this case, because of the low resistance of the heater, the current quickly rises and becomes too large.By inserting an inductor into the current path suppresses the current rising rate.In addition, by adding a diode between the FET's drain and the heater's high-side, when the FET turns off, the current flows the inductor and the heater bypath into the diode path and continues. This movement helps reduce electrical and acoustic noise. In the absence of the bypass device, the inductance causes the FET drain voltage rasing very high, damaging the FET.Parts and their specification choiceMicrocontroller Unit (MCU)5 V power supply capable: No regulators needs and higher output voltage than 3.3 V operated one.With PWM (as a hardware peripheral)One Input for a switchOutputs for LED indicatorsWe choose PIC16F1503.FETContinuous Drain Current: ID: >3 AGate-to-Source Voltage: VGS to gate on: < 4 VStatic Drain-to-Source On-Resistance: RDS(ON): <100 mOhms: It needs well lower than the resistance of the heater (about 1 Ohm)Maximum Power Dissipation (PD): It needs well higher than the actual power dissipation (the product of the current and on-resistance).We choose IRLML6344TRPbF.Serial resister to the FET gateIt is a trade-off between gate driver current and switching speed.The FET gate is a capacitive load for the gate driver (MCU's PWM output). Therefore, a serial resistor is needed to limit the current.On the other hand, the time constant of the series resistance times the gate capacitance restricts switching transition time, and a longer transition time causes higher switching loss.The MCU port's output current needs to meet the recommendation and ratings.We chose 270 ohms: the output current of the gate driver port has a margin for the recommended value of 25mA, including the pull-down resistor described later.Pull-down resistor for the FET gateWe should pull down the PWM output port (FET gate) to avoid turning on the FET before configuring the port.We chose 1 k ohms that the voltage division between the pull-down resistor and the port's internal pull-up is below the Gate Threshold Voltage of the FET gate.InductorRated Current -Saturation Current: >3 ARated Current -Temperature Rise Current: >3 ADC Resistance: < 100 mA: It needs well lower than the resistance of the heater (about 1 Ohm)Inductance: Larger inductance is better after satisfying the above conditionsWe choose NR10050T150M: 15 uHDiodeAverage forward current: IF(AV): >2 A(Because the diode current flow during the off-time of the FET, its average current will be lower than the FET's one)Peak forward voltage: VFM: < 0.5 V: Lower VFM is better: We choose Schottky Barrier DiodeWe choose CMS01.Determining PWM frequencyHigher PWM frequencies smooth the current with inductance. On the other hand, it increases the ratio of switching transition time to the period, then the switching loss (heat generation) increases, and power control accuracy decreases. In the current HOWICO design, 80 kHz is a good trade-off.Circuit work waveforms of HOWICOWith a lower PWM duty ratio (30%), the peak voltage drop by the heater is lower. It indicates that the inductor is correctly suppressing the current rising rate.With a higher PWM duty ratio(80%), the voltage drop by the heater is higher. It indicates that the PWM duty ratio controls the heater power properly.The fluctuation of the power supply voltage becomes higher when the PWM duty ratio is high. It is due to the followability of the power bank and the voltage drop of the USB cable.