# Design and Experiment of Discharge Control Methods for Three-Stage Coil Gun Experiments
## 4 Discharge Circuit of a Multi-Stage Coil Gun System
### Discharge Control Method Using Delay  
The second discharge control method proposed is a method used in a conventional coil gun system: registering a certain amount of time delay until the projectile reaches the intended position between the coil gun operations. The delay time is determined by calculating the time required for the projectile accelerated in the first stage to reach its discharge position in the next stage of the coil gun considering the trigger position. Once the projectile reaches the position where it needs to be accelerated for the next stage, it sends out an SCR operation signal to activate the coil gun circuit. This method requires a digital output device to send out the current to operate the SCR based on time settings.  
In this study, an external switch input activates the single-stage coil gun with no delay, the second-stage coil gun after a delay of 295 \(\upmu\)s, and the third-stage coil gun after 1860 \(\upmu\)s. Discharge control signals are sent out using an Arduino, an open-source development tool. The Arduino code used in the experiment is attached as an appendix.  
The delay values should be calculated according to the projectile position derived from the simulation considering the distance between the coils and the launcher velocity in each previous stage. The experiment was conducted based on the calculated delay values, which were adjusted to generate the maximum velocities. Because the output voltage of the Arduino is insufficient to operate the SCR used in this study, a method to amplify the control signal is required. Therefore, an amplification circuit was used using a metal oxide semiconductor field-effect transistor (MOSFET) device and an external power source. When the Arduino sends out a control signal, the MOSFET device that receives the signal connects the external power source and the SCR to activate the system. Figure 10 shows the discharge control circuit with the Arduino and the MOSFET device, and Figure 11 presents the configuration of the discharge control circuit and the SCR.  
Figure 9: Installed launch tube with infrared LEDs and phototransistor.  
MOSFET switches were used in the second and the third stages, and a 9 V battery was used as the external power source for each MOSFET switch. A discharge circuit was used at each stage to prevent damage to the entire circuit during the experiment. In the event of damage, the experiment could be resumed by replacing only the damaged circuit. In addition to the method used in this study, a discharge system can be constructed by amplifying a signal using a transistor. However, this may require the additional configuration of a freewheeling diode circuit to prevent back electromotive force from being generated in the coil gun circuit, which can cause circuit damage.  
A freewheeling diode circuit comprises a diode in parallel with the solenoid coil and in the opposite direction of the capacitor to eliminate the back electromotive force generated by the inductor. Figure 12 presents the coil gun's freewheeling diode circuit. When configuring a coil gun circuit to cut off current in the middle of operation using an insulated gate bipolar transistor (IGBT) instead of an SCR, a freewheeling diode circuit must be used to prevent damage to the coil gun circuit.  
Figure 11: SCR-MOSFET switch circuit configuration.  
Figure 12: Coil gun circuit with freewheeling diode.  
Figure 10: Schematic of the discharge control circuit with Arduino and MOSFET.