Coil Gun [Part 4] – Trigger Circuit

WARNING: A coil gun can produces voltages that can cause serious electrical burns or easily kill you if not handled properly. Please do some research before beginning this project to ensure you are taking all proper safety precautions when working with high voltage. If you are at a beginner level, I strongly encourage you to start with low voltage projects before taking on a high voltage project. This resource is a great place to start educating yourself. Then search Google for “high voltage safety tips” and read through as many links as you can.

Choosing a Switch

Now that we have a circuit that can charge a capacitor up to a high voltage, we need a circuit that will discharge that stored energy through the coil. The easiest trigger circuit would be a big mechanical switch or a relay. This is not a great idea because when the circuit is closed, it will create  a shower of sparks (trust me, I’ve tried this!). Since being bathed in a shower of sparks is not something we want, mechanical is out, leaving an electronic trigger circuit as our only choice.

Let’s consider several electronic components that can be used as a switch. The first that comes to mind is a MOSFET. A sufficiently large MOSFET would work, but it would be quite expensive and not ideal due to the on-state resistance. A solid-state relay is another option, but yet again these are also very expensive and aren’t built to handle a lot of amps.

SCR: An Electronic Switch for Power Application

One good choice for a switch is a silicon controlled rectifier (SCR). An SCR acts as a current controlled diode. There are several ways to trigger an SCR, but the ideal method for the coil gun application is gate-triggering. A voltage is applied to the gate with a series resistor to limit the current to an acceptable value. This causes current to flow from the anode to the cathode. Current will continue to flow until it falls below the rated holding current– even if the gate is pulled to ground. This is an important point to remember: if the battery is not disconnected from the charging circuit, current will continue to flow and your circuit will go up in smoke within a few seconds.

Designing the Circuit

triggercircuit

I selected the Littelfuse S4055R for my SCR. Let’s check the ratings to make sure it can handle the currents and voltages

  • The maximum peak surge current is 650A for a half-cycle of a 60 Hz wave. This corresponds to 650A over 17ms. We will keep this figure in mind when we design the coil.
  • The maximum repetitive forward voltage is 400V. We will be charging the capacitor to a maximum of 300V, which is well within the limits of the SCR.

The coil is fired using a push button switch which connects a 9V battery to the gate trigger circuit. I used a 9V battery instead of the regulated 9V since I was having some issues with spurious triggering of the SCR. In the next design, I’ll have to focus on improving filtering around the regulator.

R1 is used as a current limiting resistor. The datasheet for the S4055R states that the gate voltage is around 1.5V, so we can calculate the gate current via (9V – 1.5V) / 100Ω = 75mA. The maximum gate current required to turn on the SCR is 40mA. This is not the maximum that should be applied, but rather the minimum to apply to ensure the SCR turns on. Choosing a value of 75mA gives a little extra headroom and accounts for the 9V battery not being at full charge. R2 and C1 act as a filter on the gate to prevent any spurious noise during the charging process from falsely triggering the SCR.

The coil is connected between the capacitor bank and the anode of the SCR. D1 is included as a flyback diode to help dissipate any energy through the coil after the SCR has turned off. I’ve chosen the RURG5060 which is a heavy duty diode rated for a reverse voltage of 600v and a surge current of 500A. Since current cannot be instantaneously changed in an inductor, not including D1 would result in the capacitor bank being charged to a negative voltage. This is extremely dangerous and could cause permanent damage to the capacitor, or even cause it to explode.

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