Introduction: Coilgun SGP33 - Full Assembly and Test Instructions
This tutorial describes how to assemble the electronics of the coil gun shown in this video:
There is also a video where you see it in action on the last page of this tutorial. Here is the link.
The PCBs for this demo where kindly provided by JLCPCB.COM
The goal was to build a single stage coil gun that is lightweight, has good performance and uses commonly available parts for a reasonable price.
- Single stage, single shot
- Adjustable coil activation pulse width
- IGBT- driven coil
- Single 1000uF/550V capacitor
- Highest velocity obtained 36m/s, will greatly depend on coil and projectile properties and geometry
- Initial charge time about 8s, recharge time depends on the discharge time, in the video example it’s 5s
Total cost for electronic parts only are about $140 US, excluding the copper wire/ barrel for the coil.
In this tutorial I will only describe how to assemble the PCB.
I will also provide all the other information to get the most out of this circuit without blowing it up.
I will not give a detailed description of the mechanical assembly, as I think it could be improved / modified. You will need to use your imagination for that part.
Step 1: Warning !
Make sure you read and understand this section!
The circuit charges a capacitor to about 525V. If you touch the terminals of such a capacitor with your bare hands you can seriously hurt yourself. Also (this is less dangerous but should nevertheless be mentioned), the high current they can provide can create sparks and can evaporate thin wires. Therefore always wear eye protection!
Safety glasses is a must!
The capacitor retains charge even after the main switch is turned off. It has to be discharged BEFORE working on the circuit!!!
Secondly, we will use the energy contained in the capacitor and transform it into kinetic energy of a projectile. Even though the velocity of this projectile is low, it could still hurt you (or someone else), therefore use the same safety rules as when working with power tools or doing any other mechanical work.
So NEVER point this at a person when it’s loaded and charged, use common sense.
Step 2: Tools and Workplace Requirements
If you are completely new to electronics then this project is not for you. The following skills are needed:
- Able to solder surface mount devices including ICs, capacitors and resistors
- Able to use a multimeter
Tools needed (the minimum):
- Fine tip / large tip soldering iron
- Solder wire
- Liquid Flux or flux pen
- Desoldering braid
- Magnifying glass to inspect solder joints or a microscope
- Fine tweezers
- Multimeter to measure the DC-link voltage (525VDC)
Recommended tools (optional)
- Adjustable power supply
- Hot air desoldering station
Preparation of the workplace and general working recommendations:
- Use a clean table, preferably not plastic (to avoid problems with static charge)
- Don’t use clothing that easily creates / accumulates charge, (that’s the one that creates sparks when you remove it)
- Since nearly nobody has an ESD safe workplace at home I recommend doing the assembly in one step, i.e. do not carry around sensible components (all semiconductors once you take them out of the packaging). Place all components on the table then start.
- Some components are pretty small, like resistors and capacitors in 0603 packages, they can get easily lost, only take out one at a time from their packaging
- The charger IC in a TSSOP20 package is the most difficult part to be soldered, it has a 0.65mm pitch (distance between pins) which is still far from being the smallest industry standard but it could be difficult for somebody less experienced. If you are not sure I’d recommend you train soldering first on something else instead of scrapping your PCB
Again, the whole PCB assembly process is shown in the video mentioned on the first page of this tutorial
Step 3: Diagram
In this section I will give an overview of the circuit. Read it carefully, this will help you to avoid damages to the board you just assembled.
To the left the battery will be connected. Make sure that it is lower than 8V under all conditions or the charger circuit might be damaged!
The batteries I used are 3.7V but will have a voltage higher than 4V when under very light load, they would therefore give a voltage higher than 8V to the charger before it starts up. Not taking any risks, there are two schottky diodes in series with the battery to drop the voltage to below 8V. They also serve as a protection against inverted batteries. Also use a fuse of 3 to 5A in series, this can be a low voltage fuse like the ones used in vehicles. To avoid draining the battery when the gun is not in use I recommend connecting a main power switch.
The battery voltage at the PCB input terminals should be between 5V and 8V at all times for the circuit to work properly.
The control section contains an undervoltage protection and 3 timer circuits. Timer IC U11 with LED1 blinking indicates that the command to turn on the charger circuit is active. Timer IC U10 determines the output pulse width. The pulse width can be adjusted with potentiometer R36. With R8 and C4/C6 values as per BOM the range is: 510us to 2.7ms. If you require pulse widths out of this range these values can be adjusted as you wish.
Jumper J1 can be open for initial testing. The command to enable the charger circuit goes through that jumper (positive logic, i.e. 0V = charger disabled; VBAT = charger enabled).
The upper middle section contains the capacitor charger circuit. The transformer peak current limit is 10A, this current is configured with the current sense resistor R21 and should not be increased or you may risk saturating the transformer core. 10A peak leads to a little over 3A average current from the battery which is ok for the batteries I used. If you wish to use other batteries that cannot provide that current you will need to increase the value of resistor R21. (increase value of resistor R21 to decrease transformer peak current and consequently average current from battery)
The main capacitor output voltage is measured with a comparator. It activates the LED2 when the voltage is above about 500V and deactivates the charger when the voltage is above 550V in an overvoltage event (that actually should never happen).
NEVER POWER UP THE CHARGER WITHOUT THE MAIN CAPACITOR CONNECTED TO THE CIRCUIT. This might damage the charger IC.
The last circuit is the bridge circuit that discharges the capacitor through two IGBTs into the load / the coil.
Step 4: PCB Inspection
First inspect the PCB for anything unusual. They come actually inspected and electrically tested from the manufacturer but it’s always a good a idea to double check before assembling. I never had any problems it’s just a habit.
You can download the Gerber files here: https://drive.google.com/open?id=1qW3X0660T1McL0rmWsktKYwpjhrWA6VD
Step 5: Assembly
Download the Excel BOM file and the two pdf files for component location
First assemble the smaller PCB that holds the large electrolytic capacitor. Pay attention to the right polarity!
The 90 degree headers that will connect this PCB to the main PCB can be mounted on the top or bottom side depending on your mechanical assembly.
Do NOT yet solder the headers into the main PCB, they are difficult to remove. Connect two short wires thicker than AWG20 between the two PCBs.
On the main PCB assemble first the charger IC which is the most difficult part if you are not used to it. Then assemble the smaller components. We will first install all capacitors and resistors. The easiest method is to put a little bit of solder on one pad, then solder the component with the help of the tweezers on this pad first. It doesn’t matter how the solder joint looks at this point, this serves just to fix it in place.
Then solder the other pad. Now use liquid flux or a flux pen on the not-so-good-looking solder joints and re-do the joint. Use the examples in the video as a reference as to how an acceptable solder joint looks like.
Now move on to the ICs. Fix one terminal on the PCB using the above mentioned method. Then solder all the other pins as well.
Next we will install the larger components like electrolytic and film capacitors, trimpot, LEDs, Mosfets, diodes, IGBTs and the transformer of the charger circuit.
Double check all solder joints, make sure no component is broken or cracked etc.
Step 6: Start-up
Caution: Do not exceed 8V input voltage!
If you have an oscilloscope:
Connect a push button (normally open) to inputs SW1 and SW2.
Verify that jumper J1 is open. Ideally connect an adjustable benchtop power supply to the battery input. If you do not have an adjustable benchtop power supply you will have to go directly with batteries. LED 1 should blink as soon as the input voltage is higher than about 5.6V. The undervoltage circuit has a large hysteresis, i.e. to turn the circuit initially on the voltage needs to be higher than 5.6V but it will only turn off the circuit when the input voltage drops below about 4.9V. For the batteries used in this example this is an irrelevant feature but might be useful if working with batteries that have higher internal resistance and/or are partially discharged.
Measure the main high voltage capacitor voltage with a suitable multimeter, it should remain 0V because the charger is supposed to be deactivated.
With the oscilloscope, measure the pulse width at pin 3 of U10 when pressing the push-button. It should be adjustable with trimpot R36 and vary between about 0.5ms and 2.7ms. There is a delay of about 5s before the pulse can be restarted after each button press.
Go to step… full voltage test
if you don’t have an oscilloscope:
Do the same steps as above but skip the pulse width measurement, there is nothing to be measured with a multimeter.
Go to… full voltage test
Step 7: Full Voltage Test
Remove the input voltage.
Close Jumper J1.
Double check the correct polarity of the high voltage capacitor!
Connect a multimeter rated for the expected voltage (>525V) to the high voltage capacitor terminals.
Connect a test coil to the output terminals Coil1 and Coil2. The lowest inductance/resistance coil I used with this circuit was AWG20 500uH/0.5 Ohm. In the video I used 1mH 1R.
Make sure there are no ferromagnetic materials near or inside the coil.
Wear safety glasses.
Apply battery voltage to the input terminals.
The charger should start up and the DC voltage on the capacitor should rapidly rise.
It should stabilize at about 520V. If it exceeds 550V and still goes up, turn off the input voltage immediately, something would be wrong with the feedback portion of the charger IC. In this case you will need to re-check all solder joints and correct installation of all components.
The LED2 should now be lit indicating that the main capacitor is fully charged.
Press the trigger button, the voltage should drop a few hundred volts, the exact value will depend on the adjusted pulse width.
Turn off the input voltage.
Before handling the PCBs, the capacitor needs to be discharged.
This can either be done by waiting until the voltage drops to a safe value (takes a long time) or by discharging it with a power resistor. Several incandescent light bulb in series will also do the job, the number of light bulbs needed will depend on their voltage rating, two to three for 220V lamps, four to five for 120V lamps
Remove the wires from the capacitor PCB. To complete the module, the capacitor can now (or later) be soldered directly to the main board depending on the mechanical assembly process. The capacitor module is difficult to remove from the main PCB, plan accordingly.
Step 8: Mechanical
Mechanical mounting considerations
The main PCB has 6 cutouts to mount it on a support. There are copper traces more or less near these traces. When mounting the PCB care must be taken not to short these traces to the screw. Therefore plastic spacers and plastic washers need to be used. I used a scrap metal piece, an aluminum U-profile as the housing. If using a metallic support, it should be grounded, i.e. connected with a wire to the minus pole of the battery. Accessible parts (parts that can be touched) are the trigger switch and the battery, their voltage level is near ground. If any high voltage node would come into contact with the metal housing it would be shorted to ground and the user is safe. Depending on the weight of the housing and the coil the whole unit can be quite front-heavy so the grip needs to be installed accordingly.
The housing could also be made much nicer, 3D printed, painted etc, that's up to you.
Step 9: The Theory
The working principle is very simple.
The two IGBTs are activated at the same time for a time period lasting a few hundred us to a couple of ms depending on the configuration/adjustment of the monostable oscillator U10. Current then starts to build up through the coil. Current corresponds to magnetic field strength and magnetic field strength to the force exerted on the projectile inside the coil. The projectile starts to move slowly and just before its’ middle reaches the middle of the coil the IGBTs are turned off. The current inside the coil does not cease instantly though but now flows through the diodes and back into the main capacitor for some time. While the current decays there is still magnetic field inside the coil, so this should drop to near zero before the middle of the projectile reaches the middle of the coil otherwise a breaking force would be exerted on it. The real-world result corresponds to the simulation. The end current before turning off the pulse is 367A (current probe 1000A/4V)
Step 10: Coil Construction
The velocity of 36m/s was obtained with the following coil: 500uH, AWG20, 0.5R, 22mm length, 8mm inner diameter. Use a tube that has the smallest gap possible between inner wall and projectile and still allows free movement of the projectile. It also should have the thinnest walls possible while being very rigid. I used a stainless steel tube and no detrimental effects were noticed. If using an electrically conductive tube make sure to insulate it with an appropriate tape (I used Kapton tape) before winding it. You may need to temporarily mount additional end pieces while winding, because considerable side forces do develop during the winding process. I would then recommend to fix/protect the windings with epoxy. This will help to prevent the windings from being damaged while handling/assembling the coil. The whole coil assembly should be done in a way that the windings cannot move. You also need some sort of support to mount it on the main housing.
Step 11: Possible Modifications and Limitations of the Circuit
The capacitor charged to 522V contains 136 Joules. The efficiency of this circuit is pretty low, as with most simple single stage designs that accelerate ferromagnetic projectiles. The maximum voltage is limited by the maximum allowable capacitor voltage of 550VDC and the maximum VCE rating of the IGBTs. Other coil geometries and lower inductance/resistance values may lead to higher velocities/efficiencies. The maximum specified peak current for this IGBT is 600A though. There are other IGBTs of the same size that could possible support higher surge currents. In any case, if you contemplate to increase capacitance or IGBT size make sure you consider the following main issues: Respect the maximum current specified in the IGBT datasheet. I do not recommend increasing the charger voltage, too many variables need to be considered. Increasing capacitance and using longer pulse widths for bigger coils will also increase power dissipation of the IGBTs. They may therefore need a heatsink. I recommend simulating a modified circuit first in SPICE /Multisim or other simulation software to determine what the peak current will be.
Step 12: The Coil Gun in Action
Just having some fun shooting at random stuff...