Large Capacitor Spark Demo - 170V DC Charger

This project is intended to demonstrate what a capacitor is and grab the attention of an audience. This device converts 120V AC to charge a large capacitor to 170V DC and allows you to discharge it, producing a large spark and loud noise, in a safe way. The incandescent bulb acts as a resistor and charge indicator. There is a toggleable soft start function to limit current when charging the capacitor.

******Warning******

This project uses wall power which can be lethal if handled incorrectly. Do not attempt this project unless you have prior electronics experience. Proper safety procedures and protective gear must be used.

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- 3300uF capacitor rated for at least 170V https://www.mouser.com/ProductDetail/KEMET/ALS32A3...

- Full bridge rectifier https://www.mouser.com/ProductDetail/Diodes-Incorp...

-Button https://www.mouser.com/ProductDetail/TE-Connectivi...

-Switch

- 25W incandescent light bulb

- Bulb socket

-16 gauge stranded wire

-Wall plug cord

-Thick solid core copper wire

-Banana plug connectors

-Small screws

-Soldering equipment and supplies

-Drill and drill-bits

This charger uses a simple full bridge rectifier (FBR) circuit with switches to control the power to the capacitor. The schematic shows 4 diodes in a FBR configuration; when I built the circuit I replaced them with a FBR chip. I used a toggle button as the power switch and a flip switch as the soft start switch.

How it works

The FBR takes the 60Hz 120V sine wave from the wall and converts it into positive humps that peak at 170V. If you live somewhere with a different wall power voltage, the charger will output a different voltage. For example, 240V AC wall power would be converted to 340V DC. If attempting this project, you must consider the voltage of the wall power in your home and ensure the components you choose are rated for the voltage and power parameters of your setup.

When the power button is pressed, the capacitor starts charging to the voltage peak of 170V. The soft start function restricts the current flow into the capacitor. This is completely OPTIONAL. When the switch is open, all of the current flows through the 220 ohm resistor and is greatly reduced. When the switch is closed, the current flows around the resistor and through the switch which increases current and charging rate. The incandescent bulb acts as a resistor to limit current and indicate charge (~40 ohms). When the power is turned on, the bulb will illuminate and start to dim. The capacitor is near full charge when the bulb goes out completely. After the bulb goes out, I flip the soft start switch to the closed position to finish charging the capacitor faster. In the graph, you can see after 1s the power switch is flipped and the cap starts to charge. After 5s, the soft start switch is closed and the cap starts to charge faster. It is finished charging after around 10s.

I used Fusion 360 to model the capacitor charger and the capacitor itself. I then rendered it in a top and bottom view. I uploaded the .stl files for you to download and print.

3D Printing

I used Chroma Strand PETg and a Lulzbot Taz 6 to 3D print everything. I first tried printing with PLA, but it didn't turn out very well. Your mileage may vary. 3D print at least the box before soldering.

Notes on the schematic and soldering

The pulse sources in the diagram are just for simulation purposes, so pretend like they aren't there when you are assembling the circuit. The full bridge rectifier in the schematic is modeled using four diodes, if you use the part in the supplies list, you can just solder the wall plug cord directly to the AC input of the chip and the positive/negative leads to their respective components shown in the diagram.

*****WARNING******
Never solder something that is plugged into the wall. Always insulate solder joints with shrink wrap tubing or electrical tape before powering anything on. Always wear insulating gloves and eye protection when handling wall power.

Soldering Steps

1. Solder the banana plug connectors with the negative side going to the negative output of your rectifier and the positive going to the soft start switch and 220 ohm resistor as shown in the schematic.
2. Drill a hole big enough for your discharge leads to go through the box. I chose to put mine on either side of the capacitor slot.
3. Insert the discharge leads through the box and solder the ends in parallel to the capacitor charging leads (banana plugs).
4. Solder the 220 ohm resistor and soft start switch (in parallel) to the light bulb socket.
5. Solder the other side of the light bulb socket to one end of the power switch.
6. Solder the other end of the power switch to the positive side of the rectifier.
7. Finally, solder the AC input of your rectifier to the power cord. Thread the power cord through hole on the back of the box first.

Testing the circuit

****Wear gloves and eye protection. Spectators should stand at least 6ft away*****

1. Make sure the power switch is off, plug banana plugs into the capacitor being cautious of polarity.
2. Flip the soft start switch to the open position.
3. Plug the charger into the wall.
4. Press the power button. The light should turn on and start to dim. Wait about 5 seconds
5. Flip the soft start switch to the closed position. The light should turn off after a second or two. Wait an additional 5 seconds.
6. Press the power button again (off position). Touch the discharge of the leads together.
7. If there is a loud pop and spark, it is working

Once everything is printed, we need to integrate the circuit into the container. See labeled cross section image.

1. Route the banana capacitor connectors through the box tunnel to the round capacitor hole.
2. Push the banana plugs through the holes in the wire holder. Depending on fitment, you may need to use glue or widen the wholes with a drill.
3. Slide the wire holder into the capacitor hole. The notch on the wire holder part should be oriented toward the box tunnel
4. Drill pilot holes with a drill from the outside of the round capacitor hole into the wire holder. Put a screw through the outside of the capacitor hole into the wire holder. This will hold the wire holder down.
5. Press the bulb socket and power button through their respective holes in the lid.
6. If using the soft start feature, drill a hole through the back of the box and press the switch through it.
7. Insert the capacitor into the capacitor hole/slot. Make sure you match the polarity of the capacitor to the polarity of the charging leads.
8. Using a marker, mark the spot on the capacitor that lines up with the groove of the capacitor hole.
9. Remove the capacitor and slip the capacitor sleeve onto the capacitor with the knob lined up with the mark on the capacitor.
10. Re-insert the capacitor into the slot to check alignment. The banana plugs should line up with the lead holes in the capacitor.

You can put a piece of cotton ball soaked in alcohol clipped to the discharge lead for an added wow factor.