High voltage Switching Power supply

After experimenting with various voltage multipliers, I decided to build a high voltage switching supply capable of charging my capacitor banks in a reasonable amount of time. On this page, you will find information about what switching power supplies are, how they work, and how to build one.



Below is my high voltage switching power supply. This power supply is capable of supplying 440 volts at a resonable amperage. A power supply is critical for a coilgun project, which is why I'm spending so much time on it.

Power Supply

Inside Switching PS

As you can see, this power supply is significantly more complicated than simple voltage multipliers, but is still relatively simple to build and operate. First, I will take you through how switching power supplies work, then I will tell you how to design and build one!

How switching power supplies work

Switching power supplies rely on three things: an inductor, a power transistor and a timer. Inductors can change voltage instantaneously, but change current continuously. So what does this mean to you? Simply put, if you have an inductor with current flowing through it, and all of the sudden you stop conducting current through the inductor, the inductor's magnetic field will begin to collapse. This collapse creates a sudden and short period of time where the voltage across the inductor rises tremendously. The voltage spike which the inductor creates can even be over a thousand volts! Switching power supplies take advantage of inductors because for short periods of time, inductors can create voltages which are larger than the original voltage being applied to the inductor. If one connects a diode on the inductor, the current will flow through the diode only when the voltage on the inductor side is higher than the other side of the diode. Of course, on the other side of the diode, there is a capacitor or capacitor bank which will accumulate all of the charge. Since having an inductors magnetic field collapse doesn't produce very much power by itself, the simple solution is to cycle current through the inductor many times a second. For my power supply, I used a frequency of 3.4 kHz, however many switching power supplies have frequencies which are even higher.

In order to control the current going through the inductor so many times a second, you have to flip a switch on the power supply very quickly on and off. I'm just kidding! You can use a standard timer such as the famous and cheap LM555, and a power NPN transistor which are easily available from Digikey or Jameco. The LM555 timer outputs a signal which goes from 0 to 1, and this signal is connected to the gate of the power transformer. That's all there is to it!

How to control the output voltage

Other than selecting your inductor, you have very little native control of the actual output voltage. High voltage spikes from inductors can be several thousand volts, so it's important to halt the power supply before it gets to that voltage or else your capacitors will explode. The voltage protection circuitry for this power supply is very simple. When looking at the front faceplate of the power supply, the upper left switch is a potentiometer. Using this potentiometer and a standard  LM393N voltage comparator. When the measured voltage crosses the desired voltage as set by the potentiometer, the LM393N outputs a signal to the LM555 timer which effectively stops the timer. Once the timer is stopped, no more power sill flow through the inductor, and the output voltage will remain stable. Occasionaly, the capacitor bank will leak some chrarge, but this power supply will detect any drop in output volage and begin charging until the target voltage is reached again.

Simple modifications

There are a couple modifications which you could make in the basic design. For example, you could add LED lights to the faceplate to indicate whether or not the power supply is one, and whether or not the target voltage has been reached. For the power supply which I built, this isn't necessary. Because it operated at about 3 kHz and the inductor was made slightly loose, an audible high pitched ringing can be heard when the power supply is charging the capacitors.

The Inductor

There are a couple places you go try to obtain an inductor. Inductors may be found on old power supplies, and transformers can even be used as inductors. If all else fails, you could build an inductor out of magnet wire, which is widely available and commonly used for radio electronics. Try to find an iron core or metal bolt and wind the magnet wire around the metal, which will give it greater inductance. The inductor I built was certainly not the worlds greatest inductor. In fact, so much energy was lost in eddy currents that I decided to add a cooling fan to keep the power supply from melting. I recommend you find 20 gauge or thicker magnet wire to avoid too much resistive loss. This was another main factor affecting the total output power this power supply could deliver. You can easily see the inductor I built which is the red coil on top of the power supply.

Schematics comming soon!

LEGAL DISCLAIMER
Coilgun projects can be extremely dangerous if you don't know what your doing. Capacitors can unleash massive amounts of electricity which can seriously injure or kill. Please use this information with caution, as I can not be held responsible for your actions.