Saturday, February 20, 2016

Electric and Magnetic Field Demonstration Box

I've been active in several maker clubs around Olympia, WA. Sometimes we get invited to display things at science fairs, maker faires, etc. It is always a mad scramble as our members may or may not have a project far along enough to display, yet not installed somewhere permanently yet.

I've been saying for a while that we need to have some things on hand just for those events. When we got asked to do some teaching displays related to magnets for a local children's museum for their adults only event called Love and Magnets, I decided it was time to put my money where my mouth is.

I have done science demos for my wife's kindergarten class over the course of a few years. The electricity and magnetism demos required that I bring a selection of power supplies, parts, and wires. So this first project is to replace all those separate power supplies with one box that can be just opened and used.

It is being built into an old plastic shell carrying case for a camcorder. Clear acrylic panels are used so that you can see the inner workings. A Crookes Tube is going into the lid. A Crookes Tube has a vacuum with an electron beam that hits a phosphor coated angled surface, so that you can see how the beam is bend by electric and magnetic fields.

Early sketch of the case. Proposed electromagnetic ring launcher on the right.

I did a lot of measuring of parts and some drawings in CorelDraw, then they were cut and engraved with a LASER cutter. I mismeasured the corners, so I had to round them out a bit more on a belt sander.

Control panel with some parts installed. Center top is a cap to allow a rather large transformer to fit.

At top left is the power input, including an On-Off switch, fuse, and internal LC noise filter. The two pairs of 30A rated banana jack 5 way binding posts on the left are high current adjustable AC and DC outputs. The analog meter is AC current from the battery charger, the digital display just under it is DC voltage and current. Just below is a variac (variable transformer) good for about 2.5A. The variac then connects to a transformer out of a Harbor Freight battery charger that has a 75A starter boost function. The red pushbutton is so the high current output cannot be left on. I don't know which limit I'll run into first, but I'm hoping for a few 10s of amps into very low resistance.

That might sound impossible with a variac rated for 2.5A, but that will be before the transformer. The voltage drops by the turns ratio, but current rises by the turns ratio. So a 10:1 transformer with 2.5A coming in will net 25A out of the secondary. The transformer is wound with the primary on the inner core, then spacing, then the secondary around the outside of the core. So if the windings present are not sufficient for my needs, I could remove the secondary and rewind it with fewer turns of much heavier gauge wire. Power is my limiting factor, the variac is rated at a max of about 250VA.

The two sets of banana jack 5 way binding posts along the bottom are for medium voltage, medium current use. Both with have round black power switches once another order of them comes in. These outputs are for lower power things like small coils, electromagnets, motors, etc.

I think the blank area on the bottom right is going to get a hole cut for a cooling fan, and I now have a short-resistant class C audio amplifier to add, although I think I'll place a 2 ohm 10W resistor inline so as not ot unnecessarily stress the amplifier. I have a coil wound around a film canister and hot glued to a paper plate. It is very effective to demonstrate how a speaker works by turning up the amplifier and then lowering it over a neodymium magnet.

Continuing clockwise, two more switching power supplies with digital readouts. Both will be connected to 3V to 7kV high voltage converters that will be added to the feedback path with voltage dividers. The one at the top right is to supply the Crookes Tube, the one just below it will have both a positive and negative high voltage up to about 5kV to be used to bend the electron beam with an electric field, and for external devices that require high voltages. The outputs of the positive/negative high voltage are in the panel in the lid. I have determined that the outputs of the 3V to 7kV high voltage converters are isolated, so I can merely connect the positive of one to the negative of the other and ground that junction.

The panels in the case. Nothing else is installed yet.
The Crookes Tube is going in the top just under the safety banana jacks. The lone red jack in the middle will connect to a smooth, flat electrode just above the Crookes Tube and to a needle electroscope. This can then be used to illustrate that like charges repel, unlike charges attract, and how sharp points and ultraviolet light can drain charge. The safety banana jacks above are to be changed so they will be left yellow for negative, center green for ground, right red for positive from the 0 to 5kV supply. This will then also be available for electroscopes, static electricity motors, etc.

Everything but the high current outputs will be powered from an open frame 12V@15A switch mode power supply which I bought surplus. I bought several of them a few years ago to power a solid state cooler, and bought a few extra.

Another carrying case will store things used with this to illustrate the concepts. Magnets, electromagnets, coils, various kinds of DC and AC motors, etc.

I will post updates as the project progresses.