Notes on Experiment 11

This experiment was great: making the HVPS worked as well and was as satisfying as making the LVPS, and the spark gap you make in week 2 was fun to play with. Week 1 is rather construction heavy: plan the layout of your circuit carefully, and then be prepared for an hour or two of careful soldering. If you are careful, everything should work correctly. The measurements you will take with the HVPS in week 2 are pretty simple.

Note that contrary to what I originally said in my Lab 10 notes, everyone should build the HVPS, doing Lab 11 essentially solo. Sorry if I misled anyone; see my Lab 10 notes for comments on this.

Don't be scared by the fact that the HVPS can put out a very large voltage (mine puts out about 1400 volts when it is cranked up). Because of the way it is constructed, the amount of current it can produce is highly restricted. If a load tries to draw too much current, something has to give in order to keep the current low. (What does this say about the internal resistance of the HVPS?)

Because the current output is so restricted, you have nothing to worry about when you put your hand across the output leads. It feels a bit like a static electricity zap, no big deal. (Just to be safe, do NOT use both hands: the closed circuit might go through your heart! Even though the current is very low, you don't want to screw around in a way that can cause cardiac problems.)

If you are working ahead, it will probably be difficult to understand what is going on in the circuit - you really need to understand induction. Read ahead and talk to a TA. The key point is that the voltage in the secondary coil on the HVPS depends on the derivative of the current in the primary. The transistor makes the primary's current vary with time; the inductance turns this into a rather large AC voltage (which the diodes and capacitors then turn into a rather large DC voltage).

I don't have too many specific recommendations; Zap was fairly clear this time around. Here are a few notes which might help:

1. Careful layout is very important. In particular, as noted in Zap, many of the connections must be kept as short as possible. (This is because long leads create large ``stray'' capacitances, which changes the oscillation frequency of the circuit.) What I did was place the various components that required short connections fairly close on the breadboard. Then, I didn't need to use any wire to solder everything together: I just bent the leads over so that they were right next to each other, trimmed off the excess, and soldered them together. If you do this, there won't be much insulated wire on the bottom of the HVPS. Make sure you work on a non-conducting surface. (I made little legs out of pushpins to keep the power supply about 1/2 an inch off my working surface.)

2. It is extremely important that the coil you wind around the inductor be in the same ``sense'' as the inductor. In other words, it must wrap around in the same direction as the inductor's wires. However, you don't need to be too careful about how close your windings are to one another. I put four windings in one gap, a fifth around the ring between the two gaps and five-ish windings in the other gap.

3. Somewhere in Zap, it tells you to have your instructor explain the markings on the ceramic capacitors. Here's the way it works: ignore everything but the number. (In particular, most of the capacitors have an ``M'' on them. This does not mean micro or anything useful like that; it is related to the tolerance rating of the capacitor. According to David Beckman, M means it is 20% accurate.) If the number has a decimal point, that is the reading in microfarads. If the number does NOT have a decimal point, that is the reading in picofarads. Logical, eh? It gets better: only two significant digits are given. If a third digit is present, it represents an exponent. Thus, the squarish capacitor you use on the LVPS, labeled 105, is 10 × 105 picofarads, which is 1 microfarad. I think it's safe to say that the genius who dreamed up this scheme could use a swift kick in the ``lower back''.

4. The spark gap apparatus is pretty easy to make. You will modify it somewhat when you do Lab 12. To facilitate these modifications, don't attach the speaker wire too securely to the tacks. I originally wrapped a few turns of wire around the tack and then soldered it into place. This was not a really good idea: it was a real pain to remove when I started making the transmitting antenna in Lab 12. Just solder it in place well enough that it doesn't fall off; that will be good enough to do Lab 11's measurements and allow you to set up Lab 12 pretty easily.

5. A lot of people are finding that their HVPS doesn't work, even though the circuit is properly made, or else it works well for a few minutes, freaks out, then stops working. In some cases at least, I suspect that your primary and secondary coils are shorting out. The primary (the magnet wire) is insulated by just a thin coating of some goop; if the insulation is weakened or cracked, it can easily short with the secondary (which has several hundreds volts circulating through it). To reduce this problem, wind the primary very carefully, and perhaps try improving the insulation between the two. (One successful idea was to use a stronger wire for the primary; another was to paint over the exposed conductors with clear nail polish.)

Once you've got it made, everything you need to do is pretty easy. Have fun, and try not to give yourselves cardiac arrest. Feel free to email me if you have any questions.