Notes on Experiment 10

I found this experiment to be rather construction heavy - you do a lot of prep work without a whole lot of measurement at the end. The real payoff is in Lab 12 when you take the amplifier and use it to make your microwave receiving antenna. In this experiment, you will directly measure the mutual inductance of the coils and compare with the theoretical prediction. I found it worked out pretty well - my measured value was within 6% of the theoretical expectation.

Be prepared to do lots of soldering, and plan ahead. I did a pretty lousy job laying out the circuit elements and blew a lot of space on my board.

I found that the way this is set up over the two weeks is kind of imbalanced: week 1's stuff took me about 4 hours, while week 2's stuff took only an hour, maybe even less. Once you've got everything done for week 1, you might want to just blast ahead and finish week 2 as well; it's up to you.

Here are some specific recommendations:

1. The 8 pin socket leads ain't the strongest critters in the universe: if you bend them too much, they will break, easily. Because fate hates us, they will probably wait until you've soldered most of the circuit together before breaking. (That's what they did to me, and the local Radio Shack didn't have similar sockets in stock - grrr.) If you don't bend them too much (no more than about 45 degrees, say), they will probably be OK.

2. The bottom figure on page 77 is WRONG. The bottom figure claims that pin 7 is the output and pin 6 the +DC input; the circuit in the top figure reverses this. The top figure is correct. A version of the bottom figure that isn't wrong is given at the end of the chapter (page 88).

3. Pins 1, 5, and 8 aren't used at all. To keep them from shorting anything out or otherwise interfering, slip some of the insulation you strip from the wire over the leads.

4. You need a 2.7 ohm resistor to complete this lab. For some strange reason, there is no such thing in your kits; however, there's a big supply of them in SAC 34. You might want to get your ombuds to grab a bunch for you.

5. The discussion of ``ground'' in the circuit is kind of confusing. Ground, by definition, is the zero point of the circuit. You should solder the tail ends of the 360 ohm resistors together; that little tail will be ground for this circuit. In other words, all voltages are measured relative to it. You don't have to hook it to ``ground'' (ie, a pipe in your room or something like that).

6. 1/4 watt resistors are the skinny little ones; they burn up if you try to dissipate more than about 1/4 watt of power through them. (It smells pretty foul when this happens.) What Zap means when it suggests using a lead from a 1/4 watt resistor for checking the sockets is to poke one of the leads into the socket, then make the measurement on that lead. Clip the negative terminal of the meter to the ground point, put the lead of a skinny resistor in the socket, and clip the positive terminal of the meter on the lead (between the socket and the resistor itself - you don't want the resistor in the circuit).

NOTE ADDED AFTER FINISHING LAB 11 AND STARTING LAB 12: The purpose of Lab 11 is to turn the low voltage (2V - 15V) power supply into a high voltage (500V - 1000V) power supply. To do this, you have to break many of the connections you made in Experiment 10, so that this round of circuit construction seems wasted. However, if you look ahead at Lab 12, you will see that the circuits made in both Lab 10 and Lab 11 are used: the amplifier you make in Lab 10 is used to construct a microwave receiver, and the HVPS you make in Lab 11 is used to construct a microwave transmitter.

As it turns out, you need to rework many of the connections to the amplifier in Lab 12 anyway, so breaking things in order to make the HVPS is not such a big deal. In Lab 12 you will use the amplifier for a rather different application, and the circuit thus has to be somewhat different.

Here's how you should do Labs 10 - 12:

1. Everyone constructs an amplifier circuit, doing Lab 10 more or less on their own. There isn't much to do here that a partner can help with anyway, aside from some hand-holding on the crap work. It is helpful to have two meters for the final measurement, though: you need to measure the voltage across the 2.7 ohm resistor (which tells you the current in the primary coil) and the induced voltage in the secondary coil (which goes through the amplifier; you measure the amplified output). You don't need two meters for this, but I found it convenient. In particular, when you put a big chunk of iron in the coils (I used a hammer), you can easily compare how the two readings behave.

2. Everyone constructs the HVPS on their own in Lab 11. Leave as much of the amplifier circuit intact as possible. You can remove many of the leads that aren't needed any longer, and you must disconnect the LVPS leads for use in the HVPS. However, the bulk of the amplifier can be left intact.

3. Partner up on Lab 12. One of your high voltage power supplies turns into the microwave generator; the other apparatus reverts to being a LVPS/amplifier, which in turn becomes a microwave receiver. Contact the mothership, tell them that Earth is ripe for conquest. Enslave humanity. Live out the remainder of your days as the Demon King and/or Queen of Humankind. Bwah ha ha ha.

An earlier version of this page recommended partnering up to do Lab 11. I take this back - everyone really should make their own HVPS. It's a good lab, so time-wise it's not such a bad thing; and, as discussed above, you don't break the amplifier circuit any worse than Lab 12 asks you to anyway. Everyone really should have their own HVPS, just in case it is needed for the lab quiz.

Feel free to email me if you have any questions.