Page 33 - 15 Dangerously Mad Projects for the Evil Genius

14 15 Dangerously Mad Projects for the Evil Genius

If you have an oscilloscope, you can use it to
measure the duration of the pulse through the coil.
You will need to set your scope as follows:

■ Channel sensitivity: 5V/div. Make sure the
scope is okay with 35V.

■ Trigger mode: single shot, rising edge.
■ Timebase: 2.5 ms/div.
You can see a firing of the gun in Figure 1-24.
This shows that at 2.5 ms/division, the pulse is
about 15 ms in duration.
Figure 1-24 Oscilloscope trace of a test firing

electronics, and eBay usually has a selection of
Summary
high-power SCRs
This is a fun project to make. As we discovered in You may find that taking apart the soft iron
the theory section, the gun is very inefficient. This laminations from a transformer and layering them
is probably just as well, since it could do some around the coil will increase the efficiency
damage if it operated at anything like 100 percent considerably.
efficiency.
Whatever techniques you decide to employ, be
Improving the efficiency at least to one or two careful, and expect to destroy the occasional SCR.
percent is quite possible, but will absorb quite a lot As well as being cautious, you need to be scientific
of your time. in your approach to improving the performance of
To do this, you will probably want to buy a the coil gun. Keep a log book and take speed
spare SCR and test the one you have to destruction measurements after each change you make. Don’t
with a shorter coil using thicker wire. You may forget that if you change the coil, or the capacitors,
also want to experiment with bigger and better you may find that the best starting position for the
capacitors, or a higher voltage circuit. projectile may have changed. You will probably
have to do tests at several different positions after
You can find much higher power SCRs, but they
each change.
will be expensive, unless you get them from a
surplus store. Certain online stores sell surplus In the next chapter, we stay with the weapons
theme and build a trebuchet.