Homebrew PCBs

High-end professional PCB drills may be pneumatic motors (capable of RPM's on 
up into the hundreds of thousands of RPM!), but the need for a compressor and 
the air-driven "Mess" preclude this thought for many.

Brush-type "universal" motors such as found in the "Dremel tool" have their 
shortcomings.  

A solution to some of these problems, with its own disadvantages, would be 
the use of  a "high frequency" instrument-motor.  The "400 Hz. aircraft 
instrument-motor" can be found as "surplus" in various catalogs of vendors of 
that kind of thing, from tiny things of no use as a "machine-tool part" on up 
to fractions of a hp.  

I found a "95 Watt, 400 Hz" 12,000 RPM motor in a precision "servo housing" 
of anodized aluminum.  Strangely, it had an input-voltage of 115 VAC (@ 400 
Hz).  So, the trick remaining would be to supply it with that 115 VAC at 400 
Hz.  It's actually a quasi-three-phase motor, and that "third phase" is 
derived simply by using a capacitor from one of the "incoming legs" to that 
third "odd" leg.  But this must be to only that one specific "odd" leg, not 
"any of the three", as it is an "unbalanced 3-phase" kind of motor.   Good!  
Coming up with THREE PHASE 400 Hz would be a project!  Single-phase is just 
about the limit of MY patience for such a project!

See the file, "400 Hz. Generator".  For those who might attempt constructing 
this circuit, some description:  What we have from beginning to end is a 
(nearly-) square-wave oscillator with a trim-pot making it easy to set on 400 
Hz.  This feeds both monostables of a 74HC4538, each side adjusted so that 
the output-pulse-width is just-under the 1+ millisecond width of a sinusoidal 
400 Hz. AC.  Yes, the result is a square-wave with "gaps" between the + and - 
going transitions.  The transformer and the 1.1 uF capacitor make that 
very-nearly sinusoidal, and the motor has NO clue it is being fed with an 
"approximation".

The transformer I used was totally custom-made on a stack of E-I punchings 
with a 1.75 sq.-in. center-leg.  There were 20 turns, total, of #14 copper 
for the primary, with a center-tap (per the schematic).  The secondary had 
100 turns of #22, with a tap at 80 turns.  I "experimented".  The DC-in is a 
point you may-well wish to also experiment.  I would suggest you keep it 
between 8 and 15 VDC, and the supply you use be able to source up to 14 or 15 
amps, depending on how much loading you will be placing on the motor as you 
"breadboard" things.  

Clearly, few of us tinkerers wind their own transformers, so I suggest you 
find a 12 VCT "filament transformer" rated for ten amps at 12.6 VAC.  Since 
6.3 VAC is also a popular tube-filament voltage, it should be easy to find a 
12.6 V. secondary with a center-tap (or DUAL 6.3 V. secondaries)    A 60 Hz 
transformer will work "stress-free" on the higher frequency, of course, 
provided both TIP-140 transistors are switching on/off (so steady DC never 
flows into the xfmr's primary!  Obviously, you would be using this 
transformer "backwards", that is, the input would be the "push-pull DC"  to 
the transformer's "secondary", normally the low-VAC out,and the output from 
the transformer's "primary".

If anyone has specific questions, ask!            Jan Rowland      
JanRwl@...


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