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In article <10837@bloom-beacon.MIT.EDU> tytso@athena.mit.edu (Theodore Y. Ts'o) writes:
>I am trying to design an alternative power supply for a photometer (the
>GR1501-A, if anyone carse).  The problem is that it requires 3 45 Volt
>batteries which are almost impossible to obtain, and when they are
>available, they are extremely expensive.
>
>So what I'm trying to design is a DC-DC inverter that will take power
>from a 6V latern battery (or something similar) and covert it to a +90
>and -45 volt output.  

I've been building this kind of inverter for years in order to satisfy 
my peculiar perversion with keeping old tube-type portable radios working.
My circuit is very easy to understand and build and is fairly efficient.

Rather than try to enter a schematic in ASCII, I'll describe the circuit
in enough detail to reproduce.

My circuit uses a toroidal or cup-type transformer whose center-tapped primary 
is driven by a square wave oscillator push-pull style.  I apply power 
to the center-tap and take each end to ground through a power FET transistor
of appropriate rating.  In series with Vcc applied to the center-tap is
another power FET series pass element used for regulation.

The oscillator is built around a hex schmitt trigger in a feedback 
configuration.  This connection uses 3 gates hooked end to end with
the output fed back to the input via resistors and caps.  See the 
National linear databook for a discussion of this circuit.  I trim the 
components to generate a square wave at about 5 khz.  Feed the output 
into another of the triggers in the package to clean the wave up.  Connect
the output of this gate to the gate of one of the FETs.  Also connect
the output of this gate to another trigger to generate an inverted signal.
Connect this to the other FET.

Take the last gate and connect a feedback resistor from input to output
to bias it as a linear inverting amp.  About 10k or so should do.
Connect the output to the series pass FET.  This is the regulation
circuit.  You will later develop a voltage from the output of the
inverter that controls the pass element.  You will have to play around 
with scaling resistors in order to establish an operating point for
the pass element.  This is not a very precise regulator but it works fine
for most tube applications.

Winding the transformer.

I build up the previous ciruit before winding the transformer so I can use
it as a test circuit.  You can get design manuals from core vendors such
as Feroxcube but for one-off construction, you can usually do as well
characterizing the core experimentaly which is nice if you find some 
uncharacterized cores.  

What I do is make a guess as to the proper number of primary turns.  
Experience says that 30 turns per side is a good starting point.  I then
connect the core to the driving circuit and measure the idling current
draw. Set the applied voltage to at least the minimum operating voltage 
pluse some margin.  For 12 volts, I usually use 10 volts.  I increase the 
number of turns until the idling current ceases decreasing.  The core is 
saturating at this point.  

Record the number of turns and calculate the necessary turns ratio 
for each output voltage.  I wind a separate coil for each 
voltage rather than trying to tap one coil.  Greatly reduces the incidence
of ground loops, a real problem with high impedance tube circuits.  Put
a convential full wave bridge rectifier and sufficient filtering as needed.

I usually select the highest current winding for regulation.  Establish
a voltage divider from this output to ground such that a voltage of the
proper level is developed for feeding back into the regulator circuit
described above.  The easiest way to find this voltage is to connect a
variable power supply to the input of the regulator and determine what
voltage is necessary to drive the pass transistor to the operating
level.

Since the transformer core is saturating, the other outputs should 
regulate very well.  In fact, for most radio applications where I run
the inverter from a gel-cell, I rarely need a regulator.  I simply apply
battery voltage to the CT of the primary and use the output without any
active regulation.

This circuit is very non-critical and easy to configure.  A schematic 
should not be necessary.  Just get some parts from radio shack and go to it.

John



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John De Armond, WD4OQC                     | Manual? ... What manual ?!? 
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