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Port-a-Power for a TC
Hi All,
I believe it was Gary Weaver that asked about inverters (a while back)
to power a portable TC. There are several ways to go about this:
1.) Use two power transistors in a self oszillating configuration.
2.) Use a quartz stabilized clock to drive a 1st and 2nd stage power
driver.
3.) Using a simple IC multivibrator, which in turn, drives a few power
MOSFETs.
4.) Run the inverter at high frequency (20-100khz) and go from there in
designing the output stage (simlar to a SMPS)
Personally, I like variation #3. In case one, you have almost no control
over the running frequency and if your circuit runs away (heating) you
will most certainly fry every component inside (I believe you actually
encountered this). Case two is overkill, because you donīt need
50/60.000000000 hz. Also, this circuit uses normal bipolar transistors,
which will require big cool sinks in order to survive. Variation #4 is
used in professional inverters, because it is very efficient (85-90%)
and VERY compact. However, the design is beyond the scope of the
normal hobby electronics guy and require precise thought to construct.
Also, as they require more electronics, I donīt know how sensitive they
might be to junk RF floating around an operating TC. I might be able
to design one, but it would take me days to figure it out. The circuit
down below is simple, fast and easy to build and or repair.
Variation 3 really takes care of it all. The IC can be a simple 7400 (or
7404, 4047, etc) that runs at ~500/600Hz. Using polyester (2%) and
metal film resistors for the oszillator, will keep the frequency pretty
stable. As the current requirement is low (more in a minute), no heating
occurs. The circuit runs at 10x the required frequency, so small
deviations are of zero concern. Take the output of this oszillator and
feed it into a 4017 rigged as a 1:10 divider. The two outputs from this
IC (+/- phase) are ample to drive two darlingtons (one takes care of the
"upper" part and the other of the "lower" part of the wave form -->
push pull). The outputs of these two darlingtons drive two sets
(depending on the power you want) of MOSFets. The IRF540 is a
great choice. These are rated for 100V, 27A, 125W dissapation and
have an "on" resistance of 0.085 ohm. They are housed in a
TO-220A case. Here in germany, they cost 70 cents new. The
internal heating of these device is VERY low, because of their low
"on" resistance. This means that only small heatsinks will be
necessary. For more power, just parallel more MOSFets (donīt
forget to add a ferrit bead in each gate lead to prevent parasitic
oszillations). A simple set of MOVs across the D-S will prevent
kickback from killing the MOSFets. A third MOV across the supply
lines will prevent junk from frying the darlingtons or the ICīs. For the
really cautious, you can add a 20V Zenerdiode across the C-E of the
driving darlingtons, although they are pulled down hard. While
MOSFETs only need small voltages to be driven, one must remember
they present a large capacitive load. This is why you need the
darlingtons in the pre-stage.
Your real problem is finding a suitable xformer. But, help is as far
away as your junk parts box. Find a good sized MOT. In germany,
the biggest I could find was a 1650VA unit. Make sure the primary
is intact. The MD filament and the HV winding can be removed
(using a Zona saw or a very sharp = Xacto knife). To find the correct
number of turns, wind a few turns (~50) of some wire through the
old secondary window in the core. Plug in the MOT (120V) and
measure the output voltage of the new secondary. This will help you
determine the needed number of turns. The number of turns depends
(indirectly) on the wanted output power. For a high power unit I would
suggest you run the inverter off two 12 batteries in series. If
something like 1kVA is enough for you, you could get away with 12V.
Increasing the voltage will reduce I^2R losses. For 1kVA output be
expecting to pull around 100-120 amps on the primary side at 12V.
Running this at 24V would decrease your current to 50-60A. After
deciding on the input voltage, rewind the MOT with a center tap
(needed for the push-pull configuration of the driver stage). For a
12V input I would suggest you shoot for a 9-10V range from one leg
to the centertap and 18-20V across the outer legs. The gauge of the
primary (new winding) will be dictated by the amount of current
flowing and, of course, be limited to what the MOT was originally
rated for (in VA). For a really BIG inverter (3-5kVA) I would suggest
running 3-4 12V batteries in series for 36-48V input power and using an
old welding xformer as the power transformer. For 48V input and 5kVA
output expect to run around 150-180A primary current. Depending on
the actual usage, you might need to find other MOSFets and, of course,
your run time will be limited to the Ah capacity of your batteries (and
it might not be "portable" any more). However, if I remember it
correctly, you were looking for 600-1000VA. Easily done with the
above circuit.
BTW: The above circuit is called a "pseudo sine wave inverter" because
the amplitude of the square wave is ~ equal to the peak to peak
amplitude of the normal mains sine wave. Depending on what exact
parts you have to buy, this project will run you from 0-$150. If you add
a small AND gate between the multivibrator and the 1:10 divider you can
remote control your inverter. HOWEVER, use this only to start/stop the
coil, when you are not near it. NEVER !!!! trust electronic components
to really cut out the power to the inverter while adjusting the coil.
Unplug the 12V (etc) power source first!
If you decide to build this or something similar, hard wire the high
current lines (i.e. everything behind the MOSFets) to prevent I^2R
losses and smoke from rising (PCB material is way too thin at
these current levels.) Everything in front of the MOSFets can be
mounted on a single small PCB
Inverting greets from germany,
Reinhard