Fig. 1 Compact DC-to-DC Converter.

IÛ0V 10 ma frequency, so as to achieve fast rise time of the square wave. )

However, for this application we are not constrained to use germanium power transistors, since we wish to convert only a watt or two. For switching currents of this order there are many inexpensive germanium computertransistors that have cutoff-frequencies of several megacycles. Now, it is the core-material that is the determining factor in limiting our frequency; and so we can almost immediately forget "C-cores" and tape-wound types. We are now in a good position to exploit the wonderful world of ferrite-cores, which will function on up into the megacycles. An additional bonus presented to us by the ferrite manufacturers is that such cores are not only available in torroid forms but in pot-core forms which are much easier to wind. The combination of pot-core form (where one simply winds a small plastic bobbin right from the wire spool, with no shuttle needed) and high frequency operation (few turns are needed), really makes this an easy job.

The converter to be described runs at about 20 kc, meaning that the job of filtering, after the rectifier, is simplified. Further, the 20 kc note, due to any magnetostriction, is above audibility and shouldn't bother anyone but the family dog. The circuit is shown in Fig. 1; note the 1500 mmfd de-spiking capacitors (yes the de-spiking capacitors get smaller too as operating frequency goes up). The unit uses 2N1305's and 1X4005 s for a total semiconductor cost of about §3.50. The core with bobbin was obtained locally at a surplus electronics dealer0 but a standard Indiana

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