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RE: 20V power supply

> Wayne replies:
> I also need 20 Volts to power my TranSystem downconverter and 28V
> coaxial relay.  Last night I discovered the National
> Semiconductor LM2577T-ADJ "Simple Switcher step-up voltage
> regulator".  I'm going to order one ($5.72 from the Radio Shack
> commercial catalog) to build a boost regulator with 13V input,
> 18-20V output.
> The regulator only needs 10 external components.  The most
> critical components are the inductor, high-current Schottky
> diode, and low-ESR filter capacitor.  In my junkbox I have a
> ferrite cores to wind an inductor, 1N5822 diodes, and filter
> capacitors from a salvaged switching power supply.  The inductor
> design would be very critical if I needed 1.5A output current.
> But it shouldn't be extremely critical to supply 350 mA to my
> coaxial relay, DEM preamp, TranSystem downconverter and PTC
> thermistor (initial surge current is about 700 mA when the PTC
> thermistor is cold).
> The switching frequency is 52 kHz.  I will put the regulator
> circuit in an aluminum box, but that won't do much to attenuate
> the magnetic field from the inductor.  I will put .01 uF RF
> bypasses on the input and output cables to minimize conducted
> RFI.  I suspect that the 52 kHz harmonics won't go all the way up
> to 2401 MHz.  If harmonics go up to 123 MHz (downconverter IF
> frequency), hopefully 55 dB gain of the preamp and downconverter
> will make the AO40 signal much stronger than any RFI.  Are these
> reasonable assumptions?  Does anybody have construction tips for
> minimizing RFI from homebrew switching power supplies?


I currently use a LM2587T-step-up voltage regulator in my 144 to 10386MHz
1W transverter as a 28V coaxial relay power supply. Only normal bypasses
are used.
I find no harmonics or harmful magnetic fields. The inductor is 3C6-material
toroid ring with 15 turns of copper wire. (Not critical at all).
There is no shield needed. The switching frequency is about 100KHz.
Do I have some luck though?

73 Jari OH3UW

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