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20V power supply

Dave Guimont wrote:

I can use 12v battery power, but require two batteries to supply the 20v needed
by the CalAmp converter

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?

Go here for more information about the LM2577T-ADJ switching regulator.
http://www.national.com/pf/LM/LM2577.html <http://www.national.com/pf/LM/LM2577.html> 

Wayne Estes W9AE
Mundelein, IL, USA
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