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G8MNY  > TECH     12.02.19 16:00l 57 Lines 2510 Bytes #6 (0) @ WW
BID : 58897_GB7CIP
Subj: Simple 20A PS
Sent: 190212/1455Z @:GB7CIP.#32.GBR.EURO #:58897 [Caterham Surrey GBR]

By G8MNY                                     (New Sep 11)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

This is a positive regulator followed by power transistors design. It's a
simple cheap low component count design, but does have drawbacks.

  1/ Unregulated 18-25V needed to give the 3.8V voltage overhead.
  2/ Hazardous exposed unregulated rail voltages on the transistor cases.
  3/ Transistors will get very hot as there is a lot of voltage drop!

     / \   100mF+   敖陳朕   /  /  /  /  /  / 6x
  __/   \__ 30V===  青7815団堕田陳田陳田陳田陳田陳 2N3055
  ~ \   / ~    C     青賃潰  \e \e \e \e \e \e
 on  \ /            case団潅   o   o   o   o   o   o Soldered
 Case 藩様様様様        u1    |   |   |   |   |   | in 3A
                              o   o   o   o   o   o Fuses
                          1k   藩様瞥冤瞥様瞥冤瞥様瞥用>+14V
                               17V __    u1    100u  @ 20A
                              Zener/_\'    ===      ===
                            Crowbar            25V 

The transformer needed is 17V-19V @ 25A for an 20A rated PSU. The high pulse
current AC wiring to bridge & to the reservoir C, should be in thick & twisted
wire to keep the losses & magnetic pulse radiation down.

If the transformer has 2 identical windings (e.g. 2x 13A), consider 2 diode
bi-phase rectifier instead of the single bridge or use 2 bridges, this cuts the
silicon losses & heat down, & enables higher rated PS to be built.

A slow blow mains fuse of 2A should be used.

The 7815 1A regulator must be mounted on the chassis. it's internal thermal
shutdown (180C?) & current limiting are used to limit the drive to the 6 low
gain 15A 2N3055s. This ensures they are not over driven to destruction, if a
large heatsink or case is used. Each pass transistor has 3A fuse as the emitter
equalising resistor & identical base & emitter wiring length to ensure all are
given equal drive voltage.

Off load the PSU voltage will be high, a load R (20R 10W) can be put across the
output to keep this down if important. Voltages over 17V may damage equipment,
so a high power 17V zener is needed, capable of blowing a 3A fuse! Current
peaks & RF will be absorbed by the caps.


Why Don't U send an interesting bul?

73 De John, G8MNY @ GB7CIP

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