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G8MNY > TECH 04.08.26 15:50l 56 Lines 2523 Bytes #10 (0) @ WW
BID : 1091_GB7CIP
Subj: Simple 20A PS
Path: JH4XSY<IW0QNL<HB9ON<HB9ON<PI8ZTM<VE3CGR<VK2RZ<GB7CIP
Sent: 260804/0643Z @:GB7CIP.#32.GBR.EURO #:1091 [Caterham Surrey GBR] $:1091_GB
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
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!
Bridgeﾉﾍﾍﾍﾍﾍﾍﾍﾍﾍﾋﾍﾍﾍﾑﾍﾍﾍﾍﾍﾍﾍﾍﾍﾍﾍﾍﾋﾍﾍﾍﾋﾍﾍﾍﾋﾍﾍﾍﾋﾍﾍﾍﾋﾍﾍﾍｻ
/ \ 100mFｺ+ ｳ ﾚﾄﾄﾄﾄｿ ｳ/ ｳ/ ｳ/ ｳ/ ｳ/ ｳ/ 6x
__/ \__ 30V=== ﾀﾄｴ7815ﾃﾄﾂﾄﾅﾄﾄﾄﾅﾄﾄﾄﾅﾄﾄﾄﾅﾄﾄﾄﾅﾄﾄﾄｴ 2N3055
~ \ / ~ Cｺ ﾀﾄﾄﾂﾄﾙ ｳ ｳ\e ｳ\e ｳ\e ｳ\e ｳ\e ｳ\e
on \ / ｺ caseﾃﾄｴﾃｴ o o o o o o Soldered
Case ﾈﾍﾍﾍﾍﾍﾍﾍﾍﾍｹ ｳu1 ｳ | | | | | | in 3A
ｺ ｳ ｳ o o o o o o Fuses
ｺ ｳ 1K ﾈﾍﾍﾍﾊﾍﾑﾍﾊﾍﾍﾍﾊﾍﾑﾍﾊﾍﾍﾍﾊﾍﾍﾑﾍ>+14V
ｺ ｳ ｳ 17V _ｳ_ u1ｳ 100uｳ @ 20A
ｺ ｳ ｳ Zener/_\' === ===
ｺ ｳ ｳ Crowbar ｳ ｳ 25V ｳ
GND>ﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾊﾍﾍﾍﾍﾍﾍﾍﾍﾏﾍﾍﾍﾏﾍﾍﾍﾍﾍﾍﾍﾍﾍﾏﾍﾍﾍﾍﾍﾍﾍﾏﾍﾍﾍﾍﾍﾍﾍﾍﾏﾍ>-VE
AC
The transformer needed is 17V-19V @ 25A for an 20A rated PSU. The high pulse
current AC wiring to bridge and to the reservoir C, should be in thick and
twisted wire to keep the losses and 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 and heat down, and enables higher rated PS to be built.
A slow blow mains fuse of 2A should be used.
PSU
The 7815 1A regulator must be mounted on the chassis. it's internal thermal
shutdown (180?) and 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 and identical base & emitter wiring length to ensure all
are given equal drive voltage.
PROTECTION
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 and RF will be absorbed by the caps.
Why Don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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