Message:18684 In: TECH.WW
From: G8MNYDate: Thu, 12 Mar 26 10:07:00 Z
Newsgroups: TECH.WW
Subject: Simple 20A PS
Message-ID: <54204_GB7CIP>
Path: N2NOV|VE2PKT|PY2BIL|GB7CIP
R:260312/1009z @:N2NOV.#RICH.NY.USA.NOAM $:54204_GB7CIP
R:260312/1009Z 63632@VE2PKT.#TRV.QC.CAN.NOAM LinBPQ6.0.25
R:260312/1008Z 40042@PY2BIL.SP.BRA.SOAM LinBPQ6.0.25
R:260312/1007Z @:GB7CIP.#32.GBR.EURO #:54204 [Caterham Surrey GBR]
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!
BridgeÉÍÍÍÍÍÍÍÍÍËÍÍÍÑÍÍÍÍÍÍÍÍÍÍÍÍËÍÍÍËÍÍÍËÍÍÍËÍÍÍËÍÍÍ»
/ \ 100mFº+ ³ ÚÄÄÄÄ¿ ³/ ³/ ³/ ³/ ³/ ³/ 6x
__/ \__ 30V=== ÀÄ´7815ÃÄÂÄÅÄÄÄÅÄÄÄÅÄÄÄÅÄÄÄÅÄÄÄ´ 2N3055
~ \ / ~ Cº ÀÄÄÂÄÙ ³ ³\e ³\e ³\e ³\e ³\e ³\e
on \ / º caseÃĴô o o o o o o Soldered
Case ÈÍÍÍÍÍÍÍÍ͹ ³u1 ³ | | | | | | in 3A
º ³ ³ o o o o o o Fuses
º ³ 1K ÈÍÍÍÊÍÑÍÊÍÍÍÊÍÑÍÊÍÍÍÊÍÍÑÍ>+14V
º ³ ³ 17V _³_ u1³ 100u³ @ 20A
º ³ ³ Zener/_\' === ===
º ³ ³ Crowbar ³ ³ 25V ³
GND>ÄÄÄÄÄÄÄÄÄÄÄÊÍÍÍÍÍÍÍÍÏÍÍÍÏÍÍÍÍÍÍÍÍÍÏÍÍÍÍÍÍÍÏÍÍÍÍÍÍÍÍÏÍ>-VE
AC
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.
PSU
The 7815 1A regulator must be mounted on the chassis. it's internal thermal
shutdown (180øC?) & 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.
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 & RF will be absorbed by the caps.
Why Don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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