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G8MNY > TECH 12.04.01 23:58l 74 Lines 3784 Bytes #-8843 (0) @ EU
BID : 42329_GB7CIP
Read: GUEST OE7FMI
Subj: High Current PSU design
Path: DB0ZKA<DB0SAA<DB0TTM<DB0FP<DB0CWS<DB0ZDF<DB0GE<LX0PAC<LX0HST<HA3PG<
HA8FY<ON1CED<ON0CK<GB7SXE<GB7CIP
Sent: 010412/0954Z @:GB7CIP.#32.GBR.EU #:42329 [Caterham] #:304473 $:42329_GB7C
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@EU
SIMPLE HIGH CURRENT CROWBAR PROTECTED PSU.
By G8MNY
--------------------------------------------------------------- +VE
! ! ! ! ! ! !
/ \ +! 300,000u ! ! ! x !
/ \ ----- 25V +! ! 16v ---\ ! 40A !
~ \ /~ ===== ----- ! / \ x link !
35A \ / ! 470u ===== ! --- ! !
bridge ! ! 16V ! ---\ ! ! -----
H/S !---------! ! / \ 14.3V ! ----- =====
!--TH---O-------- --- ! \ / ! 1000u
x lamp ! ! 100R ----- ! 16V
3x13A fuse ! ! ! ! /! SCR !
x lamp O ! !--/ ! !
3x e \ ! c ! ! ! 0u1 ! !
2N3055 >!----\ ! ! ! !-!!--! !
H/S / ! >!---------- 1K ! !
!c e / ! TIP H/S ! ! !
GND>----------------!-------!------------------------------------------ -VE
-VE REG CROWBAR
AC
The transformer needed is 15V-18V @ 25A for an 18A PSU. The high pulse current
AC wiring to bridge & to the reservior C, should be thick & twisted, to keep
the losses & magnetic pulse radiation down. A slow blow mains fuse of 1.5A
should be used.
PSU
TH is a thermal switch mounted on the pass transistor heatsink (H/S) or
rectifier, 60 deg C is recommended. The 2 lamps are 12V 60mA pilot lights, that
give a fairly constant current into the 1 Watt zener, the 470uF decouples any
hum from the zener supply. The 14.3V zener voltage can be made up with several
zeners or conducting diodes to adjust the O/P voltage.
TRANSISTORS
Three paralleled 2N3055 pass transistors are needed for 18A, each are each
fused with a current sharing emitter resistor "a 13A mains fuse", which limits
the failed transistor current to a safer 26A peak for the over voltage crowbar.
The driver PNP can be any >2A TIP type device with insulation kit.
The -ve regulator design gives much lower thermal mounting resistance as well
as no hazardous exposed unregulated rail voltages. The pass transistors are all
mounteä (with thermal paste) directlù oî aî externaì groundeä heatsink oò
better still they caî be mounted on differenô sideó oæ aî allù boø makinç up
the -ve connection as well.
OVER VOLTAGE CROWBAR
The crowbar is any SCR of >26A peak (most are). Thick leads are needed from
Anode & Cathode to the PSU O/P terminals. The false trip 1K & 0.1uF should be
mounted on the SCR. The 40A fusible link is for protection against fire if the
PSU is used for battery charging & the trip should attempt to S/C a car
battery!
An alternative to the conventional crowbar circuit is a special protection
zener diode, eg RS 239-494 which is a 15V diode, designed to blow S/C.
BELLS & WHISTLES options:-
Mains neon across the mains after a switch.
O/P LED with 1K can be added.
O/P Voltmeter can be added, any suitable meter with a cal R. 0-15V or with a
10V zener in series as well a more meaningful 10-15V scale can be done.
O/P current can be added, by sampling the average voltage drop across the 3 13A
fuses. Use a 470R from each emitter to a meter cal pot (100R), connect the
other end of the cal pot to the -ve of the fuses. Put a 1mA (or better) meter
movement from the slider to the fuse -ve. Calibrate the meter with a PSU load.
73 de John G8MNY on 2001-APR-12 @ 10:54 local time
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