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G8MNY > TECH 31.12.02 17:22l 83 Lines 4136 Bytes #999 (0) @ WW
BID : 35317_GB7CIP
Read: GUEST OE7FMI
Subj: High AMP Crowbar Protected PSU
Path: DB0MRW<DB0PV<OE2XOM<OE5XBL<OE3XSR<OK0PBX<OK0PHK<OK0NAG<OK0PPL<RZ6HXA<
SP7MGD<GB7YKS<GB7PZT<GB7MAX<GB7COV<GB7CIP
Sent: 021231/0955Z @:GB7CIP.#32.GBR.EU #:35317 [Caterham] $:35317_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY
This is a negative regulator design, which gives several advantages over the
nolpositive rail design.
1/ much lower thermal mounting resistance,
2/ no hazardous exposed unregulated rail voltages on transistor cases,
3/ transistors can be easily mounted around an ally box spreading the heat,
4/ high current -ve O/P connections can be via the earth chassis.
--------------------------------------------------------------- +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 rated PSU. The high pulse
current AC wiring to bridge & to the reservior C, should be in thick & twisted
wire 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 feed supply. The
14.3V zener voltage can be made up with several zeners or conducting diodes to
set the best O/P voltage or thermal stability.
TRANSISTORS
Three paralleled 2N3055 pass transistors are needed for 18A, each is fused with
a current sharing emitter resistor "a 13A mains fuse", which limits the failed
transistor current to a safer 26A max for the over voltage crowbar to handle
under a fault condition.
The driver PNP can be any >2A TIP type device mounted on the heatsink with an
insulation kit.
The pass transistors are all mounted (with thermal paste) directly on an
external grounded heatsink or better still they can be mounted on different
sides of an ally box making up the high current -ve connections as well.
OVER VOLTAGE CROWBAR
The crowbar is any SCR of >26A peak (most are, eg 8A RMS). Thick leads are
needed from Anode & Cathode to the PSU O/P terminals. The false trip protection
components of 1K & 0.1uF should be mounted on the SCR.
The 40A fusible wire 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 special 15V diode zener, designed to
always blow up S/C & can handle pulse currents of 75A.
BELLS & WHISTLES options:-
Mains neon across the mains after a switch.
O/P LED with series 1K can be added.
O/P Voltmeter can be added, any suitable meter with a calibration R. 0-15V or
with a 10V zener in series gives a more meaningful 10-15V scale.
O/P current meter can be added, by sampling the average voltage drop across the
3 13A fuses. Use a 470R from each emitter to a meter calibration pot (100R),
connect the other end of the calibration 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 known PSU load.
73 De John, G8MNY @ GB7CIP
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