High AMP Crowbar Protected PSU
This is a negative regulator design, which gives several advantages over
the normal positive rail design.
1 much lower transistor thermal heatsink mounting resistance,
2 no hazardous exposed unregulated rail voltages on transistor
cases,
3 transistors can be easily mounted around an ally box to spread
the heat,
4 high current -ve O/P connections can be made via the earth chassis.
AC
The transformer needed is 15V-18V @ 25A for an 18A 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. A slow blow mains fuse of 2A should be used.
PSU
TH is a thermal switch mounted on the pass transistor heatsink (H/S) and/or
rectifier, and/or transformer, a 60øC 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 rail connections as well.
CURRENT LIMIT
Another PNP monitors the voltage drop across the shunt R (0.033ê=18A)
when this is greater than 0.6V the Zener voltage is reduced to keep the
current at the limit max. Make the shunt R from a length of thick steel
wire and do high current to voltage drop test to determine the R.
OVER VOLTAGE CROWBAR
The crowbar is any SCR of >26A peak (most are, eg 8A RMS). Thick leads
are needed from Anode and Cathode to the PSU O/P terminals. The false
trip protection components of 1K and 0.1uF should be mounted directly
on the SCR. The 40A fusible wire link is for protection against fire if
the PSU is used for battery charging and 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 and can handle pulse currents of 75A.
BELLS and 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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