By G8MNY (Updated Sep 08)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

This is a negative regulator design (upside down), which gives several advantages over the normal positive rail designs...

1/ Much lower thermal resistance on the transistor heatsink mounting.
2/ No hazardous exposed unregulated rail voltages on the transistor cases.
3/ Transistors can be easily mounted around an ally box to spread the heat.
4/ High current -ve output connections can be made via the earthed chassis.
5/ Independent zener supply rail gives low drop out voltage.

35A +ÉÍÍÍÍÍÍÍÍÍÍËÍÍÍÍÍÍÑÍÍÍÍÍÍÍÍÍÍÍÑÍÍÍÑÍÍ\/\/\/ÍÍÑÍÍÍÍÍÑÍÍÍÍÍËÍÍÍÍÍÑÍ>+VE
Bridge/ \ 100mFº+ ³ ³ ³ shunt ³ ³ x ³
__/ \__ 25V=== ³ +³ ³ ³ ³ |40A ³
on ~³ \ / ³~ Cº +³ 470u=== e\³ b ³ ³ x ³
case³ \ / ³ -1 toº === 25V ³ PNPÃÄÄÂÄ100Ä´ ³ º ³
³ ÈÍÍÍ)ÍÍÍÍÍ͹ ³1mF ³ c/³ ³ ³ _³_15v º ³+
_³_2x 1N_³_-9V /³\ ³30V ³ ³ +³ ³ /_\' º ===
/_\ 4001/_\ / ³ \ ³ ³ ÃÄ´ÃÄÙ _³_ ³ Aº ³1mF
ÀÄÄÄÄÄÄÄÁÄÄÄ)ÄÄ)ÄÄ)ÄÄÄÁÄÄTHÄÄÄOÄÄÄ´ ³ 10u /_\' ³ _º_ ³30V
-2 to -10V / ³ \ lamp ³ ³ 14.3V³ 4R7 \ / ³
x x x ³ ³ ³ ³ GÄÒÄ ³
3x 13A | | | lamp O ÀÄÄÄÄÄÄÄÄÄÄ´ ³____/º SCR ³
Fuses x x x ³ ³ ³ u1 ºK ³
e\³ e\³ e\³ b c ³ ³ ÃÄ´ÃÄĶ ³
3x 2N3055 ÃÄÄÄÅÄÄÄÅÄÄÄÄÄÄ\³ b ³ ³ ÀÄ1KÄĶ ³
on case /³ /³ /³ PNPÃÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ º ³
ºc ºc ºc e/³ TIP on case º ³

ÍÍÍÍÍÍÍÍÍÍÊÍÍÍÊÍÍÍÊÍÍÍÍÍÍÍÏÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÊÍÍÍÍÍÏÍ>-VE

chassis -VE REG I LIMIT CROWBAR

AC
The transformer needed is 15V-18V @ 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 a large 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.

Small 100nF capacitors from the secondary AC to ground (to a tag on the bridge bolt) are recommended in RF environment to decouple mains RF leaking across the transformer.

A slow blow mains fuse of 2A should be used.

PSU
TH is a thermal cut out switch/s mounted on the pass transistor heatsink, &/or rectifier, &/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 from the independent negative rail. 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 output voltage or thermal stability. There is no opamp control to go wrong (with RF!) in this simple highly reliable PSU design.

TRANSISTORS
Three paralleled 2N3055 pass transistors are needed for 20A, each is fused with a current sharing emitter resistor "a 13A mains fuse" soldered straight on the emitter leg. This limits the failed transistor current to a safer 26A max for the over voltage crowbar to handle under a fault condition. Wiring to the common -ve cap point should be in equal length wire.

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.

The driver PNP can be any >4A TIP type device mounted on the heatsink/case with an insulation kit.

CURRENT LIMIT
A small PNP monitors the voltage drop across the shunt R (0.030ê=20A) when this is greater than 0.5V the Zener voltage is reduced to keep the current at the limit max. Make the shunt R from a length of thick steel wire & do high current to voltage drop test (with a car headlamp passing 5 Amps) to determine the R. The 10uF makes the current limiting action stable with no added hum.

OVER VOLTAGE CROWBAR
The crowbar is any SCR of >26A peak (most are, e.g. 8A RMS 80A Max). Thick leads are needed from Anode & Cathode to the PSU output terminals. The false trip protection components of 1k & 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 & the trip should attempt to short out a car battery!

An alternative to the conventional crowbar circuit is a special protection zener diode, e.g. RS 239-494 (obsolete), which was a special 15V diode zener, designed to always blow up short circuit & can handle pulse currents of 75A.

LAYOUT
I used a folded ally side sections to make up a box of the right size for
the components I had. I put in a perforated steel base plate to take the weight of my toroidal transformer.

Mains Fuse _2N3055
ÚÁÁÄÄÂÄÂÄÄÁÄÁÄÄÄ¿
³ /~~~\BR³~~~~~³³
³Þ Ý ³ ³³
2N3055³³ (o) ³ ³ Cap ³³2N3055
[³Þ Ý ³ ³³]
³ \___/ ³_____³³
³ tag strip ³
ÀUÄÄÄÄMeterÄÂÄÂÄÙ
Mains Sw Terminals

By using a low secondary voltage with a very large cap & a minimum drop out voltage design, spreading the heat generating components evenly around a blackened ally case with through ventilation is adequate, in this case no added heatsinks or fans for 20A intermittent load. In fact the hottest is with a light load of 6 Amps, as here is 7 Volts to drop in the pass transistors, At 20A there is little to drop (2V) & the rectifier & transformer do the work.

BELLS & WHISTLES options:-
Mains neon across the mains after a switch.
An output LED with series 1K can be added.

Output 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.

Output current meter can be added, by sampling the voltage drop across your current limiting R shunt. Feed to a meter calibration pot (100R), This can be combined with the voltage meter as here.

+ - V
ÚÄ(1mA Meter)Ä>oÄÄÄ¿10-15V
Á 20A oI ³
ÚÄ100RÄ¿ ³ 4K7
³ ³ ³ ³
Á/\/\/\ÁÄÄÄÄÄÄÄÄÄÄÁÄÄÄ)ÄÄ+
_³_
10V /_\'
_³_

Or you can use the average voltage drop across the 3x 13A fuses (use a 470R from each one). Feed to a meter calibration pot (100R). etc. If a fuse blows you will get a full scale reading.


Why Don't U send an interesting bul?

73 De John, G8MNY @ GB7CIP



-----------------------------------------
ARGEN-X BBS - IPV6 TELNET LU9DCE.DYNU.COM -----------------------------------------


--- Mystic BBS v1.12 A48 (Linux/64)
* Origin: ARGEN-X BBS (4:902/6)