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G8MNY  > TECH     17.02.19 09:15l 66 Lines 2768 Bytes #4 (0) @ WW
BID : 59414_GB7CIP
Subj: Simple Variable PSU
Sent: 190217/0800Z @:GB7CIP.#32.GBR.EURO #:59414 [Caterham Surrey GBR]

By G8MNY                                            (New Nov 06)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

This is a simple circuit that can go to quite high voltage & currents depending
on the values used.

50-0-50V             +70V
(  D1         LIMIT  4k7                      
(_    __       LED    2W                      
(    /_\    Ĵ<Ŀ    /       __
(  D2        RED     Z1__   22k<ĴT1      /_\ D4
 )  +           50V/_\`  Pot   \e  /    
         ===  \      Ĵ    __     ĴT2   
    1000uF  T347>Pot     ON\_/        \e     0.5A
      100V  e/     100     LEDGREEN   C2 o-o> 0-50V
       C1                           10u+   __      Variable
                     1W             80V===  /_\ D3   50-200mA

T2 2N3055 on a heatsink, or other 100V 50W NPN TAB or MOSFET instead of T1 & T2
T1 & T3 BFX84 or other 100V 500mA NPN
Diodes all 1N4003
Zener 50V 1W (can be several diodes in series)

The transformer & rectifier diodes D1 & D2 charge up C1 to 30% more than the
zener voltage. The 4k7 puts current into the zener Z1, the 22k voltage setting
pot pick a fraction of this voltage & feds it to the high gain darlington T1 &

The ON LED voltage drop compensates for the darlington loss, giving approx 0V
at the low end of the voltage pot. A voltage scale can be drawn for the pot if
additional metering is not needed.

T2 emitter is the output voltage & the circuit is protected from sudden load
with output capacitor C2 & reverse diode D3 & load greater than supply by D4.

Output current is limited by T3, it is turned on when 0.5V is across the 2R2
(Max current) plus the set fraction across the 100R pot (down to 50mA). T3 on
reduces the voltage until the limiting current is reached & also lights the
warning LIMIT LED. In addition if T3 turned hard on e.g. a shorted output, then
the ON LED is extinguished! As with the voltage pot, a current pot scale can be
drawn up if additional current metering is not used.

Green only  = Output at set voltage.
Red & Green = Output less than set voltage & current at set limit value.
Red ON only = Output heavily overloaded, NO output volts, PSU overheating?

The current pot is connected to the output 0V, with the ON LED then to the
voltage pot.
C2 & D3 connect between the 0V & the output fuse holder.
D4 needs to be wired across T2.
T2 case is not exposed, as it is at +70V!
Use a sensible mains fuse. e.g. 100mA slow blow.

Why Don't U send an interesting bul?

73 De John, G8MNY @ GB7CIP

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