By G8MNY (Corrections Nov 19)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
Here is a very unusual circuit principles, of this Italian 0-35V 0-5A PSU. It uses a transformer variac secondary to make an efficient linear regulated PSU, together with variable current limiting & dissipation power fold back.

ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
³ÚÄÄÄÄÄÄÄ¿ ÚÄÄÄÄÄÄÄ¿ ( ) FL-305 ³
³³ \ V ³ø³ \ A ³ Fine V ___ ³ It was all blown up &
³³ \ ³ ³ \ ³ .- -. ³ modified into a simple ³ÃÄÄÄÄÄÄÄ´ ÃÄÄÄÄÄÄÄ´ / Course\ ³ raw PSU. So I did a lot ³ÀÄÄÄÄÄÄÄÙøÀÄÄÄÄÄÄÄÙ \ V / ³ of reverse engineering,
³ overload ( ) '-___-' ³ to find out it how it
³ on trip sw A ³ originally worked, before
³ ! ÚÄÄÄÄ¿ + - e + - ³ attempting the restoration.
³ ø ø ÀÄÄÄÄÙonø (o)(o)(o)(o)(o) ³ ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ

SCHEME 15A Power NPN 15A LÄÄo-oÄ¿ 6-42V AC Darlington__ Amp Trip
2A )||(<ÄÄÄÄÄÄÄ¿ ÚÄÄÂÄÄÄÄÄÄ´~~~\_/e ÃÄRshuntÄMeterÄÂÄÄÄÄÂÄÄÄÄÂÄÄ\ÄÄ +ve Mains _)||( \ ~³15A³+ ³8-60V ³ ³ ³ ³ Control ³
)||( Var Bridge ³ ³ Drive ÃÄÄÄÄImonÄÄÄÄÄÄ´ PSU 0V! ³ 0-35V
ON )||( R ~³ ³- ³ ³ ³Out ³ Volt
N___ \_)||(________³ ³ ===16mF ³+In Out³ Vmon Meter 0-5A
|| ___ ³ ³ 65V ÀDrop MonÙ ³ ³
300W ||(30V Control ÀÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÁÄÄ\ÄÄ -ve EÄÂÄÄe ÀÄÄÄ +/-12V PSU
///

The coarse Voltage Variac control has stops on it to give between 6V - 42V AC, & a 50K Var R to control the course regulated Volts. Unusually a separate 30V winding is used to make a floating +/-12V @ 40mA zener PSU on the +ve O/P rail, for 5 Op amps, 6 transistors & 3 LEDs of the complex control circuit.

The 1st Op amp with input protection diodes & 1K Rs, monitors the output Volts, compares PSU -ve through VarR & Fine VarR to +ve from 5K preset & 5K6 using +6V zener reference. And with 2 transistors it shunts away the 3mA drive to the darlington, & lights up the green OK LED.

+12VÄÄÄÂÄÄÄ2K7ÄÄÂÄ470RÄ>Darlington Base 0-3mA
Voltage Comparitor ÚÄÄÄÄÄÄÄÄ)ÄÄÄÄÄÄÄÄ´ (Or a Mosfet)
³\ ³ ³ ³ /³
+6V 0VÄ1KÄÂÄÄÂÄ´+\ ³/e 1K2 e\³ /+Ã
³ preset /_\\~/³1 >Ä´<ÃÄ´PNP ÚÄÄÁÄÄ¿ PNPÃÄÂÄ´>ÃÄ< 2³ Current Limit
À5K6ÄÄ5KÄÄÂÄ1KÄÁÄÄÁÄ´-/ ³\ OK OVERLOAD /³ ³ \-Ã Comparitor
2K ³/ ³ LED LED ³ ³ \³
Fine VarR 10K ³ ³ 10K ³
³ ³ ³/ \³ ³ ³ /³
50K ÃÄÄÄ´NPN NPNÃÄÄÄ´ ³ /+Ã
Course VarR ³ ³\e e/³ ³ ÀÄ´>ÃÄ< 5³ Power Foldback
³ 10K ³ ³ 10K \-Ã Comparitor
-Ve -12VÄÄÁÄÄÄÄÄÁÄÄÄÄÄÁÄÄÄÄÄÙ \³

A 2nd Op amp monitors the output current & compares it to the current set pot & -6V ref zener. Overloads, limit the drive current & with 2 more transistors, by again shunting away the 3mA darlington drive, & lighting the red OVERLOAD LED, instead of the green OK LED.

A 3rd Op amp monitors the voltage drop across the darlington pass transistor. And converts this into current to feed the emitters of a "dual transistor long tailed pair" mixer, with a sample of PSU O/P current into one of the bases.

Darlington ÚÄÄÄÂÄÄÄ+12V Power Dissapated ³\
Volts Monitor 10K 10K ³\ 0VÄ1KÄ´+\
ÚÄÄÄ68KÄÄ¿ ÃÄÄÄ)ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄ´+\ ³5 >Ä Power
³³\ ³ ³ ³ ³ ³4 >ÄÂÄ680RÄÂÄÄ´-/ Foldback +OutÄ680KÁ´+\ ³ ³ ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ)ÄÂÄ´-/ ³ 3K3 ³/ Comparitor
³3 >Â6K8´ ³/ V&I \³ ÚÄ1K5Ä+Out ³ ³ ³/ ³ ³
+InÄÄ680K´-/ ³ ³ ÚÄ´ Mixer ÃÄ>Ipot ³ ÀÄÄÄ10KÙ 5K
³³/ ³ ³ ³ ³\e e/³ 100R 10K preset Set Max Power
ÀÄ68KÙ ÀÄ)ÄÄÄÁÄÄÄÙ ³ ³ ³ Dissipated
ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÁ<+ve & 0V -6V

The resultant balanced mix product, feeds the 4th Op amp, to give an accurate signal proportional to the power being dissipated. And that then feeds the 5th Op amp & also uses the -6V to set the Max Power Dissipated, & also light the OVERLOAD LED.

CALIBRATION
A high power DC load is needed, I used a large tapped WW resistor 2+2+4R 1kW. Set the start position of the Variac VarR, & then the 4 presets are used to determine, 1 Max overhead voltage, 2 Max Current range, 3 Current foldback sample Ipot, & 4 Max foldback power. When correctly set the head voltage is always just enough for a cool regulator at all output voltages & currents!

DOUBLE CURRENT MODIFICATION
As the PSU has a huge mains transformer & a roomy chassis, I decided to see if it could be made to do 10A! (350W Regulated DC O/P!)

1/ Transfer the 2 heatsink reverse voltage protection tab diodes to the PCB,
as they don't need heatsinking.

ÚÁÁÁÁÁÁÁÁÁÁÁÁÁÁÂÄÄÄÄÄÂÄÄÄÂÄÄÄ¿
³³ pcb ³³ C ³potÃÄBr´³ 2/ Re-use the 2 spare heatsink holes for 2 ³~~~~~~~~~~~~~~ÀÂÄÄÄÂÙ~T~ ³ more NPN insulated Darlington 15A
³ _ _ _ _ Rs ÚÄÄÄÄÄÁÄÄÄÄÄ¿³ transistors, pointing away from each other, ³(_)(_)(_)(_) Ý ³ VARIAC ³³ to spread the heat.
³ Cs Ý ³TRANSFORMER³³ File rear case hole to give clearance, &
³ ÚÁÄÁ¿ ÚÁÄÁ¿Ý ³___________³³ wire them up in parallel to PCB, with a 3A ÀÂÁÄÄÄÁÂÄÂÁÄÄÄÁÂÂÂÄÄÄÂÄÁÄÂÄÄÄÄÙ fuse in each emitter, as current share Rs.
~~~~~ ~~~~~ ÀÙ ÀÄÄÄÙ
3/ Remount 16mF C & bridge, from PCB metal panel, on to the rear case either
side of the VarR, making sure the pillar mounted PCB will fit back OK.
4/ Remove the PCB metal panel, & add more main 65V smoothing (I used 4x 5mF) in
the new space, to maintain low DC ripple @ 10A before regulation.
5/ Shunt 5A meter with resistance wire to now read 10A FSD, & re-label 2x/10A. 6/ Shunt the large current sampling 0.1R with resistance wire to bring it down
to < 0.06R, & mount it with the left over pillars behind Amp meter mounting
screw.

Then do the calibrations again with current limit set to @ 10A max, & the fold back current to 1.5A when set @ 35V. It is all just doo able, right on the limit of the PSU & still not running too hot.

CONCLUSION
This is now quite useful bench test PSU, able to test low current circuits
with the new current limit from "10mA to 10A" @ voltages from "75mV to 35V". Used for testing prototypes, zapping & charging batteries, run modest power rigs (30W) etc.

Be aware not to over voltage kit, as it is a Lab PSU not just a Rig one!

If it is often to be used at this "higher power limit", a 60øC thermal trip glued on the transformer might be a good safety feature to add!


See also tech buls "Lab PSU", "Simple 20A PS", "High AMP Crowbar Protected PSU"

Why Don't U send an interesting bul?

73 De John, G8MNY @ GB7CIP



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