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From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW

By G8MNY (Updated Oct 17)
(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 regulator PSU, together with variable current limiting & dissipation power fold back.

┌────────────────────────────────────────────┐
│┌───────┐ ┌───────┐ ( ) FL-305 │
││ \ V │°│ \ A │ Fine V ___ │ It was all blown up &
││ \ │ │ \ │ .- -. │ modified into a simple │├───────┤ ├───────┤ / Course\ │ raw PSU. So I did a lot │└───────┘°└───────┘ \ V / │ of reverse engineering,
│ overload ( ) '-___-' │ to find out it how it
│ on trip sw A │ originally worked, before
│ ! ┌────┐ + - e + - │ attempting the restoration.
│ ° ° └────┘on° (o)(o)(o)(o)(o) │ └────────────────────────────────────────────┘

SCHEME 15A Power NPN 15A L──o-o─┐ 6-42V AC Darlington__ Amp Trip
2A )||(<───────┐ ┌──┬──────┤~~~\_/e ├─Rshunt─Meter─┬────┬────┬──\── +ve Mains _)||( \ ~│15A│+ │8-60V │ │ │ │ Control │
)||( Var Bridge │ │ Drive ├────Imon──────┤ PSU 0V! │ 0-35V
ON )||( R ~│ │- │ │ │Out │ Volt
N___ \_)||(________│ │ ===16mF │+In Out│ Vmon Meter 0-5A

|| ___ │ │ 65V └Drop Mon┘ │ │
300W ||(30V Control └──┴──────────────────────────────┴─────────┴──\── -ve E─┬──e └─── +/-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.

+12V───┬───2k7──┬─470─>Darlington Base 0-3mA
Voltage Comparitor ┌────────)────────┤ (Or a Mosfet)
│\ │ │ │ /│
+6V 0V─1K─┬──┬─┤+\ │/e 1K2 e\│ /+├
│ preset /_\\~/│1 >─┤<├─┤PNP ┌──┴──┐ PNP├─┬─┤>├─< 2│ Current Limit
└5k6──5K──┬─1K─┴──┴─┤-/ │\ OK OVERLOAD /│ │ \-├ Comparitor
2K │/ │ LED LED │ │ \│
Fine VarR 10K │ │ 10K │
│ │ │/ \│ │ │ /│
50K ├───┤NPN NPN├───┤ │ /+├
Course VarR │ │\e e/│ │ └─┤>├─< 5│ Power Foldback
│ 10K │ │ 10K \-├ Comparitor
-Ve -12V──┴─────┴─────┴─────┘ \│

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 emmitters of a "dual transistor long tailed pair" mixer, with a sample of PSU O/P current into one of the bases.

Darlington ┌───┬───+12V Power Dissapated │\
Volts Monitor 10K 10K │\ 0V─1K─┤+\
┌───68K──┐ ├───)────────────────┬───┤+\ │5 >─ Power
││\ │ │ │ │ │4 >─┬─680──┬──┤-/ Foldback +Out─680K┴┤+\ │ │ ├────────────────)─┬─┤-/ │ 3K3 │/ Comparitor
│3 >┬6K8┤ │/ V&I \│ ┌─1K5─+Out │ │ │/ │ │
+In──680K┬┤-/ │ │ ┌─┤ Mixer ├─>Ipot │ └───10K┘ 5K
││/ │ │ │ │\e e/│ 100 10K preset Set Max Power
└─68K┘ └─)───┴───┘ │ │ │ Dissapated
└─────────────┴──────────┴<+ve & 0V -6V

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

CALIBRATION
A high power DC load is needed, I used a large tapped WW resistor 2+2+4Ω 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 hugh mains transformer & a roomy chassis, I decided to see if it could be made to do 10A! (350W Regulated DC O/P!)

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

┌┴┴┴┴┴┴┴┴┴┴┴┴┴┴┬─────┬───┬───┬┐
││ pcb ││ C │pot├─Br┤│ 2/ Re-use the 2 spare heatsink holes for 2 │~~~~~~~~~~~~~~└┬───┬┘~T~ │ more NPN insulated Darlington 15A
│ _ _ _ _ Rs ┌─────┴─────┐│ transistors, pointing away from each other, │(_)(_)(_)(_) ▌ │ VARIAC ││ to spread the heat.
│ Cs ▌ │TRANSFORMER││ File rear case hole to give clearence, &
│ ┌┴─┴┐ ┌┴─┴┐▌ │___________││ wire them up in parrallel to PCB, with a 3A └┬┴───┴┬─┬┴───┴┬┬┬───┬─┴─┬────┘ fuse in each emmitter, as current share Rs.
~~~~~ ~~~~~ └┘ └───┘
3/ Remount 16mF C & bridge, from PCB metal panel, on to the rear case either
side of the VarR, making sure the piller 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 pillers 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 usefull 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 60°C 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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