I might be asking a dumb question but why is D2 on the negative rail and will light or do anything and why ???
Sorry but I am trying to learn here and to better understand..

 

Well I found some info and Mastech made the original and Velleman just re badged it..
Hope it helps
http://www.eevblog.com/forum/testgear/vellemanhq-power-ps1503sb-(mastech-hy1503d)-review/
http://forum.onliner.by/viewtopic.php?t=6070662

Hope it help and let me know how it works out or need more info
Jay Sr

 

Well, yes, but the functions of board 2+3 and a bit from board 4 could be merged into one switcher, that would be current limited to 2A at the input and provide a voltage limited current source, and feed that to the final linear reg.
Also, you mentioned that the linear regs should not be dissipating more than 15W, but I think that at high output current and low load impedance you might be getting more than that in Q4.

I thought of that, more work than I care to do. I'm following a block diagram approach, which is more likely to work the first time. There is also the thing that the designs on boards 1,3,and 4 can be used for other applications, other power supplies with minor parts changes.

 

I might be asking a dumb question but why is D2 on the negative rail and will light or do anything and why ???
Sorry but I am trying to learn here and to better understand..

It is a pseudo zener, nothing more. I'm pushing the power supply limits on some of the chips, their max voltage being 36V total.

The -3.5V is used to allow the final regulator (and the pre-regulator) to sense all the way to ground on the inputs of the comparator and op amps.

 

OK, since board 2 is likely not to be used in future projects like this, I have re-sequenced the part number to put it last. This is the current schematics I am working from.



Signals

+48V / Input
+28V / Output
+51.5V / Output
-3.5V / Output
Ground
+DVM1 PM PS / Output
-DVM1 PM PS / Output
+DVM2 PM PS / Output
-DVM2 PM PS / Output


Bill of Materials

U1 - LM317
U2 - 555
CR1.2 - 1N4454 Silicon Diodes
D1,2 - Blue LEDs
ZD1 - 15V Zener Diode
SD1-8 - Shottky Diodes, Low Power, Optional
C1,4,6,8 - 0.1uF Caps
C2 - 1uF Cap
C3 - 220uF Cap
C5 - 1nF Cap
C7,9 - 10uF Cap
C10,12,14 - 10uF Cap, Optional
C11,13 - 0.1uF Cap, Optional
XFM1 - Torrid Transformer, Hand Wound, Optional
R1 - 120 ohm
R2 - 56Kohm
R3 - 2.7Kohm
R4 - 1Kohm
R5 - 10Kohm
R6,7 - 330 ohm



Signals

+48V / Input
+51.5V / Input
+48VLIM Out / Output
Ground


Bill of Materials

U7 - LM393
U8 - 7555, CMOS 555
Q4 - P-Channel MOSFET
Q5 - NPN, 2N2222A
D4 - Red LED, Indicator
ZD5 - 36V 5W Zener
SD11 - Shottky Diode, 50PIV 2A Min
C21,23,24,26,28 - 0.1uF
C22,27 - 220uF
C25 - 10uF
L2 - 220uH Hand Wound Torrid
R20 - 100 ohm Variable Trimmer
R21 - 22Kohm
R22 - 2.4Kohm
R23 - 10Kohm
R24 - 10 ohm
R25 - 0.1 ohm 1W
R26,28 - 1Kohm
R27 - 4.7Mohm



Signals

+48VLIM / Input
PreReg / Output
PS Output / Input
-3.5V / Input
Ground


Bill of Materials

U3 - LM393
U4 - 7555, CMOS 555
U5 - LM317
Q1 - P-Channel MOSFET
CR3,4 - Silicon Diode, 1N4454
ZD3 - 5.1V Zener
ZD4 - 12V Zener
SD10 - Shottky Diode, 50PIV 10A
C15,17 - 220uF
C16,18,19 - 0.1uF
L1 - 220uH, Hand Wound Torriod
R8,9,13 - 10Kohm
R10 - 100Kohm
R11 - 10 ohm
R12 - 2.2Kohm
R14 - 120 ohm



Signals

+28V / Input
-3.5V / Input
PreReg / Input
+PS Output / Output
Ground
+ DVM, Current Meter
- DVM, Current Meter
+ DVN, Volt Meter
- DVM, Volt Meter
Q4 Base
Q4 Emitter
Q5 Base
Q5 Emitter
R2 +, Current Adjust
R2 -, Current Adjust
R6 +, Voltage Adjust
R6 -, Voltage Adjust
+D4. Indicator
-D4. Indicator


Bill of Materials

U6 - 1465, Fast Linear Op Amp
Q2 - PNP Darlington
Q3 - NPN Darlington
D3 - Green LED, Front Panel
C20 - 0.1uF
R15 - 0.1 ohm
R16,20 - 10Kohm Pot, Front Panel
R17 - 500Kohm Trimmer
R18,21 - 10Kohm
R19 - 13Kohm
R22 - 10Kohm Trimmer
R23 - 5.1Kohm

 

Quick demonstration of why a diode is needed as return path when cap. coupled sig. is fed into a diode.
Test #1 has no output- LED dark, test #2 LED lit.