ChipAmp Dual Voltage Power Supply with Soft-Start & Fan Regulator
This is a bare PCB, not a kit. Constructors of DIY power amp systems can assemble this PCB as power supply for a range of IC-based power amps, or for a very capable dual output bench power supply. It will supply regulated +/- 8VDC to +/- 30VDC with enough power to drive the power amp stage to maximum output with appropriately sized CT transformer secondaries. The design includes build options for soft-start to reduce power-on in-rush current and a separate voltage regulator for a small DC fan.
Continuous power is limited by transformer (VA rating and VRMS output) and by thermal management. When constructed using part recommendations, this design can deliver +/- 30VDC at 2.5A load on each output with the appropriate transformer and large heatsinks. It's a dual regulator design, but outputs share a common ground. For a variable output bench supply, wire panel mount potentiometers to this PCB.
Bare board and assembled prototype photos are shown. Buyer will receive only the bare board. The assembly photos are shown for reference only. A full parts list is shown in the listing.
PCB specs: 101mm x 61mm, 2 layer, FR4, 1.6mm thick, 2 oz copper, green soldermask, white silkscreen, HASL finish (not lead free).
The PCB was professionally designed and fabricated by a volume PCB manufacturer. The circuit was simulated with SPICE to confirm function/voltage/current/power ratings/bias points.
The prototype was built with parts list components, and tested with a 29-CT-29VRMS transformer. Outputs were set for +/- 25VDC, and no more than 100mV peak to peak ripple was observed at 2.5A continuous output. Higher capacity electrolytics will further reduce ripple.
The design is primarily through-hole construction, though some SMD components are present. You need good soldering skills, PCB soldering/assembly tools (including SMD), to be comfortable with AC mains wiring safety, and to be willing to research/experiment to resolve transformer selection and thermal management issues. The constructor will provide the transformer, wiring, heatsinks, enclosure, and fasteners needed to complete the project. So your project requires you to source parts, assemble/solder/test/calibrate the PCBs, fabricate your chassis/heatsink and wiring, and assume responsibility for system design and wiring safety decisions. You will also need a multimeter and an oscilloscope if you're evaluating ripple voltage under loads. If any of this is uncomfortably vague, then don't buy this PCB.
Soft-Start Control (optional)
Soft-start controls limit the high inrush current associated with large VA transformers connected to linear supplies with large filter capacitors. This is often a surge of multiple-tens of amps.
What does this circuit do? A power-on cycle is activated by +5V or +12V power applied to this circuit by way of the XV+ and XG terminals. This activates a relay, which switches AC power on, but during a start-up delay (100ms to 150ms), AC current flows through a large power resistor to limit in-rush. After the delay the power resistor is bypassed with a second relay.
The design is tested with +5V or +12V activation power only. If higher DC voltages are required, the constructor will need to evaluate component ratings and availability of relays with appropriate coil voltages that fit the footprints. For this application, the activation power is usually supplied by a small stand-by 12VDC power source. That's how consumer audio consumer components do it. The stand-by source is always on.
Transformer selection
A voltage, power dissipation spreadsheet is available (no charge, upon request) to guide CT transformer, heatsink, and component selection.
AC Wiring & Safety Warnings:
(i) If the soft-start control is implemented, AC line voltages (120/240VAC) ARE connected to this PCB, and the line out to the transformer primary is fuse protected. Otherwise, AC mains protection is not provided, which means your wiring design must include an AC chassis mount or inline fuse. Either way, transformer secondaries are connected - THESE CAN BE AS LETHAL as AC line voltages. If you are uncomfortable with AC mains wiring, either seek expert advice or abandon this project.
(ii) AC EMI line filtering is NOT provided. This feature, if required, is a constructor supplied chassis mount feature.
(iii) Earth ground from the AC line is NOT connected to this PCB, but your harness should connect it to the chassis in which this PCB is mounted.
(iv) This design provides NO current protection on DC outputs (DC outputs are not fused). Any circuits powered by this PCB that don't have supply current protection will mean that the wiring harness includes a chassis mount or inline fuse holder and appropriately rated fuse.
(v) DC output over-voltage indication or protection are not provided. These features, if required, are constructor designed and integrated.
Surface Mount Parts
QS2 (optional - Soft-Start Control) N channel MOSFET; 12V option: 2n7000 TO-92 or 2n7002 SOT23-3; for +5V, 2n7002 may be preferred. For SOT23-3 use QS2ALT pads.
FC1 (optional - DC fan) 0.1uf 0805 SMD 50V 20% X7R cap, such as C0805C104M5RACTU, back side
FC2 (optional - DC fan) 1.0uf 0805 SMD 50V 20% X7R cap, such as C0805C105K3RACTU, back side
C4 0.1uf 0805 SMD 50V 20% X7R cap, such as C0805C104M5RACTU
C+1 0.1uf 0805 SMD 50V 20% X7R cap, such as C0805C104M5RACTU
Soft-Start Control Parts
CS3 0.1uf 2.4 X 4.4mm, 5.0mm lead spacing 50V Z5U bypass cap, such as KEMET C410C104M5U
RS1 see note above for 5V/2N7002 operation; otherwise, 47k metal film resistor - thru-hole 1/4W 1%, 0207 package or equiv.
RS2 27k metal film resistor - thru-hole 1/4W 1%, 0207 package or equiv.
D1 1N4001 or higher DO35-7 diode. Don't omit.
D2 1N4001 or higher DO35-7 diode. Don't omit.
CS1 electrolytic cap, 5mm diameter, 2mm lead space, 16V or higher; see notes above for value
CS2 47uf electrolytic cap, 5mm diameter, 2mm lead space, 16V or higher
QS1 2N4403 or 2N3906 PNP TO-92 transistor
SSFUS Fuse holder clips, thru-hole
R1 10 ohm 50W Wirewound Resistor, Chassis Mount, such as HSA5010RJ or Dale RH-50
ACREL DPST relay 12V option: FTR-F1AA012V, OSA-SS-212DM3, G5PA-28-MC-12VDC OMRON; 5V: FTR-F1AA005V, OSA-SS-205DM3. If substituting verify footprint.
SSREL DPST relay 12V option: FTR-F1AA012V, OSA-SS-212DM3, G5PA-28-MC-12VDC OMRON; 5V: FTR-F1AA005V, OSA-SS-205DM3. If substituting verify footprint.
other 4.8mm spade 5mm lead space, see TE 63860-1 (locations LINEI, LINEO, XG, XV+, CHASSIS1, CHASSIS2)
5 x 20 mm glass fuse (try 4A fast blow fuse)
Power Supply Parts I
ZF (DC fan option) BZX79C12 12V zener diode
RF (DC fan option) 1.8K metal film resistor - thru-hole 1/4W 1%, 0207 package or equiv.
R+1 5.6k metal film resistor - thru-hole 1/4W 1%, 0207 package or equiv.
R+2 470 metal film resistor - thru-hole 1/4W 1%, 0207 package or equiv.
R-1 5.6k metal film resistor - thru-hole 1/4W 1%, 0207 package or equiv.
R-2 470 metal film resistor - thru-hole 1/4W 1%, 0207 package or equiv.
R-3 22k metal film resistor - thru-hole 1/4W 1%, 0207 package or equiv.
C-1 0.1uf 2.4 X 4.4mm, 5.0mm lead spacing 50V Z5U bypass cap, such as KEMET C410C104M5U
C+2 0.22uf 2.4 X 4.4mm, 5.0mm lead spacing 50V X7R cap, such as K224K20X7RF5TH5
C-2 0.22uf 2.4 X 4.4mm, 5.0mm lead spacing 50V X7R cap, such as K224K20X7RF5TH5
C5 0.1uf .4 X 4.4mm, 5.0mm lead spacing 50V Z5U bypass cap, such as KEMET C410C104M5U
Z3+ BZX79C3V3 3.3V DO-35 package, 0.5W or equiv.
Z3- BZX79C3V3 3.3V DO-35 package, 0.5W or equiv.
VP TL431CLP TO-92 2.5V reference
VN TL431CLP TO-92 2.5V reference
Q+1 2N4403, 2N3906 TO-92 PNP transistor up to 36VRMS secondaries; above that, substitute 2N5401
Q-1 2N4401, 2N3904 TO-92 NPN transistor up to 36VRMS secondaries; above that, substitute 2N5550/2N5551
C3 0.22uf 18mm, 15mm lead spacing PET film cap, at least 100V, 10%, such as WIMA MKS4J032204F00K series
R+A 25K trimpot, Piher PT10LH01-253A2020 side adjust or equiv.
R-A 25K trimpot, Piher PT10LH01-253A2020 side adjust or equiv.
RL 1/2W 10k metal film resistor - thru-hole 1/2W 1%, 0309 package or equiv.
ON 3mm thru-hole LED
B1 D5SBA20 bridge rectifier, mount small heatsink fin with thermal grease before soldering.
other 4.8mm spade 5mm lead space, see TE 63860-1 (locations L1, L2, CT, (2) V+, (2) GND, (2) V-,)
Heatsink Mounted:
Q+2 MJE243, KSC2690A, or equiv.
Q-2 MJE253, KSA1220, or equiv.
Q+3 2SC5200, 2SC3856, 2SC5242/FJA4313 or NPN TO-3P/TO-264 equiv. Not TO-220 device. Mount with mica insulator + grease.
Q-3 2SA1943, 2SA1492, 2SA1962/FJA4213 or PNP TO-3P/TO-264 equiv. Not TO-220 device. Mount with mica insulator + grease.
FQ (DC fan option) MJE243, KSC2690A, or equiv. Mount to heatsink with thermal grease.
other (2) mica insulators + thermal grease
aluminum finned heatsink
Power Supply Parts II
C1 6800uf, 63V (or higher specs) Electrolytic cap, 30mm diameter, 10 or 12.5mm lead space
C2 6800uf, 63V (or higher specs) Electrolytic cap, 30mm diameter, 10 or 12.5mm lead space