+I needed dual voltage power supply for my lab that runs on mains
+electricity (wall power) and slowly charges attached 12V Lead-Acid
+battery. Lead-acid battery in turn provides high current when needed
+as well as power in portable situations or when mains electricity is
+not available.
+
+
+
+Power supply provides about 13 Volts and 5 Volts simultaneously.
+
+
+
+!!!WARNING!!! I use USB-A connectors for power output, but IT IS NOT
+COMPATIBLE WITH USB DEVICES AND POSSIBLY WITH EXISTING USB CABLES.
+
+
+
+Normal USB devices and cables should not be plugged in! It would
+damage device/cable and likely power supply.
+
+
+
+I found USB-A connectors to be easily available, cheap, reliable and
+functional, so I re-purposed connector for my own needs but I'm using
+totally incompatible electrical wiring.
+
+
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+
-Simple charger and terminal (front-end) for 12V lead-acid battery.
+As seen, it has dual voltage output. Additionally one wire is
+dedicated to data transmission and can be used with 1-wire
+protocol. Power supply simply passively links all data wires together
+but does not participate in communication.
-Idea is to distribute power from rechargeable 12V battery to various
-appliances (soldering iron, radio transmitters, etc..) while also
-providing use-charging in the background.
+Power supply uses 3D printed body with lots of holes, for fanless
+cooling.
+
+
+
+
+
+
-Single on/off button disconnects battery and up-step voltage source
-from front panel terminal connectors.
+Various modules are realized on top of prototype PCBs that slide into
+dedicated rails within the body.
+
+
+
+
+
+
-Tecnoiot MT3608 voltage up-step is used. 3V DC external power supply
-is used to feed voltage up-step. Up-step is tuned to ~14.4 V.
+Like this:
-
+
-Current design is rather of blog (experience sharing) value. It has
-quite some drawbacks:
+When cover is closed, it blocks PCB movement:
-
-
Warning: Circuit has dangerous instability. Requires low voltage
-~3V but relatively high current external power supply. Using
-higher voltage but low current power supply could easily lead to
-situation where step-up generated load pushes power supply output
-voltage down too much. This leads to too much power being lost
-within power supply and easily leads to power supply overheating!
-
-
There is no dedicated current limiter. 3V external power supply
-and up-step are acting as current limiters.
-
+
+
+
+
-
While RCA connectors do work as DC power connectors, but are not
-intended to be used this way and are not very reliable.
-
+
+There is still some empty space inside, so why not add dummy cover on
+top that can be replaced later with add-on functionality/expansion
+board/terminal :)
+
-
Voltage up-step provided output is not very clean. Extra capacitors
-and induction coils could be added to remove high frequency noise in
-the voltage.
-
-
Metallic and shielded body would be helpful to reduce
-electromagnetic/radio noise generated by step-up.
+
+
+This is where high voltage from mains electricity enters the system.
+
+
+
+Schematic:
+
+
+
+
+For safety I kept high voltage section as minimal as possible. That
+is, wall plug runs straight into transformer. Also I used UV hardening
+glue for extra safety and isolation on PCB.
+
+
+
+Power on/off switch operates on already reduced voltage of about 30
+volts. Power switch is located on indicator panel.
+
+
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+
+
+DC current of about 30 volts is then routed to Main board.
+
+As seen from schematic, ~30 volts DC from transformer board is routed
+into first step-down converter that reduces it to about 15V. Reduced
+voltage is then directed to Current limiter circuit. Current limiter
+loses about 2 volts. Now we have current and voltage limited power at
+about 13 volts. This power is used to charge connected 12V Lead-Acid
+battery. Also the same power is routed to connector terminal to be
+consumed by connected devices.
+
+
+
+As seen from this schematic, device is not meant to provide high
+current for long periods of time. Instead it gets comparatively
+limited current to charge the battery and feed devices with low
+current requirements. Occasional current spikes are backed up by
+battery that stays in use-changing mode.
+
+
+
+Also about 13V output voltage is approximate and depends on connected
+battery charge level.
+
+
+
+Second Step-Down converter reduces voltage even further to quite
+precisely 5V DC. This resulting voltage is also routed to connector
+terminal.
+
+
+
+Both 13V and 5V lines are also routed to indicator panel for
+monitoring.
+
+
+
+There is single on/off switch. In off position, it disconnects battery
+and transformer from the system effectively powering everything down.
+
+
+
+On schematic above, some wires are annotated with numbers from 1
+to 13. This corresponds to output pins on the board.
+
+
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+
5 Current limiter
+
+
+Schematic:
+
+
+
+
+Simple LM317 based current limiter is used. I used 4.7 ohm
+resistor. It provides about 265 milliamps of current. See calculator.
+
+
+
+
+
+
+
+
+
+Thermal paste below and UV hardening glue on top is used to attach
+LM317 to the heatsink. There is also jumper-like solution on top
+right. This is handy to attach multimeter tap to verify/monitor
+current during initial device calibration.
+
+
+
+If attached battery is really empty, significant voltage drop can
+occur in LM317. Heatsink is needed to dissipate that power.
+
+
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+
+Note: resistor gets hot to.
+
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+
6 Indicator panel
+
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+
+Basically 2 digital DC voltmeters and main on/off switch.
+
+As seen above, most of the USB connectors are used to deliver dual
+power output and 1-wire data connectivity, except one on the bottom
+right. This is used to attach 12V battery. Some capacitors are thrown
+in as well to stabilize against smaller current spikes.
+
+
+
+Electrically schematic is realized using smaller prototype PCBs.
+
+
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+
+Those PCBs are wired to central small PCB that acts as a hub:
+
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+
+PCBs are held together by being sandwiched between front panel:
+
+Since device uses adjustable step-down modules, these need to be
+calibrated to provide correct output voltage. It is important that
+battery receives proper charging voltage otherwise either no charging
+occurs or battery starts gassing out and gets destroyed.
+
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