Simple AC power supply with battery backup
I’ve been using a store bought imported power supply for the homebrew general coverage receiver, with satisfactory results, but wanted a homemade supply for use with it as a matter of principle and appearance.
The power supply presented here is nothing special, but provides several convenient functions: A low power regulated DC output derived from the AC mains; a rechargeable battery pack and charger; both power sources can be connected to the output connectors which can accommodate multiple loads; and a digital multifunction display.
The line transformer is a Philmore miniature 7.5-0-7.5 1 amp secondary device, with the center tap unused and the ends of the secondary connected to a full wave bridge to ensure sufficient DC output voltage. 1N4007 diodes, bypassed by 100 nF disc ceramic capacitors for noise suppression, make up the bridge.
The DC regulator is an LM317 device. A 200 ohm sense resistor is used between the output pin and the adjust pin, which is grounded through a 4k7 trimpot to set the desired output voltage. The small heat sink limits power output to about 600 ma or so. The circuit is completely generic - I did not even bother to draw a schematic.
The Battery Pack consists of three overpriced pieces of Li-ion 18650 cells, each with a nominal voltage of 3.7 volts and 1800 mah capacity that I purchased locally at a Wal Mart lawn and garden center.
Probably the best way to manage Li-ion battery packs is with one of the inexpensive battery protection boards designed especially for this purpose - they limit the charge / discharge current, charge / discharge voltage cutoffs, and probably other stuff like equalizing charge between individual cells in the pack. I tried several different types and had 100% failure rate for reasons unclear to me.
So, to move forward with the project, I built a crude charger from another LM317 that limits charge current to 600 ma, about 1/3 C. More charge current would be hard on the cells, and 600 ma is about the limit from the transformer / regulator / heat sink combination. The charger module takes it’s DC input directly from the rectifier output. A trim pot sets the voltage limit on the charger, here set to about 12.3 volts, or 4.1 volts per cell ( 90% capacity ). 18650 Li-ion cells should not be discharged below a certain point, generally about 2.75 volts per cell, so a protection circuit or voltage monitoring should be used to protect the pack from over discharge.
An inexpensive front panel digital voltmeter allows for monitoring of the battery pack charging and discharge voltages, and adjustment and monitoring of the output voltage of the DC supply from the mains. The display also contains a 24 hour clock with battery backup, and a temperature sensor, as well as the DC voltmeter. It is a two wire device, meaning no independent DC supply voltage is required. The display can cycle through its various functions at a programmed interval, or continuously display one selected parameter. The attached video shows the display cycling.
A series of simple mechanical switches allow: turning the AC power on / off; connecting the battery pack to the charger or the load control switch, or complete disconnection of the battery pack; connecting the output load connectors to the battery pack or the DC output derived from the mains; and connecting the voltmeter to monitor the battery pack voltage or DC output of the mains AC supply, or disconnected ( display off ).
Banana / Binding posts are used for the output connectors - only two pairs are presently installed, but the case is fitted for three more pairs if needed. A two wire polarized line cord is used with a 1 amp fast blow fuse inline on the hot side before the switch.
The chassis and small contrasting finish bezel around the display are designed on TinkerCad and 3d printed. The display and bezel are press fit into the opening on the chassis; a green display is installed at present - I may change it out to another color. Connectors are used for a mostly modular design - the battery pack and DC supply / charger module can be installed / removed without need to solder / unsolder any connections.
73 and happy building,
Win W5JAG