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Zero-standby charger

Why

Because the phone chargers should not eat any current when not in use. There is also a (fairly negligible but anyway) risk of fire from an unloaded charger, especially the cheaper types with marginal parts. Having the phone stationed in a cradle is also more comfortable, especially if said cradle is mounted on a wall or a table side or another otherwise unused, easily accessible space.

What

The unit consists of several parts.

The phone is held in a cradle, hand-moulded from polycaprolactone (Shapelock, Polymorph, etc.) using the phone itself as a jig. A power connector is molded into the material. Due to difficult availability, the connector was cannibalized from a charger. (What idiot can choose a connector that is not easily obtainable off-the-shelf?!?) The cradle has a molded cut-out for the carrying strap of the phone. The design of the assembly allows replacement for another cradle if the phone gets decommissioned and another type is obtained.

A switch is positioned behind the cradle, actuated by the backside of the phone. It serves the role of detecting the presence of the phone and disconnect power when the phone is not present and the charger is not needed. As the hole for the switch was fairly large, and dirt, dust, cookie crumbles, and other undesirable materials could gradually accumulate inside the case, a protective seal was made from plastic foil and attached with double-sided tape between the switch and the case. The tape provides a better-than-needed degree of sealing. (The seal had to be remade; during the wiring assembly, minitorch was used to shrink the heat-shrink tubing, and the hot air instantly melted the thin plastic foil.)

The power supply board is a switching power supply extracted from a phone charger. Originally a weaker one, giving at most 350 milliamps, was used; this was later changed to an 800 mA board from another, compatible phone charger.

A switch was added to the design for selectable charging speed; selectable series resistance (10, 5, or 0 ohms) limits the current consumed by the device attached, and therefore limits the heating of the device's components, possibly reducing wear. The cost is a longer charging time, which may not be an issue when the phone spends most of its time cradled. The switch allows selecting between slow, middle, and fast charging.

An integrated ammeter shows the immediate current consumption. A cheap 10-amp one was used, with shunt replaced with a homemade one and the dial recalibrated for 0.8A range. As the charger is to be used in darkness, and also as some indication of power-on state is desired, two 3mm yellow LEDs are added to the meter's body, illuminating the scale with a gentle yellow glow, their plastic domes ground flat to modify the angle of the light.

An USB port, wired for charging, was added yet later, for the purpose of connecting any other 5V-chargeable device, e.g. a camera, a MP3 player, or another phone. A computer USB port is typically limited to 100 mA, this one allows up to almost 800 mA. The charging current limit resistors can be applied here as well, as the USB connector is connected in parallel to the phone power jack. A three-position mains power switch was also added, allowing disabling the charger (so the device serves as only a cradle), enabling the charger regardless of the phone switch state (so the USB port can be used without the phone in the cradle), and normal behavior, with the charger powered only with the phone present.

Images


Phone in the cradle

Phone in cradle, meter backlight visible

Charger, front side

Charger, bottom-right side

Inside, with lid

Inside view

Inside, low voltage side

Inside, mains side

Switch and power connector

Mockup

Case cut

Polycaprolactone, hot

Molding the cradle

Cradle

Cradle, mounted

Case inside, bare, with cradle and power lead

Mounting of mains switch

Cradle with power switch

Switch with foil seal

Switch with foil seal

Switch with seal, in position

Power supply board, old version

Power board, old and new

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