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12 Volt Distribution Box
 This
project describes a very basic 4 way distribution box that I use with my modest solar
powered backup system which outputs 12 volts DC on four 2.1mm x 5.5mm
DC sockets mounted on the front panel. The two sockets on the left
are wired as a parallel pair and can be externally switched (these
supply a couple of LED lamps). The two sockets on the right are also
wired as a parallel pair but are 'always on' (one supplies a
Wi-Fi router and other I use to charge my phone). The front panel also
hosts an LED to indicate that power is present (green for green energy,
get it!). On the rear panel there's a PG7 cable gland where 12 volts
enters via a captive cable from a LifePO4 solar battery, and another
'2155' socket which is used to connect an external switch for the
lamps. If nothing is plugged into this socket, it automatically shorts
out and the lamp outputs will also be 'always on'.
 Main Schematic
The
schematic is self explanatory, though some may question the choice of
using two fuses instead of four. In this particular application, two 2
amp fuses are perfectly fine as each load is no more than a few hundred
milliamps and the wiring is rated at 6 amps, but please be aware that
these sockets are not rated for much over 1 amp each. The value of 47K
for the LED series resistor may seem a bit high, but if a high
brightness LED is used then it's just right, and will draw hardly any
current. The external switch used for turning the lamps on and off can
be a normal mechanical type, or alternatively, a MOSFET can be used as
described next (see schematic below). WARNING: THIS CIRCUIT IS 12 VOLTS DC ONLY, NOT 240 VOLTS AC MAINS!
 MOSFET Schematic
So why use a MOSFET? Using a MOSFET is a way to switch the lamps on and off using standard
logic levels (5V = ON, 0V = OFF). I wanted the ability to control the
lamps remotely using Alexa, and I thought the easiest way to do
this would be to simply plug a USB charger
into a smart plug and then use the output of that to trigger the gate
of a MOSFET. Other methods could have been used to achieve this, but as I use Tapo smart
plugs everywhere, I wanted to have just the one app on my phone to
control my lights.
 MOSFET Stripboard Layout
If
you're wondering what the two resistors are for, the 1K is to limit any
inrush current due to the fact that a MOSFET has some gate capacitance
which can present a low impedance when discharged, and the 220R is to
ground the gate when there's no drive signal to ensure that it's
definitely off (it also discharges any gate capacitance). This could be
a higher value and still work, but 220R was chosen because the output
of most USB chargers will stay high for a while due to residue voltage
on their smoothing capacitor and this resistor helps to discharge it,
otherwise the lamps would stay on for a few seconds after being turned
off. The MOSFET board can be housed in a miniature
Hammond box with entry and exit grommets.
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