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12 Volt Distribution Box
 I
have a basic solar power back up system that supplies 12 volts for my
router and some LED lighting so I needed a simple little box to provide
some fused outputs. The output sockets on the front panel are standard
2.1mm x 5.5mm DC barrel types with two being directly connected to the
solar battery and one being fed via the changeover contacts of a relay.
When the router's AC adapter is connected to the rear DC socket, its
voltage will be appear on the switched output, but if there is a mains
power cut, the relay will switch over to the solar battery. In the
summer months the adapter can be unplugged to let the router run
exclusively from solar energy. The front panel also hosts two LEDs to
show what voltage sources are present at the input, red for mains (via
a 12 volt wall adapter) and green for solar (via a LiFePO4
battery). Green for 'green energy' get it!
 The
schematic is self explanatory,
though the reason for using different LED current limiting resistors is
because green LEDs are brighter to the human eye than red, so with
these values they appear equal in intensity. The values may seem high,
but they're perfect for the high brightness LEDs used here. The fuse
ratings chosen were to suit my particular needs, but be aware that the
sockets used are not rated for much over 1 amp. To this end, power from
the battery enters the box via a captive cable as this will have to
handle up to about 3 amps. Please note that due to the simplicity of
this circuit, there will be a very short interuption to the switched
supply during changeover (just a fraction of a second) which will cause
a router to reboot. As this only happens once in a blue moon, it's not
really an issue for me in this particular application. If needed
though, a UPS circuit can be found here
 The
relay is mounted on stripboard with a 3mm securing hole in the corner.
It just so happens that a small eyelet tag can be soldered onto the leg
of the relay coil (indicated on the diagram by an arrow) so its hole
lines up with the securing hole. This automatically grounds the circuit
to the metal chassis via an M3 mounting bolt and hex pillar. While on
this subject, all the sockets are earthed to the enclosure through their
metal bodies so no additional ground wiring is required. To keep with
this line of 'tidy' design thinking, I was able to obtain a 3 way 20mm
fused terminal block with a grounded middle terminal which allowed
the negative of the battery supply cable to connect directly to chassis
upon entering the box through a PG7 gland. Also available to match the
3 way fused block were single 20mm fused blocks that can be clipped to
it and to each other to form as many rows as required
 WARNING: JUST BECAUSE I USED UK MAINS CABLE, IT DOES NOT MAKE THIS A MAINS POWERED DEVICE - IT IS 12 VOLTS DC ONLY!
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