30 Silicon chip Australia’s electronics magazine siliconchip.com.au
Instead of using the PCBs that we designed, you could copy
the approach used for the prototype and build the control sys-
tem on a piece of stripboard (Veroboard, for example) and hand-
make your own PCB to host the Mosfets and related components.
My suggested stripboard layout is shown at right. This re-
quires a board with at least 13 strips and 21 rows of holes. The
diagram is drawn looking from the top of the board (ie, from the
non-copper side). The copper tracks are shown as a visual aid,
as if you can see them through the board.
You may want to use a larger piece of stripboard so that you
have space to drill some mounting holes later. Before fitting the
components, cut the tracks in the sixteen locations shown (in-
cluding all seven tracks under IC1). It’s often easier to cut the
tracks with a 3mm twist drill, just removing the copper around the
hole. Having soldered the components in place, fit the wire links.
The shorter links can be made using component lead off-cuts,
or in some cases, by merely bridging adjacent tracks with sol-
der. Longer links are best made with solid-core insulated wire
(eg, Bell wire).
For the Mosfet board, you will need a piece of double-sided
copper laminate around 100 x 100mm (slightly smaller, if you’re
planning to fit it into the specified box; check it fits before pro-
ceeding). Ideally, this should have thicker-than-normal copper (eg,
“2oz” which is double normal PCB copper thickness).
The required layout is shown clearly in the photos below. On the
top of the board, you will need to make three straight cuts (eg, us-
ing a rotary cutting disc) to separate the copper into four islands.
The central islands should be around 25mm wide. Be careful
not to cut through the fibreglass substrate; just the copper. En-
sure the cuts are wide enough to guarantee electrical isolation.
The underside requires just one cut down the middle, separating
the copper on either side.
Next, drill two 8mm holes for the battery terminals and eight
2mm diameter holes (around the locations where the Mosfet tabs
will be soldered) for wire vias to pass through later. Now is also a
good time to drill four 3mm holes which the control board will be
mounted to later (lining up with holes on that board).
Bend pin 1 (the gate) of each Mosfet up, then solder the re-
maining five small pins to the central island. Be careful to place
the Mosfet so that the body does not bridge the cut in the cop-
per plane. Then, using a hot iron, solder the tabs in place. Join
the gates with light-duty wire; it’s easier to use stiff b ell wire, but
you could use Kynar or multi-strand wire.
The small copper island at the bottom is the ground connec-
tion point. Solder the anodes of the two TVSs to this island, with
the cathodes to the large planes on either side. You can now add
the zener diode, with its anode to the large central copper area
and its cathode to the Mosfet gate wire.Alternative construction method using stripboard and hand-cut PCBs
Next, run a strip of thick copper wire down the central island,
soldered near every pair of Mosfets, plus wires on the underside
fed through each of the 2mm holes you drilled earlier and bent
over to touch the battery terminals. Solder them near the terminals
and on both sides of the 2mm holes to form vias.
If you can’t easily get thick copper wire, you can use a bundle with
multiple pieces of 0.71mm or 1mm diameter tinned copper wire.
Solder four wires to this PCB: one to the main battery terminal
side, to supply 12V to the control board; one to the small ground
area, to connect to GND on the control board; one to the cathode
of the zener diode, which goes to the gate drive pin on the con-
trol board; and one to the central copper island (or zener diode
anode), which goes to the control board Mosfet source terminal.
Note, though, that this source terminal only connects to a 10kΩ
resistor with the other end connected to GND. So you could make
your life slightly easier by simply soldering a 10kΩ resistor be-
tween the two central copper islands on the Mosfet board and then
you won’t have to run this fourth wire.
The only part that’s left now is LED1, which can be chassis-
mounted to your box, with its anode connected to pin 4 of CON1
on the control board, and its cathode to pin 1. Make the three other
connections from your Mosfet board to CON1 on the control board,
as described above, and you are ready for testing.
The photo at left shows the original
(hand made) prototype “Mosfet
PCB” with its hand-cut breaks
between the copper sections. Note
how the gate pins here are all
connected to (the red) insulated
wire, not to the PCB. At right is
the opposite side, with 8mm
brass bolts soldered firmly
in place, with heavy copper
wires which pass through the
board and are soldered to the
top copper as well.Stripboard prototype with matching layout below. Don’t
forget to cut the tracks where indicated – you’ll have a
massive short circuit otherwise!