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External 9V power supply unit
Notes
The PT DC motor speed controller requires an input of 9V DC, and its connector is a 3.5mm mono headphone socket with a positive tip.  The original PT PSU is a very basic unit, and a beefier, regulated unit improves the pitch stability.
You can used to be able to buy a decent multi-voltage 1.2VA linear regulated PSU for about £20, but they are getting harder to find as cheaper, more efficient (but electrically noisy) switch-mode power supplies replace them.
This page lists the components I used to build a professional looking PSU in a black anodised aluminium box; not cheap, but made with good quality components.  You can reduce costs by using a plastic case, eliminating power switches, and using a cheaper toroidal transformer.  But where is the fun in that?!

Components
Case and fittings
Black aluminium heatsink case, 16510360mm (200mm long if front switch required)
8 25mm M3 allen head stainless steel bolts
Replacement front panel 10360mm in black 3mm acrylic
4 self-adhesive small rubber feet
Fused switched IEC panel socket, Maplin part
2 x 20mm 500mA quick blow fuses, Maplin part
1 8mm M3 allen head bolt, nut, washer, for grounding chassis (endplate) to IEC socket.
1x Neutrik mono panel mount 3.5mm jack.
(Optional) IP67 black latching switch with LED indicator
Transformer
100⨉165/200mm Veroboard
Nuvotem toroidal transformer, 230VAC input 15VAC, 1.25A output
Brass (non-ferrous/non-magnetic) M5 50mm nut and bolt, nylon washer – for securing transformer

Voltage regulator
1 Velleman K1823 kit - 1A PSU kit
VO220 heatsink and insulating washer
20cm of 5A+ three core wire
2 2 way terminal blocks
4 PC brass standoffs
4 PC M3 screws
4 5/32" nuts

PSU components
9V power lead
1m+ of 3A twin core cable for power lead
1m+ nylon cable sheath
2   Neutrik gold-plated 3.5mm mono plug
Heatshrink or nylon mesh cover for power leads

Construction
Do not try unless you are competent and confident; this is just a guide, and assumes you know what you are doing.

Cut Veroboard to length of case; screw on front endplate with the M3 allen head bolts, and slide the Veroboard into the bottom slot of the case to check.
In the rear endplate, cut hole for IEC socket, and drill holes for 3.5mm socket and M3 earthing bolt.
Cut section out of back of Veroboard to clear the IEC socket, typically about 40mm by 40mm
Drill hole for Transformer fixing bolt in Veroboard (allowing space for terminal block if positioned at front), and bolt transformer in place.
Slide heat shrink onto mains wire, then solder onto IEC socket terminals, cover joint with heat shrink.
Fit IEC socket in endplate, cut earth wire to length, solder on spade terminal, and bolt to endplate.  Check earth connection.
Build the Velleman kit. Fit heatsink. Fit stand-offs to PCB corners with M3 screws.
Test the kit; fit fuse to socket, attach Socket L+N to the transformer input wires with one terminal block.  Attach one of the output pairs to the input of the PSU kit.
Isolate the other output pair e.g. in the other terminal block.
If fitting the front switch, connect the LED pins to the PSU output, and connect the positive output of the PSU through the Normally Open pins of the switch before connecting to the terminal block..
Attach the kit output to a multimeter, turn the voltage trim pot right down, and switch on.  Turn the trim pot up slowly until 9VDC is measured. Attach a load, e.g. slot car motor and test. Check the heatsink gets warm, but not too hot. Disconnect all parts again.
Fit 3.5mm plug to endplate, and wire to kit output.  If front panel switch is fitted, wire it to the kit output in parallel.
Work out where to route wires and where to fit the kit.
Mark and drill holes for standoff screws.
Wire up, and check the output is still 9VDC.
Slide Veroboard into case, and check EIC for any short-circuits. Test output again.
Screw endplate carefully in place, test outside case is earthed at the IEC pin (test on one of the bolts) and test output again.
Shake and turn over to check nothing rattles, and test again.