the power lead to the transformer and connecting the PC board
All the components fit on to a small PC board with the
transistor at the edge of the board so that it can be screwed to a heatsink.
Fit the 8 power diodes first. These all align in the
one direction and the line on the board corresponds to the white or silver band
on the body of the diode.
Next fit the resistor, zener diode and 100n monoblock
capacitor. The zener looks like a signal diode and may have a number of
different markings on it. Sometimes it is marked with the zener voltage and
sometimes it has a code number. It may have 1N 5535 or 1N 5245 or IN 5861 or
even another number. Zener diode numbering is very messy, but they all refer
to a 15v zener. The mini pot is next to be fitted. If you want to mount the pot
on the front panel, it will be best to use a standard 5k pot with
shaft so you can fit a knob.
The two electrolytics are next, making sure the
positive lead goes down the hole marked on the overlay.
Finally, the BD 679 is fitted so that the heatsink goes
between the transistor and PC board (to provide the best heat transfer). See
below for more details on choosing the correct heatsink and applying thermal
grease to improve heat transfer.
An adequate heatsink is most important when
building a power supply, both to make it reliable and keep the components
operating within their temperature range.
When any of the parts get too hot you can introduce
unwanted hum (another name for ripple) or even create premature failure of
the diodes or transistor.
When all the components are fitted, and the heatsink is
in place, it can be placed in a suitable case, along with the power
Artwork for the 2-amp power supply
The new boards in the kit have added heatsinking for the diodes.
This project has two options. It can be a mains
operated project, using a 2155 or 2156 transformer or it can be connected to
a plug pack.
We recommend it be connected to a 16v
AC 1 amp plug pack as these are double insulated and provide
As we have mentioned in the introduction,
the 2 amp transformer M 2156 will not provide much more than 1.4 amps DC output
so the 1.5 amp AC plug pack has nearly the same rating.
By the time you buy a M 2156 transformer and power
cord, the total will be the same as the cost of the plug pack so it should be
one of your considerations.
a plug pack to the 2-Amp Power Supply
The heatsink is one of the most important components
in a power supply. It must provide adequate heat dissipation to protect a
heat-sensitive device, from being damaged.
All power supplies dissipate heat. Some are more
efficient than others but whenever voltage and current are present together,
heat will need to be dissipated.
The heatsink in our project fits between the metal side
of the transistor and PC board to get direct contact with the transistor.
It is most important to get good thermal contact
so that any heat generated in the transistor will be carried away by the heatsink.
Since the transistor has a very small area for this
heat transfer, (all the heat must pass through the side of the transistor) it
is most important that the gap between the face of the transistor and the
heatsink be filled with thermal grease (thermal compound). Use a very thin
layer of ordinary toothpaste. You only need
a small amount smeared over the metal side of the transistor. The bolt is then passed
though the transistor, heatsink and PC board and the nut tightened until a
small amount is squeeze out from under the transistor. This proves
the nut is tight but it must not be too tight as you may damage the transistor
itself by cracking the case.
SIZE OF HEATSINK
The way to select the correct-size heatsink is to build
the project and fit a heat sink about 4cm x 10cm as a trial experiment. Next
you need some high wattage resistors or car lamps to load the power supply to
its maximum rating (this will depend on the transformer you use). Place one
finger on the transistor and another on the heatsink, about 2cm from the
transistor, and monitor the temperature rise in both positions. You should be
able to hold your finger for the duration of the experiment. If not, a
thicker heatsink is needed, as the one you are using is not transferring the
heat away from the transistor fast enough. The heatsink supplied in the kit is
thick and will dissipate about 8 watts.
The kit comes with a "Movement."
A movement is a moving coil with a pointer
with no resistors connected to the terminals.
The movement we supply is actually a SIDE PANEL METER that you convert to an
Any Movement can be converted to an ammeter, quite
To produce 0 -1 amp reading,
two 1R and one 4R7 (all in parallel) are
connected across the two terminals. These are called SHUNT RESISTORS.
The three shunt resistors can be clearly seen in the photo above.
To produce 0 - 2 amp reading, four 1R and one 2R2 are connected in parallel
across the terminals.
The kit contains a total of 6 resistors for this conversion.
All you have to do is open the movement and change the scale. You can put a
sticker over the scale and write 1A or 2A on the sticker.
The project should be housed in a small case,
preferably plastic, with either a flying lead on the output or a set of
terminals on the front of the case, marked with positive and negative.
The power cord must be anchored inside the case.
You can place a metal plate in the inside
of the bottom of the case to act as a chassis and this can be bent to form part
of the heatsink for the transistor.
If you are going to use a plug pack, the case
will be a lot smaller and a lot cheaper. You could even leave the project out
of a case and use the mini pot on the board as the voltage adjustment. I prefer
to leave things in their skeleton form, so I can see the "works" -
that's why I hardly ever suggest fitting a project into a case.
Decide carefully which arrangement you are going to
choose. Either way it will be a handy addition for your model railway or
Here is the POWER SUPPLY module from Graham Stewart, in a case
with a digital voltage module showing the output voltage and banana plugs for
It's a nice power supply that you can build yourself and have the
satisfaction of learning about heatsinking.