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DC Distribution Box with Reverse Voltage, Over
Voltage and Transient Protection
by Mike Smith MØMTJ
Summary of key features and benefits:
A multi-way DC distribution board.
Standardised connections to protect against
accidental reverse connections.
Diode protection, incorporating a transient
An over-voltage crowbar circuit.
Option to add a digital volt meter display.
Last year I decided that I should standardise the
power connections on all my equipment. After
venturing out portable I realised that I needed a
good safe and reliable method of connecting
equipment that allowed for no errors.
I use a Yaesu FT-857D. Worryingly this and other
transceivers such as the FT-897 and FT-817 and
perhaps other similar radios do not have reverse
polarity protection. If the polarity is accidentally
reversed it's 'Goodnight Vienna', as they say!
Absolutely no more crocodile clips or other assorted
connectors I thought!
Like many other radio amateurs I settled on the use
of Anderson Powerpoles® manufactured by Anderson
Power Products (APP). These ingenious connectors are
safe, easy to work with, inexpensive and ideally
suited to amateur radio operations.
APP list a number of advantages: Powerpoles® have
genderless housings with simple assembly; they are
stackable with colour coded modular housings with
connection versatility; a self securing design with
a low resistance connection; available in four
sizes. The most appropriate type for amateur radio
are the 30 Amp Powerpoles®. These are small, neat
and available from a number of suppliers, including
those listed below.
Having settled upon the standardised connector type
I decided to design a connector panel or box that
could sit alongside my power supply unit (PSU). This
would facilitate safe connection and disconnection
of multiple pieces of equipment and accessories. I
happened to have a Box034 (MB5) from Bowood
Electronics which was ideal for the purpose, but
other enclosures could be used.
Although the Powerpoles® offer protection against
connection mistakes, there remains the the
possibility that the power distribution box itself
could be connected to the PSU incorrectly. I
therefore wanted to add further protection in the
form of a diode that would conduct and blow the in
line fuses should the connections ever be
accidentally reversed. A 'belt and braces' approach.
Having recently repaired a minor fault with my main
power supply unit (PSU) I was looking at power
supply specifications. To my surprise I discovered
that many 13.8 volt PSU's, including my own, do not
include over voltage protection circuity. Many '12
volt' radios are rated at 13.8 volts with a
tolerance of +/- 15%, and will therefore operate
within a voltage range of about 11.7 to 15.8 volts.
If the PSU was to develop a fault with the regulator
transistors a greatly increased and potentially
destructive voltage could be delivered to the radio.
A protection circuit, often known as a 'crowbar',
can be included in the output of any PSU to protect
against an over voltage occurrence. Worryingly, for
the sake of a few pence, crowbar circuits are often
omitted from PSU designs, probably due to cost
cutting measures in budget equipment, leaving
equipment vulnerable to fault conditions.
The significant parts of a crowbar circuit are a
zener diode and a thyristor. The circuit is designed
to trip at a certain voltage, in this case 15 volts.
This voltage is set by the zener diode. Should
the PSU become faulty and attempt to deliver a
higher voltage, the zener diode will trigger the
thyristor's gate (G). The thyristor will then
conduct from anode (A) to cathode (C) instantly
shorting the power supply from positive to negative.
This will blow the fuses, removing power from the
transceiver and other equipment. The electrolytic
capacitor ensures that the circuit is insensitive to
short duration transients or noise on the incoming
When searching for a suitable circuit design I found
the very useful website of Phil Salas, AD5X. Phil
detailed a crowbar circuit which, interestingly,
included the additional protection of a transient
suppressor 1.5KE15A. The transient suppressor is a
high current (70 amp) zener diode. This clamps the
supply voltage, protecting against brief transient
over-voltage spikes. Being as this diode is
connected directly across the the supply, it also
protects against accidental reverse polarity
connection. In which case the diode would conduct,
shorting out the supply so that the in-line fuses
While researching the design I noticed that Bowood
Electronics stocked a very neat panel mount digital
voltmeter. I felt this would add a useful and
attractive finishing touch to the project.
I made several versions of my DC distribution board.
An eight way version, housed in the larger enclosure
includes the digital volt meter. I use a smaller
four way version in the car without a meter.
drawn by Phil Salas AD5X
Thyristor is TYN640RG, 40A, 600V, which is
available in the UK
In other areas the S2055W thyristor may be found.
Thank you to Will Outram of Bowood
for advice concerning component
value is stated as 10uF in Phil's circuit
diagram, but his text states 1uF. I made up
these circuits using the value stated in the
circuit diagram, however I suspect that the
value is possibly 1uF. The capacitor makes the
circuit insensitive to short duration transients
or noise on the incoming DC line.
Construction is fairly straightforward. The crowbar
circuit itself could be made up on a small piece of
tag strip, or on a piece of Veroboard as I did. The
layout and track breaks are shown in the
photographs. I double checked my work against the
circuit diagram to ensure the correct orientation of
protection circuit with reverse voltage
protection and transient suppression
The ABS box requires a slot to be cut out in the top
for the Powerpoles® using a very sharp blade. A
Dremel tool can help with this. For the eight way
version the slot measures 91mm x 16mm. The slot is
45mm x 16mm for the four way version. The
Powerpoles® are retained in the slot with small 1mm
thick aluminium plates, each about 15mm wide. These
fit into the grooves on the sides of the connectors.
The two plates are held tightly in place on the top
surface of the box with four M3 nylon screws.
8 Way Version
Underside view of
the 4 Way Version before the wiring and circuit
Each pair of Powerpoles® must be spaced a small
distance apart from the next pair to allow enough
room for individual connections to be made to the
completed unit. I cut out some 11mm x 35mm pieces of
3mm thick styrene board to use as spacers. As long
as the slot is cut accurately in the box, the fit
will be tight enough to hold the spacers in place by
friction. If the fit is not tight enough each spacer
could be held in place by lightly glueing each one
to the enclosure – not to the Powerpole housings.
I used thick copper wire to connect the Powerpoles®
together. A neater method could be to use 0.15
strip-board as two rails (+ve and -ve) to connect
each pole to. The crowbar circuit is wired across
the +ve and -ve rails taking care with correct
polarity. The wires of the in-line blade fuse
holders enter the enclosure through a strain relief
grommet. One in-line blade fuse holder is
soldered onto the negative side, the other to the
The other ends were fitted with colour coded
connectors of a size that matched the terminals of
the power supply – noting polarity. Because the
in-line fuse holders are supplied with red wire I
used black heatshrink to identify the negative
wires. These wires are quite short, so an additional
length of 30 amp DC power cable could be used as an
extension if the distribution unit is to be placed
at a greater distance from the PSU.
8 Way Version
wiring and circuits
Underside of the compact 4
Ensure that there
is a fuse on both of the substantial power
cables that connect to the Power Supply Unit
The finished 8
Way DC Distribution box with Crowbar
over-voltage protection, reverse
protection and transient suppression and Volt Meter
The finished compact 4 Way DC Distribution box with Crowbar
over-voltage protection, reverse
protection and transient suppression
Before applying power I double checked all wiring
and construction for errors. Before connecting any
equipment, I connected the distribution box to the
PSU and ensure that the correct voltage appeared at
each pair of Powerpoles. The function of the crowbar
circuit could be tested if the PSU allows adjustment
to over 16 volts – such a condition will blow the
fuses when the thyristor is triggered. (Do not have
any equipment connected during testing).
1 x TYN640RG Thyristor, 40A, 600V, TO-220
1 x 1.5KE15A Transient suppressor diode, TVS, 15V,
1 x 1N5245B Zener diode, 15V, 0.5W (£0.12)
1 x 1uF, 25V electrolytic capacitor (£0.08)
1 x 10 Ohm resistor (£0.02)
1 x 27K Ohm resistor (£0.02)
1 x strip-board (Vero board) 20 holes by 8 holes or
tag strip (£0.30)
8 x Anderson Powerpoles® Red (4 for smaller version)
8 x Anderson Powerpoles® Black (4 for smaller
version) (£11.00 for 8 red
and 8 black)
Connecting wire (£ probably
in junk box)
Length of thick copper wire (to wire up Powerpoles®
) (£2.00 if not in junk box)
1 x ABS Project box e.g. MB5/Box034 150 x 100 x
60mm (£3.55) or Box021 75 x 51 x 27mm
4 x small rubber
1 x Strain relief grommet (£0.15)
4 x M3 nylon screws, nuts and washers
2 x In-line standard blade fuse holders
Length of black heatshrink to identify negative
cable on in-line fuse holder (£0.40)
2 x eyelet lugs preferably colour coded (to connect
to PSU) (£0.40)
2 x Blade fuses (e.g. 25 amp)
Small piece of 1mm thick sheet material, e.g. sheet
aluminium (£ probably in junk box)
3mm thick spacing material – I used black styrene
Plasticard from Station Road Baseboards ( £ ? )
30 Amp DC cable (if
1 x Digital voltmeter (if
About £32.00 to £37.00 for larger 8 way box with meter.
About £22.00 to £27.00 for small box without meter.
Bowood Electronics : http://www.bowood-electronics.co.uk/
Anderson Power Products : http://www.andersonpower.com/products/singlepole-connectors.html
Torberry Connectors : http://www.torberry.co.uk/superbasket/category/92/Powerpoles+-+15+to+180+amps
Sotabeams : http://www.sotabeams.co.uk/
Phil Salas AD5X : http://www.ad5x.com
Station Road Baseboards : http://www.stationroadbaseboards.co.uk/
available from Bowood Electronics:
Links to Electronic
Component Suppliers >>
DISCLAIMER: If you attempt any of
these projects proceed with due caution with
own safety and the safety of the equipment
that you are working with!
I cannot be held responsible for any
accidents, injuries or damage
caused to any equipment that may result.