Radio Pages on MDS975
MDS975 Home Page
My Home Page

Amateur Radio Button
My Station button
Antennas button
Accessories button
Projects button
Data Modes button
Information button
Portable Operating - /P - button
QSL button
RSGB button
Links button
QRM - Home Plug Networking
                        Interference and other QRM
Photos and
                        bits 'n' bobs
Contact button

Mini Site Map

MDS975 Home



G Whip Antenna Products
Get on the air with

Site Search

powered by FreeFind

                        Distribution Box with crowbar protection
                        circuitPROJECTS (07)

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 suppressor.
An over-voltage crowbar circuit.
Option to add a digital volt meter display.

M0MTJ DC Distribution Box with crowbar
                        protection circuit


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 DC line.

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 will blow.

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.

                        Distribution Box with crowbar protection
Circuit diagram 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 Electronics for advice concerning component selection.

The capacitor 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 each component.

                        Distribution Box with crowbar protection

M0MTJ DC Distribution Box with crowbar
                        protection circuit

M0MTJ DC Distribution Box with crowbar
                        protection circuit
Crowbar over-voltage 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.

                        Distribution Box with crowbar protection
8 Way Version

                        Distribution Box with crowbar protection
Underside view of the 4 Way Version before the wiring and circuit is fitted

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 positive side

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.

                        Distribution Box with crowbar protection
8 Way Version wiring and circuits

M0MTJ DC Distribution Box with crowbar
                        protection circuit
Underside of the compact 4 Way Version

M0MTJ DC Distribution Box with crowbar
                        protection circuit
Ensure that there is a fuse on both of the substantial power connecting
cables that connect to the Power Supply Unit

                        Way Version
The finished 8 Way DC Distribution box with Crowbar over-voltage protection, reverse
protection and transient suppression and Volt Meter

M0MTJ DC Distribution Box with crowbar
                          protection circuit
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).

Parts list
1 x TYN640RG Thyristor, 40A, 600V, TO-220  (£2.60)
1 x 1.5KE15A Transient suppressor diode, TVS, 15V, 1500W  (£0.60)
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 (£1.20)
4 x small rubber feet          (£1.00)
1 x Strain relief grommet   (£0.15)
4 x M3 nylon screws, nuts and washers   (£1.40)
2 x In-line standard blade fuse holders   (£2.70)
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 required)     (£1.50)
1 x Digital voltmeter (if required)    (£4.00)

Estimated cost (£GBP  2014)

About £32.00 to £37.00 for larger 8 way box with meter. About £22.00 to £27.00 for small box without meter.

Components available from Bowood Electronics:
Suppliers of
                          Electronic Components - Bowood Electronics

References and links

Bowood Electronics :
Anderson Power Products :
Torberry Connectors :
Sotabeams :
Phil Salas AD5X :
Station Road Baseboards :

June 2014

Suppliers of
                            Electronic Components - Bowood Electronics
Components available from Bowood Electronics:

Links to Electronic Component Suppliers >>

: If you attempt any of these projects proceed with due caution with regard your
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.

Mike Smith - © 2003 - 2016