Expanded Range Voltmeter
Next to an aerial and a radio you will require a source of power.
Many radios nowadays will operate on 12 Volts so a sealed lead acid battery to avoid local mains borne QRM may seem attractive.
An alternative is a lithium battery but they're more expensive than lead acid.
An expanded range voltmeter makes it easier to read the critical readings 11·5 Volts (safe minimum) to 14·5 Volts maximum (if on charge) for a 12 Volt nominal deep cycle leisure battery (not a car battery that will easily damage if used this way) for amateur radio use.
Car batteries are designed to deliver a large cold cranking amps then rapidly recharged from the alternator. Leisure batteries such as used for golf carts are ideal for extended radio use e.g. national field day contests.
Take a 0 to 5 Volt voltmeter and rework the scale in your head from 0 to 10, the 1 to 11, the 2 to 12, and so on until 15. In other words you add a 1 prefix to each scale reading and label the meter as such.
The attached circuit diagram below is adapted from: http://www.thebackshed.com/windmill/articles/ExpandedScaleVoltMeter.asp
The black plastic project box I used measures: 11·5 x 9·5 x 4·5 cm.
The expanded range meter above (10 Volts to 15 Volts) is reading 13·2 Volts.
Components: potentiometer, Zener Diode & resistor.
It took me a while to source the PowerPole panel mount kit because I did not know what to call it when using a search engine but eventually found it at:
All the other components I sourced from E-bay. Expect to pay something in the order of £30 to make this project. Any 5 Volt DC meter will do that will fit in your project box. Although I used a large meter the pointer is thin and I later found that a smaller meter with a thick pointer may be easier to read.
If I had a second go at this project I would attach the PowerPole panel mount through 90 degrees because it was difficult to secure the lower nut and bolt.
To create the circuit diagram I used CircuitSymbol font with OpenOffice Writer and an image editor but it was most tedious so here is an alternative option using Circuit Diagram Symbols:
My first attempt was to solder the components onto a strip of Veroboard but despite all my efforts it did not work. I tried again with a fresh diode, resistor and potentiometer using a chocolate block connector and it worked first time. If I had my time again I would have soldered the Zener Diode last rather than first as I suspect I may have damaged it with heat.
Here is a precis of the explanation:
"The Zener Diode ZD1 is chosen to suit the minimum meter voltage you want to display. I used Zener Diode 1N4740 to operate at 10V. Resistor R1 is chosen to supply a current of a couple milliamps through the Zener Diode to make sure it's regulating properly. The variable resistor VR1 is the meter scale current adjustment. I used a variable power supply set at 13 volts and adjusted VR1 to calibrate the meter."
The 1N4740 was fragile so eventually I replaced it with robust 5Amp 1N5347B which worked fine for me.
If the meter behaves in an unusual manner try putting a 10M Ohm resistor and 0·1uF capacitor both in parallel across the zener diode.
So that's the theory.
Small piece of red insulating tape indicates critical 11·5 Volts limit.
Thank you to Norman G8ATO of Verulam ARC for technical advice.
73 Bob Houlston G4PVB MA3053SWL
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