RADIO HABANA CUBA
EXPANDED SCALE VOLTMETERDxers Unlimited for 14 August, 1999
with Arnie Coro, radio amateur CO2KK
A listener in the United Kingdom writes: "Arnie, is there any way to make a DC voltmeter measure just the critical operating area of my batteries, that is from about 10 to 15 volts, instead of using the full scale for the 0 to 15 volts???" Sure amigo, and this is known in the trade as an EXPANDED SCALE VOLTMETER. I must say that it is one of the most useful and practical instruments that one can think of. For example, as you well know, the most popular type of storage batteries are so-called 12 volt types. If they are made by series-connecting lead acid cells, six of them in series connection will provide, when fully charged, 2.2 volts times six, so here it is 13.2 volts. But in order to CHARGE that battery, the charger must provide no less than 14 or better yet 15 volts DC. Here is how an EXPANDED SCALE VOLTMETER helps you a lot. You design your meter so that the extreme left end of the scale, instead of reading 0 volts as usual, reads say 10 volts, and then the extreme right or maximum is made to read 15 volts or 16 volts. This is easily achieved by using a very simple circuit requiring just 2 fixed and one variable resistors and a zener diode... and of course the METER itself:
ARNIE'S EXPANDED SCALE VOLTMETER Parts List: For my EXPANDED SCALE voltmeter, I use a standard 0 to 50 microampere meter movement, a 2.7 kilo-ohm resistor, a 68 kilo-ohm resistor, a 22 kilo-ohm pre-set type potentiometer or variable resistor, and a 10-volt zener diode. If you cannot find a 10-volt zener diode, just go ahead and use a 9.1 zener diode; for some some reason, these are much more easily available. If you use the 9.1 volt zener, then your meter scale will go from 9 to 14 volts and the 15 volts will probably fall off scale, right at the end of the scale, but still useable. As you may realize, NOW you have a meter that really HELPS to SEE how much voltage the battery is really delivering. Your meter will measure from 9 or 10 volts to 14 or 15 volts, so you can tell when the battery REALLY needs recharging, without having to resort to the digital multimeter. As a matter of fact, I use several of these expanded scale voltmeters in my equipment.
Now here is how to interconnect the components of the expanded scale voltmeter:
Assembly: The meter's PLUS terminal connects to the PLUS terminal of the battery you are monitoring. From the meter's PLUS terminal, you connect one lead of the 2.7 kilo-ohm resistor. The other end of the 2.7 kilo-ohm resistor goes to one lead of a 68 kilo-ohm, 5 percent tolerance resistor. The other end of the 68 kilo-ohm resistor goes to one terminal of the 22 kilo-ohm variable preset resistor. The two other terminals of the 22 kilo-ohm preset resistor are connected together and, with a wire, to the MINUS terminal of the meter. Now you connect cathode end of the 10 volt or the 9.1 volt ZENER diode to the junction of the 2.7 kilo-ohm resistor and the 68 kilo-ohm resistor. The ANODE of the ZENER diode is connected to the MINUS TEST LEAD of the meter. Again -- this is important -- the PLUS test lead of the METER goes directly to the PLUS TERMINAL of the meter itself. But the MINUS TEST LEAD is connected ONLY to the ANODE terminal of the ZENER DIODE.
Calibration: The calibration is fairly simple... connect a DIGITAL MULTI METER to the battery that you want to test... measure the voltage, and then connect your EXPANDED SCALE VOLTMETER to the same battery at the same time. Let's say you have a voltage of 12 volts. Then, turn the pre-set control until the 50ua movement gauge is about mid scale... The final adjustment is done by using a variable power supply set for 10 volts. This should cause the gauge's needle to move to the low end of the scale. Now, carefully crank up the power supply's voltage and mark your scale at the 11, 12, 13, 14 and 15 volts and that's it. You are the proud owner of an EXPANDED SCALE VOLTMETER!
Arnie Coro CO2KK
Havana, Cuba
14 August, 1999Postal address:
Arnie Coro, "Dxers Unlimited"
Radio Havana Cuba
PO Box 6240, Havana, CUBA 10600
phone: 53-7-814243
phone res: 53-7-301794
e-mail: arnie@radiohc.org
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