Expanded Scale Voltmeter - Fiberglass RV


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Old 05-13-2008, 10:28 PM   #1
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Technical Help Needed!!!

I'm working through the remodel of our 1985 U-haul Camper Trailer, and I started to install an inexpensive RadioShack analog panel voltmeter (0-15 VDC / $12.99) to keep track of the battery voltage. But I realized that with a little more work I could modify the circuitry of the existing 0-15 VDC voltmeter to make it into an Expanded-Scale Voltmeter that would be much more useful over the scale of 10-15 VDC.

My question: Does anyone out there know how to modify the circuitry of my voltmeter to create an Expanded-Scale Voltmeter? I know that I can simply purchase an expanded-scale voltmeter (i.e., BlueSea) at a boating supply store for about $35.00, but I think that it's possible to modify my existing 0-15 VDC voltmeter by inserting a "Zener diode" and a couple of resistors somewhere in the circuit.

Has anyone completed this modification to create an expanded-scale voltmeter that's functional over the range of 10 to 15 volts?

Any help will be greatly appreciated!

Steve
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Old 05-13-2008, 11:15 PM   #2
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To actually get the needle to swing farther you would have to amplify the voltage after you cut off the bottom 10 volts with the diode. Then you would have to make a new scale card to read it.
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Old 05-13-2008, 11:30 PM   #3
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Quote:
Technical Help Needed!!!


My question: Does anyone out there know how to modify the circuitry of my voltmeter to create an Expanded-Scale Voltmeter? I know that I can simply purchase an expanded-scale voltmeter (i.e., BlueSea) at a boating supply store for about $35.00, but I think that it's possible to modify my existing 0-15 VDC voltmeter by inserting a "Zener diode" and a couple of resistors somewhere in the circuit.

Has anyone completed this modification to create an expanded-scale voltmeter that's functional over the range of 10 to 15 volts?

Any help will be greatly appreciated!

Steve
My suggestion would be to buy a 0-5V meter and use a 10V Zener in the negative lead to give the 10V offset. Of course a series resistor to the supply voltage input would be needed since the internal impedance of a modern digital meter is way too high to provide a reasonable diode current. The only other problem you could face is that most of the inexpensive stand alone meters require an isolated power source. You could use a 9V transistor radio battery for the power source if you are only doing the occasional test but for continuous monitoring you would probably want to use an isolated DC-DC converter.

Of course, by the time you go to all that trouble you might as well buy the BlueSea unit.
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Old 05-14-2008, 12:49 AM   #4
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You might look for an automotive volt meter that has real numbers on the scale. It's already calibrated for the range you need. Don't know about the cost though.
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Old 05-14-2008, 11:53 AM   #5
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Try one of those cheep digital voltmeaters at www.harborfreight.com
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Old 05-14-2008, 01:13 PM   #6
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This is what I use.

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I adapted a Cigarette Lighter male plug on the end.


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Then plug into your Cigarette Lighter to test battery voltage.
Sold at Harbor Freight---> http://www.harborfreight.com/cpi/ctaf/disp...temnumber=92020
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Old 05-14-2008, 03:40 PM   #7
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Quote:
Technical Help Needed!!!

I'm working through the remodel of our 1985 U-haul Camper Trailer, and I started to install an inexpensive RadioShack analog panel voltmeter (0-15 VDC / $12.99) to keep track of the battery voltage. But I realized that with a little more work I could modify the circuitry of the existing 0-15 VDC voltmeter to make it into an Expanded-Scale Voltmeter that would be much more useful over the scale of 10-15 VDC.

My question: Does anyone out there know how to modify the circuitry of my voltmeter to create an Expanded-Scale Voltmeter? I know that I can simply purchase an expanded-scale voltmeter (i.e., BlueSea) at a boating supply store for about $35.00, but I think that it's possible to modify my existing 0-15 VDC voltmeter by inserting a "Zener diode" and a couple of resistors somewhere in the circuit.

Has anyone completed this modification to create an expanded-scale voltmeter that's functional over the range of 10 to 15 volts?
Any help will be greatly appreciated!
Steve
Attempting to alter a cheap DC voltmeter to work expanded scale is pretty difficult. First you need to understand how the meter works. Most meters of this type have somewhere around 50 microamp meter movements, which means that the current through the actual meter requires 50 microamp for full scale. (50 microamp = .00005 amps).

To make a 15 volt full scale meter a resister is selected to put in series with actual meter. The resistance is selected so that when put in combination with the resistance of the meter will cause 50 microamp of current to flow when the whole thing is connected to 15 volts.

Now that you're throughly confused, the bottom line is it would take a bit of engineering and a some parts to turn that meter into expanded scale. The cost of the parts alone would probably more expensive that purchasing one off the shelf. Electronic parts are priced by volume. You go off to Radio Shack you'll pay a $.20 each for a resister. Electronic manufacturers will pay $.001 or less for that same part by purchasing 10,000 pieces at a time. DIY is pretty much cost prohibitive.

Now that's my $.02 worth

Byron
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Old 05-16-2008, 12:37 PM   #8
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Many thanks to all for your good advice and council! I do have several digital multimeters but I'd like to install a switched panel voltmeter inside my U-Haul that will always be connected to my 12 VDC electrical system.

I found an excellent article with a detailed description of how to convert a 0-15 VDC analog panel voltmeter into an expanded-scale 10-15 VDC analog panel voltmeter at the following website. The article (part 5.0) includes the circuit diagram and the specs for the zener diode and resistors.

Parts needed:
10 V 1 W zener diode
4.8 K ohm resistor
100 ohm resistor


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I made a trip to my local RadioShack but they didn't have the parts in stock. I found them online at Partsexpress.com (10 V zener diode = $0.39; 4.7K ohm resistor $0.69; 100 ohm resistor $0.69 / total parts = $1.77) but the minimum shipping and handling cost was $12.50! This would bring my total cost to purchase and modify the 0-15 VDC RadioShack meter ($12.99) to about $27.30.

So... I went ahead and bought the marine grade BlueSeas expanded-scale analog voltmeter from my local boating supply store for $32.00. The BlueSeas voltmeter covers the range of 8-16 volts and also includes an internal LED backlight as an added bonus. As you can see the expanded-scale voltmeter gives you a much better indication of the status of your battery with line increments of 0.2 volts.


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Thanks again for your help!

Steve
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Old 05-17-2008, 10:10 AM   #9
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The schematic you have is for a very low impedance meter. It'll work but, as has been mentioned, It's expensive to buy small quantities of electronic components today. The other problem is that the digital meter is a very high impedance unit. Unfortunately an apple and orange comparison. I think the Blue Sea unit will serve you very well. I'll bet if you popped the bonnet on it you would find a circuit very much like the one in the schematic.
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Old 05-17-2008, 03:40 PM   #10
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Camping World sells a digital voltmeter that plugs into a cig lighter and goes beyond 15VDC.
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