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Voltmeter Needs Independent Power Supply

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Dennis Johnson

Joined: 10 Jan 2006
Posts: 89
Location: N. Calif.

Posted: Wed Jun 23, 2010 12:07 pm Post subject: Voltmeter Needs Independent Power Supply

Greetings,

  

 I'm adding a digital voltmeter to a 12 volt system.  It's not an  airplane, but I think my question also applies to airplanes.  The system is  a single cylinder engine with a 12 volt starter, alternator, and  battery.  I don't really need a voltmeter on it, but I wanted to play with  a cheap digital voltmeter for the educational value and this seemed like as good  an application as any.

  

 I bought this voltmeter:  Model XL3-20V, about two-thirds of the way  down the following page:

  

 http://www.futurlec.com/Panel_Meters.shtml

  

 The instructions that came with the meter say, "Power Source:  DC 12V,  must have separate power supply."

  

 It has four terminals.  One pair is for 12 volts DC positive and  negative power supply and the other pair of positive and negative for the DC  input signal.  I wanted to connect both pairs to the 12 volt starting  battery that I want to measure.  But the note that the meter requires a  separate power supply sounds like that won't work.

  

 I have a general question:  Why would a digital voltmeter need a  separate power supply?

  

 I have a specific question:  Is there a simple workaround, like  putting resistors or diodes in the power supply wires?  I know that I can  always go with a relay and flashlight batteries, but I'm hoping for a more  elegant solution.

  

 Thanks,

 Dennis

  

  

  

   [quote][b]

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retasker(at)optonline.net
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Posted: Wed Jun 23, 2010 1:44 pm Post subject: Voltmeter Needs Independent Power Supply

Some of these meters are designed such that they will not work properly 

unless the supply is isolated from the voltage being read.  I would 

guess that they can get away with a cheaper solution that way.  It is 

probably that the grounds cannot be connected but I don't know exactly 

what the circuits are.  Unfortunately, there is no work-around - simple 

or otherwise.  The power source MUST be totally isolated from the 

voltage to be measured.  Either use batteries or use an isolated DC-DC 

converter.



If it is possible, I would try to exchange it for one where the power 

supply does not have to be isolated, such as the PM1029B or the LCD 

versions CX101B or PM128E.  Of course, all of these require a 5V supply 

so you have that to deal with, but at least it can be derived from the 

12V you have with a simple regulator (LM7805) and a couple of capacitors.



If you really want to eliminate all the hassle (but pay more) try the 

Lascar Electronics EMV 1200-40 at

http://www.alliedelec.com/search/productdetail.aspx?SKU=5720050#tab=Specs



This is a self-powered meter.  Just connect it to a voltage between 4 

and 40 volts and it reads the voltage - no power supply required.  The 

current consumption spec says 3 mA, but I have two of them for my two 

batteries and the actual consumption is less than 1mA.



Some of the other meters that Lascar make can run from the 12V you have 

available and read whatever voltage you want to (non-isolated supply).



Dick Tasker



Dennis Johnson wrote:

 	  Quote: 	
		   Greetings,

 I'm adding a digital voltmeter to a 12 volt system.  It's not an 

 airplane, but I think my question also applies to airplanes.  The 

 system is a single cylinder engine with a 12 volt starter, 

 alternator, and battery.  I don't really need a voltmeter on it, but I 

 wanted to play with a cheap digital voltmeter for the educational 

 value and this seemed like as good an application as any.

 I bought this voltmeter:  Model XL3-20V, about two-thirds of the way 

 down the following page:

 http://www.futurlec.com/Panel_Meters.shtml

 The instructions that came with the meter say, "Power Source:  DC 12V, 

 must have separate power supply."

 It has four terminals.  One pair is for 12 volts DC positive and 

 negative power supply and the other pair of positive and negative for 

 the DC input signal.  I wanted to connect both pairs to the 12 volt 

 starting battery that I want to measure.  But the note that the meter 

 requires a separate power supply sounds like that won't work.

 I have a general question:  Why would a digital voltmeter need a 

 separate power supply?

 I have a specific question:  Is there a simple workaround, like 

 putting resistors or diodes in the power supply wires?  I know that I 

 can always go with a relay and flashlight batteries, but I'm hoping 

 for a more elegant solution.

 Thanks,

 Dennis

 *

 *

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user9253

Joined: 28 Mar 2008
Posts: 1908
Location: Riley TWP Michigan

Posted: Thu Jun 24, 2010 7:00 am Post subject: Re: Voltmeter Needs Independent Power Supply

I searched the internet for XL3-20V and found a posting that says it is OK for that meter to share a common ground for the power supply and the input signal.  http://tinyurl.com/2eul2xf

I would go ahead and connect the power and input pins in parallel and try it out.  Even if it smokes, you do not have much to lose.  If Futurlec accepts returns, it might not be worth your time and shipping expense to return it.

  I am guessing that the instructions "must have separate power supply" mean that the meter requires 12 volts to operate, as opposed to some meters that are powered by the input signal.

Joe[/quote]

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Joe Gores

Dennis Johnson

Joined: 10 Jan 2006
Posts: 89
Location: N. Calif.

Posted: Sat Jun 26, 2010 7:12 am Post subject: Voltmeter Needs Independent Power Supply

Greetings,

  

 Thanks, guys, for the helpful answers to my problems with cheap digital  voltmeters that require a power supply independent of the electrical  system I want to measure.  For my application, which isn't far from a  120 volt AC power outlet, I'll use a 120 VAC to 12 VDC transformer to power the  instruments.  Of course, that would require an unusually long extension  cord if this were an airplane.

  

 If other people are looking for cheap digital voltmeters, ammeters, etc.  you might want to first verify if they need an independent power supply or  not.  Some of the ones I looked at do, and some don't, and it doesn't seem  to be a matter of price.

  

 Thanks,

 Dennis  

   [quote][b]

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rampil

Joined: 04 May 2007
Posts: 870

Posted: Mon Jun 28, 2010 4:40 am Post subject: Re: Voltmeter Needs Independent Power Supply

It would almost certainly work fine to tie the grounds together and the 

+12 supply and the +measurement together.

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