[MRCA] Panel Meter Circuit?

[Al Klase]

OK, lets assume it's a 360 uA full-scale meter movement.

To make it work as a 6-volt full-scale voltmeter it needs to be in 
series with a resistor:

R = e/i = 6/0.00036 = 16,666 ohms - 280 ohms  (Of course your going to 
use something like a 10 K resistor in series with a 10 K trimmer.)

Verify that applying 6.0 volts gets a reading of 30.

Then, put a 24-volt Zener, or combination of Zeners in series with the 
resistor.  (remember positive goes to the cathode of the Zener.) That 
should get you the desired expanded scale.

Al

On 10/22/2012 7:35 PM, W2HX wrote:
> Ok, I've updated the chart so the data is correct (that's what happens when you aren't careful!)
> http://www.w2hx.com/x/1967LRS2/MeterRange.JPG
>
> John, what I can report is that even with 1 volt directly across the meter, it pegs beyond 30V display super fast.
> Thanks for indulging my mistakes
>
>
> 73 Eugene W2HX
>
>
> -----Original Message-----
> From: J. Forster [mailto:jfor at quikus.com]
> Sent: Monday, October 22, 2012 6:58 PM
> To: W2HX
> Cc: mrca at mailman.qth.net; boatanchors at mailman.qth.net
> Subject: RE: [MRCA] Panel Meter Circuit?
>
> That confuses things. I still think it's a 6V meter in series with a zener.
>
> Do this measurement:
>
>
>   -------------- A.M.-------------------O-----|
>    +                                    |     |
> Adj DC                                 |     |
> Supply                                V.M.   Meter
>                                         |     |
>    -                                    |     |
>   --------------------------------------O-----|
>
> Plot the A.M. v. the V.M. If the thing is a voltmeter and zener, as I suspect, the current should be about 0 until the supply exceeds 24 VDC.
>
> If the current increases linearly from 0, the meter is a voltmeter with the zero mechanically suppressed by preloading the hair spring.
>
> -John
>
> ===========
>
>
>
>
>
>
>> Just a little clarification based on the replies so far. First, I have
>> the milliamps wrong. The vertical axis is not milliamps, but
>> hundredths of amps. So 30 on that scale is really 0.3A. Second, I
>> connected a power supply in series with a few hundred k ohms and
>> measured the current in the circuit to come up with those figures
>>
>> Sorry for the error
>>
>>
>> 73 Eugene W2HX
>>
>>
>> -----Original Message-----
>> From: J. Forster [mailto:jfor at quikus.com]
>> Sent: Monday, October 22, 2012 6:12 PM
>> To: W2HX
>> Cc: mrca at mailman.qth.net; boatanchors at mailman.qth.net
>> Subject: Re: [MRCA] Panel Meter Circuit?
>>
>> Gene,
>>
>> It may not need any external curcuitry.
>>
>> Try just hooking the meter to a variable voltage supply, and increase
>> it from zero slowly.
>>
>> It looks to me like you have a 0 to 6 VDC meter in series with an
>> internal
>> 24 volt Zener diode. (It looks like your 25 V data point is a bit
>> off.)
>>
>> Best,
>>
>> -John
>>
>> ================
>>
>>
>>
>>
>>> Hi all,
>>>
>>> I am doing a battery project, installing 28V battery system in a 12V
>>> vehicle with DC-DC converters and charging circuits, etc.  I want to
>>> use some traditional round analog meters to monitor voltage and
>>> current.  So I picked up a nice expanded scale (24V to 30V) panel
>>> meter which can be seen
>>> here:
>>>
>>> http://www.w2hx.com/x/1967LRS2/V-Meter.JPG
>>>
>>> I guess in my naiveté I assumed that this would read volts directly.
>>> Well, no such luck!  Here's what I know about the meter. First, its
>>> resistance reads about 230 ohms. Second, full scale (30V reading)
>>> appears to be about
>>> 360 mA (more precisely, 0.36 A - my ammeter does not have any more
>>> resolution than hundredths of an amp). It is very hard to measure the
>>> low end of the scale, perhaps 0 mA.  To the best I could, I measured
>>> the current that corresponded to each voltage marking on the meter.
>>> Here is the result:  http://www.w2hx.com/x/1967LRS2/MeterRange.JPG
>>> It looks pretty linear.
>>>
>>> First Question is. How do I build a circuit that when attached to my
>>> battery will show the right voltage?
>>> Second Question is. Are there expanded scale meters that will read
>>> direct volts? Or do they all behave in this way?
>>>
>>> Thanks!
>>>
>>> 73 Eugene W2HX
>>>
>>>
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>>
>>
>>
>>
>
>
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-- 
Al Klase - N3FRQ
Jersey City, NJ
http://www.skywaves.ar88.net/