[R-390] Another ballast question

[Cecil Acuff]

I don't think the problem is one of concern for damage to the tube but more 
the fact that the results is inaccurate because unless you get the tube 
filaments full up the resistance measurement won't be correct...it will be 
very much low.

A better way to measure the resistance would be to put a milliamp meter in 
series and measure the current at the rated voltage of the tube and do the 
math.

Cecil....
----- Original Message ----- 
From: "Barry" <n4buq at aol.com>
To: <R-390 at mailman.qth.net>; "Roy Morgan" <roy.morgan at nist.gov>
Sent: Wednesday, January 04, 2006 9:55 AM
Subject: Re: [R-390] Another ballast question


> Yes, I realize how an ohmmeter works (at least the simple, old-style ones)
> and I realize the tube won't light when connecting the VOM this way.  From
> your original reply, it appeared you were warning me not to test the 
> tube's
> filament this way because it might damage the filament.  I assume that as
> long as it is a 12V tube, it won't damage the filament, right (again, it
> would depend on the scale selected for an old-style VOM)?
>
> Mine is an auto-ranging DMM and uses a 9V battery as a source so I'm not
> sure how much voltage is applied when testing like this, but I assume it
> would not be more than 9VDC.
>
> Thanks!
>
> Barry - N4BUQ
>
> ----- Original Message ----- 
> From: "Roy Morgan" <roy.morgan at nist.gov>
> To: "Barry" <n4buq at aol.com>; <R-390 at mailman.qth.net>
> Sent: Wednesday, January 04, 2006 9:47 AM
> Subject: Re: [R-390] Another ballast question
>
>
>> At 09:36 AM 1/4/2006, you wrote:
>> >... My ohmmeter uses a 9V supply so there shouldn't be a problem
>> >doing this, right?  Of course, 9VDC isn't the same as 9V RMS so some
>> >conversion must be made to ensure 9VDC isn't too much for 12V RMS, but
>> >this shouldn't be an overvoltage situation, should it?
>>
>> Barry,
>>
>> Ohmmeter circuits are quite simple usually. If this is an old style VOM,
>> such as the Simpson 360, it works like this:
>>
>> The test leads, the meter, a range resistor and the battery are all in
>> series.  With the test leads shorted, the meter reads full scale,
>> calibrated at zero ohms. If a resistor the same value as the range
> resistor
>> is at the test leads, the thing reads half scale.  So the current through
>> the test leads depends on the range resistor - set by the ohms scale
>> selected.  The open circuit voltage might well be the 9 volt battery
>> voltage, but will drop when the leads have a resistor connected to
>> them.  The current available (max with the leads shorted) depends on the
>> range resistor selected.  The range resistors are chosen depending on the
>> sensitivity of the meter movement. In a Simpson 260, I think this is some
>> 50 microamperes. The tubes you test will never light up.
>>
>> VTVM's and digital DMM's work on similar principles.
>>
>> Roy
>>
>> - Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
>> 7130 Panorama Drive, Derwood MD 20855
>> Home: 301-330-8828 Cell 301-928-7794
>> Work: Voice: 301-975-3254,  Fax: 301-948-6213
>> roy.morgan at nist.gov --
>>
>>
>
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