[R-390] Another ballast question

[John Kolb]

Modern DMM's will generally measure resistance by passing a constant
current through the unknown R and measuring the voltage drop. Thus with
a 1 mA current, a 0.250 V measurement would = 250 ohms.  The max
voltage presented is usually limited also to prevent turning on semi*******
junctions.  The diode test position allows a higher voltage so that the
junction V drop can be measured.  Thus the 9V battery is to danger,
even to low voltage tubes (to get back towards on topic.  :)

John

At 07:55 AM 1/4/2006, Barry wrote:
>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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