I think you're half right! Yes, the divider string results in a voltage
that is a fixed percentage of the voltage at the top of the string, e.g, 5V
versus 5000 V. However, you'd need a high impedance voltmeter to measure
that voltage. More typically, a milliameter panel meter is used, and these
require a series resistor to give., e.g., 1mA full scale. That series
resistor is basically in parallel with the bottom resistor in the divider
string, which changes the voltage across the bottom resistor. The impedance
of the panel meter is usually low enough it can be ignored.
73,
Jim w8zr
Sent from my iPhone
On Jan 12, 2017, at 2:54 PM, "gudguyham@aol.com" <gudguyham@aol.com> wrote:
Many folks think that most amps use the current to read voltage in the HV
string. Often times there is another resistor of lower value on the bottom
of the string and the current passing through that resistor creates a
voltage drop that the meter actually reads. Sometimes you can use a small
pot in place of that last resistor and you can dial in your HV right on the
money. The arraignment in the SB-220 comes to mind.
-----Original Message-----
From: MU 4CX250B <4cx250b@miamioh.edu>
To: Paul Christensen <w9ac@arrl.net>
Cc: Amps group <amps@contesting.com>
Sent: Thu, Jan 12, 2017 4:24 pm
Subject: Re: [Amps] Alpha Seventy HV Meter Readings
To compute the value of the bottom resistor in the divider string,
don't you need to compute the parallel resistance of the bottom
resistor and the resistance in series with the 1mA panel meter?
Presumably, the internal resistance of the meter is low enough to
ignore.
73,
Jim w8zr
Sent from my iPhone
> On Jan 12, 2017, at 2:09 PM, Paul Christensen <w9ac@arrl.net> wrote:
>
> I've recently acquired an air-cooled version of the Alpha Seventy
amplifier.
> I notice that HV reads about 500V low when comparing it to a Fluke DMM
with
> Fluke HV probe. The supply is the same as my vapor-cooled version. The
> vapor amp's HV meter tracks in line with the Fluke.
>
> My first thought was that the meter was out of calibration. It's a common
1
> mA DC movement. A NOS replacement meter also reads low by 500V. That
> leaves the five 1-meg HV metering divider resistors but they all measure
> within 1% tolerance as they should -- metal film resistors rarely drift
out
> of spec.
>
> So, my plan is to change the value of only the 1-meg resistor on the cold
> end of the 5 megohm string. By computation, I get 500K instead of 1-meg
for
> a total string value of 4.5-meg instead of 5.0-meg. Seem like a reasonable
> approach to y'all?
>
> Paul, W9AC
>
>
>
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|