I replaced two sets of meters and they have all been around 2400 ohms and 1
ma full scale. at full scale its 2.3 to 2.4 volts - I have seen no issue
whatever with the zeners. One could use 3.9 or 4.3 volt zeners if they were
concerned about a soft knee.
I also talked to the meter manufacturer about whether the surge could in
fact demagnetize the moving magnet - they also confirm that the meters are
2300 - 2400 ohms.
K7HP
----- Original Message -----
From: "Dr. Gerald N. Johnson" <geraldj@weather.net>
To: <tentec@contesting.com>
Sent: Wednesday, December 15, 2010 7:13 PM
Subject: Re: [TenTec] Centurion 422
>I can't tell from the downloadable manual, but all my life 1 ma meters
> have been more like 50 ohms if quality meters.
>
> 3.3 volt zeners may induce some modification of the meter reading
> because they tend to have a soft knee, not going into strong conduction
> at low current.
>
> 73, Jerry, K0CQ
>
> On 12/15/2010 7:27 PM, pfizenmayer wrote:
>> Nope - the meters drop 2.38 volts at 1 ma - 2380 ohms and are fed from
>> fairly large series resistors so the 3.3 volt zeners are fine - in
>> addition
>> the forward diode tends to damp the meters some as they are pretty
>> undamped.
>>
>> Hank K7HP
>>
>>> Most meter coils drop less than 0.1 volt so 3.3 volt zener's aren't much
>>> protection. Better to use 1N4001 single junction silicon diodes, two in
>>> parallel opposite polarity. They won't leak any current until the drop
>>> gets up to about .55 volt.
>>>
>>> 73, Jerry, K0CQ
>>>>
>>>> 73 de hank K7HP
>>>>
>>>>
>>>>
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