[volt-nuts] Finally got around to modifying my Fluke 845ab with LED's

[Dallas Smith]

Hi Chuck,


Its only 84Hz, It had the offset
problem long before I made this mod. The new meter at work many decades ago
also had this problem.


I pick white so the full spectrum would
be covered by the Csd's. Checked waveforms with scope, has sharp rising and
falling edges. This mod may not be as elegant as a Linear Devices copper mod,
but at lest I will not have to mess with the shielded front end.


The
offset may be due to the bias current in the front end reacting with the different
polarities.


Dallas


 
 
> Date: Tue, 9 Sep 2014 15:48:01 -0400
> From: cfharris at erols.com
> To: volt-nuts at febo.com
> Subject: Re: [volt-nuts] Finally got around to modifying my Fluke 845ab with LED's
> 
> One thing that might be hosing things up a bit is white led's don't
> switch quickly.  They are really UV LED's with a bit of fluorescent
> material between the LED and the world.  The fluorescent material will
> also luminesce for hours after the LED is turned off.  Perhaps because
> the led never shuts off completely, it is biasing the meter up a few
> tens of microvolts?
> 
> -Chuck Harris
> 
> Dallas Smith wrote:
> > Finally got around to modify my Fluke 845ab with LED 's for the chopper circuit. Used
> > the 17 volt windings for LED's (Mouser 941-C513AMSNCW0Y0511 Warm White Round LED)
> > instead of the 130 volt, move red wire on transformer pin 9 to pin 7.This winding is
> > 180 degrees out of phase, so I reversed the steering diodes (CR106 & CR107) I left in
> > to help make sure the phase was correct for the LED's when connecting. Change R154 to
> > 6K to set the brightness, selected for good operation of the zero control. Then
> > install jumper  to replace C119. Also changed the filter integration response caps
> > C111 to .022uF and C116 to 47uF, this stabilized the jitter to a manageable mode of
> > operation. Meter now works as well or better when the original neon's worked.
> >
> >   As the meter originally had this problem, why is the offset reading different when
> > polarity is reversed at the meter input? About 10uV's.
> >
> > Lamp Blocks.
> >
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