Mark Sims | 15 Sep 23:41 2014
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Finally got around to modifying my Fluke 845ab with LED's

BTW,  you can get LEDs that are designed specifically for flash applications.  They can handle high peak
currents,  have decent color spectra,  and/or fast rise/fall times.   There are also LED driver chips made
for flash applications. 		 	   		  
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MARVIN | 12 Sep 18:14 2014
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Re: HP 3456A AC cal

The 3456a is ~20x less accurate than the 3458a in VAC. That's as good as it gets. 

Here's a comparison of all the top DVM as provided in the 1993 HP equipment catalog, 1993 was unique in that it
covered many generations of DVM/DMM many of which were soon to be dropped from sale. 

Click or go to the link: 

http://www.imagebam.com/image/f4f16c350823319 

The 3456a shines in DCV. 

A used 34401a typically sells for $300-700, a 3457a ~ $200-400, 3458a ~ $2,000-5000 but a 3456a is ~$70, it
offers very good bang for buck in DCV. 

----- Original Message -----

From: "Richard Moore" <richiem5683 <at> gmail.com> 
To: volt-nuts <at> febo.com 
Sent: Thursday, September 11, 2014 7:30:26 PM 
Subject: [volt-nuts] HP 3456A AC cal 

I'm trying to get my head around the AC cal procedure for the HP 3456A 
-- there is no full-scale cal, just some extremely finicky tweaks for 
the bottom ends of three of the four ranges. I really don't understand 
how this is supposed to work, especially since it is very accurate on 
all DC ranges (up to 500V -- above that some heating of the divider R 
seems to occur and it loses accuracy as the voltage increases). But on 
AC, the tweaks don't result in anything like an accurate level reading 
compare to my 3458A -- at 10VAC it is off by hundreds of ppm. Anybody 
shed any light on this? Is there a way to get full scale cal points in 
(Continue reading)

Richard Moore | 12 Sep 01:30 2014
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HP 3456A AC cal

I'm trying to get my head around the AC cal procedure for the HP 3456A 
-- there is no full-scale cal, just some extremely finicky tweaks for 
the bottom ends of three of the four ranges. I really don't understand 
how this is supposed to work, especially since it is very accurate on 
all DC ranges (up to 500V -- above that some heating of the divider R 
seems to occur and it loses accuracy as the voltage increases). But on 
AC, the tweaks don't result in anything like an accurate level reading 
compare to my 3458A -- at 10VAC it is off by hundreds of ppm. Anybody 
shed any light on this? Is there a way to get full scale cal points in 
AC mode? Any mods to fix this -- lovely meter otherwise; I much prefer 
it to the 34401A I used to have, except for the overall bigness....
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Mark Sims | 11 Sep 07:56 2014
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Finally got around to modifying my Fluke 845ab with LED's

I don't think you will find any white LEDs that are based upon UV LEDs.  UV LEDs are horribly inefficient.  All
white LEDs that I know of have a "royal blue" LED as their pump.  The phosphor is typically a YAG based
phosphor.  
Alsp, the warmer white the LED is,  the less efficient it is.  Same goes for high CRI LEDs.  They need to enhance
the output towards the red end of the spectrum...  either using a less efficient phosphor configuration 
and/or filtering out the shorter wavelengths.  In some high-CRI leds you can see red/colored particles in
the dome/phosphor that filter down the shorter wavelengths.
BTW,  those big royal blue LEDs are insanely bright and your eye is not even very sensitive/responsive to
them.  You can damage your eyes and not know it.  Plus there is an effect known as the "blue light hazard."  Be
particularly careful with "remote phosphor" lighting LEDs that have damaged/missing phosphor panels. 
10+ watts of royal blue goodness can be nasty stuff...  but then, I have designed LED lighting systems that
put out 500,000+ lumens of white light into a rather small area...  I'm rather careful around bright LEDs.
		 	   		  
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Mark Sims | 10 Sep 06:31 2014
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Finally got around to modifying my Fluke 845ab with LED's

It should work fine.  The phototransistor does have some sensitivity in the blue region.  It uses a fast
comparator circuit (with a little hysteresis).  One input is the average/filtered phototransistor
output,  the other is the raw signal.  It can detect the PWM signal from a small flashlight (a couple of
lumens) in a room lit by 10,000+ lumens of overhead LED lighting.  It can also detect the light output
fluctuations caused by DC-DC converter switching.
---------------------

What would your PWM sensor circuit do if all of the AC was in the blue, and the
white light appeared as a DC bias on the phototransistor? 		 	   		  
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Mark Sims | 10 Sep 03:56 2014
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Finally got around to modifying my Fluke 845ab with LED's

No web page,  but a little description here.  It has changed quite a bit from the original description.  Now
uses TAOS color sensors.     Suppors Melexis IR thermometer chips.  Has 16 bit A/Ds.  Processor is an ATMEGA
1284.  The control program is based on Lady Heather.  Besides LEDs it can also charge/discharge/analyze
batteries,  measure power converter/supply efficiency, etc
http://budgetlightforum.com/node/12296
CIrcuit board:
http://img69.imageshack.us/img69/3782/luxor.png
Sphere in action:
http://img515.imageshack.us/img515/4288/spherew.jpg
BTW,  the PWM sensor is a silicon phototransistor with not much blue light sensitivity...

---------------------

> Do you have a web page on the LED analyzer/integrating sphere?

 		 	   		  
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Mark Sims | 10 Sep 00:33 2014
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Finally got around to modifying my Fluke 845ab with LED's

I would probably use green or yellow LEDs,  but the white ones should not be a problem. 
 I built an LED analyzer/integrating sphere and one of the features is a circuit that optically measures the
LED driver PWM frequency.  It can also detect the minute variance in LED intensity from an LED driven by a 950
kHz boost converter.  It also had no problems with a white LED driven at 4 MHz from a signal generator.  You see
the long persistence phosphors mainly in large lighting LEDs and not in small indicator LEDs. 		 	   		  
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Dallas Smith | 9 Sep 17:55 2014

Finally got around to modifying my Fluke 845ab with LED's

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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Pete Lancashire | 8 Sep 23:43 2014

OT: For the R, L & C Nuts

A table of the known (to me at least) variations of the General Radio 1409
Capacitor Standards

http://petelancashire.com/files/GR1409.pdf

-pete
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Randy Evans | 5 Sep 17:02 2014
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732A and 3458A stability

I ran a stability test on my  "new" 732A and 3458A units.  I had both
connected together (using copper wire leads) and I randomly ran
measurements of the 3458A reading of the 732A 10 VDC output.  I did an ACAL
DCV before each reading and used NPLC of 100 and 8 digit readings , and
recorded the room temp, 3458A internal temp , and the MIN/MAX/MEAN/STDDEV
of the 3458A readings, and the thermistor value of the 732A.

The room temp varied from 22.1 to 27.9 C over the several days of
 measurements.  The 3458A internal temp was, on average, 13.7 C higher than
the room temp.  The end result was a temperature sensitivity of 1.15uV per
degree C of the 3458A internal temp, or .115ppm/C.  I am assuming the 732A
is relatively stable over the changing temperature range (the thermistor
varied from 3666.6 to 3670.7 ohms) and attribute most of the variance to
the 3458A.

The absolute value of the readings are around 10.000058 VDC but neither
unit is calibrated.  I assume they can be calibrated but I wanted to make
sure they are stable first.

Do these numbers look reasonable and do they indicate any issues with the
units?

Thanks,

Randy Evans
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Charles Steinmetz | 4 Sep 15:16 2014

More grounding & guarding references

Two more grounding & guarding references that should be available on the web:

HP Application Note 123 -- floating measurements and guarding

Analog Devices Application Note 347 -- shielding & guarding

Best regards,

Charles

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Gmane