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John, awesome idea. So just build the higher resistance side of the divider with resistors that were all measured from the same range that the low side will be on. That's a great idea. The sum of that would then just plug in to the normal divider formulas to compute the other missing resistor. Once the computed value is known a resistor would be trimmed until the reading on the meter equals the value of the missing resistor. In the end you would have a divider with a ratio that has precision as good as the highest resolution of the meter you used to build it. The meter loading could be added to the formula during computation for the resistor the voltage reading will be taken from. I've made up an excel sheet that I can just plug in my precision reference voltage, the needed test voltage,the value of one resistor and the meter load. It then gives me the missing resistor value. A second part of the same sheet lets me plug in the values I'm getting to see how far off in percentage it'll be.

I think this idea would work really well for what it is. I suspect with a little patience 0.01% could be achieved. Beyond that would require a higher resolution meter and a lot more time. Perhaps with a 6 1/2 digit meter 0.001% could be done. At the very least it's a nice sanity check exercise. Of course any results are only as good as the precision of the reference used. I think for my needs getting even 0.01% would be just fine. It would be better then most of what I would buy brand new in the price range of the used gear.

Thanks,

Jeff

On 12/19/2012 12:21 AM, johncharlesgord wrote:

Jeff,

You can still make the idea work by using multiple resistors of the same value. You can make a 10:1 divider by using, say, 10 each 10K resistors; all can be checked on the same resistance scale. You need to correct for any loading by the meter input, of course, but the input impedance of the 3478A is very high on the 3V and lower DC ranges.

--John Gord

--- In hp_agilent_equipment@... <mailto:hp_agilent_equipment%40yahoogroups.com>, Jeff Machesky <jeff@...> wrote:

Found the hole in my own idea so I thought I would post it just in case
someone attempts it.

The problem is the "uncalibrated" meter is going to have different
amounts of error in different resistance ranges. So measuring the
divider resistors will result in an error percentage not less then the
differential between two ranges used to measure the resistors. That's
not to say a calibrated meter wouldn't work to create the references

but

at that point you may as well just measure the voltages. Ohhh well.
Sorry for the wasted posts.

Jeff

On 12/18/2012 8:14 PM, Jeff Machesky wrote:

Just some food for thought and criticism. Let's say all you can get is
a 2.5 or 5.0 volt precision reference. You could in theory use the
high resolution of your 5 1/2 or 6 1/2 + digit meter to trim resistors
to get precision divider ratios. Even if your meter was not in cal the
ratio would still be accurate. So for each calibration range you could
divide down your reference or source as needed to match your precision
reference. I think this method would get most of us well in the
ballpark. It would require a whole lot of patience however and I'm
sure would become less accurate at the higher voltages. Perhaps even
use a very low offset op-amp to buffer your readings. Once voltage is
accurate you could check resistance ranges with a small amount of
precision resistors and again trim the other half of a voltage divider
until your voltage equates to a value that matches your desired
resistance.

So one uncalibrated meter plus one high precision voltage

reference and

a bit of patience and a good calculator and you can get a fairly close
cal. Well in theory at least.

Any thoughts ?

Jeff

On 12/18/2012 8:54 AM, Chuck Harris wrote:

The 3478A is a nice meter, but bear in mind that it is 300V max.
It is accurate, stable, and works smoothly. It is easily controlled
over HPIB.

In its deficiency corner, is the 300V max,and a non back lit LCD

that

tends to get lost in the shadows.

I have never needed to calibrate mine, but I am pretty sure that you
need to complete the calibration of all steps before you leave a
particular function, say DCV, or ACV, or Ohms....

The calibration procedure takes a measurement of a known standard
on a particular range, and creates an offset that it used when it
calculates the voltage to display. The 30mv standard only calibrates
the offset for the 30mv range. The 3V standard, the 3V range...

You shouldn't have to pay more than about $100 for a working

calibrated

3478A. Tucker has them for sale periodically.

9 times out of 10, a voltage measured by a scope's trace is good

enough.

Did I mention that the 3478A only measures up to 300V?

-Chuck Harris

Jeff Machesky wrote:

So it's about the time of year where I'm allowed to buy one

piece of

test gear and I'm thinking I want a bench meter with higher
resolution. The best I've got so far is the Tek 2465BDM which

has a

20,000 count 0.03% accuracy DMM. It's however a little out of

cal and

I just don't want to spend the cash to get it back in perfect

spec.

It's also a bit crazy to have a scope running just to use the DMM.
This brings me to the 3478A. While I could technically afford

to buy a

new 120,000 count DMM I just can't justify the cost for the

hobby type

work that I do. Plus I love working with the older gear and saving

money.

Couple quick questions about the 3478A:

Can one range be calibrated at a time? This is not highly

clear in the

service manual. Can I calibrate just the 3V range for example?
Are they reliable, or will I have to fix up most of what I

would buy

on say ebay?

The problem I have with the meter on my Tek is that you have to

run the

entire cal for each measurement type, If you start with DC you

have to

go all the way up the range from millivolts up to several hundred
volts. As we all know it's easy to get a precision calibrated low
voltage references such as a DMM check board, it's another thing

to get

500 volts or higher reference at the spec of the meter.

So is this really a meter to buy if your willing to send it

off to be

calibrated or have a calibration transfer standard meter

that's above

the specs of the 3478A? In other words if all you have is just

a few

standards are you wasting your time?

Has anyone had any luck using a precision reference, some

precision

resistors and an op-amp to generate other reference voltages

with any

degree of accuracy? Also has anyone found a good source of say 500
volts 60 Hz AC? The right audio amp and a sig gen can get you

some of

the other odd cal standards. High voltage DC is just so much

easier to

obtain then a nice high voltage sine wave.

Sorry for the random questions, and thanks for any replies. The
54111D is still running great. My thanks again for everyone

who helped

me fix the ROM's.

Jeff



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