Q on useage of Fluke 8845A

Robert Baer wrote in news:F9QVh.6482$ snipped-for-privacy@newsread3.news.pas.earthlink.net:

Use separate meters for I and E. OR,use a fixed 100 ohm R and use the relay to alternately measure E across the R and the Zener. Calculate the I.

Use an AM503/P6302 current probe/scope in place of the Fluke for current measurement.It works down to DC.Then you can use a very low R wire in place of the 100ohms of the DMM.

15 volts across the 100ohms of the 100ua scale will be FAR over the scale. Thus not good. Will the Fluke autorange? That may be another problem.

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Jim Yanik
jyanik
at
kua.net

1) I cannot afford *two* Fluke 8845As at $1K each (i am Socially Insecure - on Uncle Sugar fixed "income"). 2) I cannot afford a 0.01% 100 ohm resistor to get a similar number of significantly reliable digits. 3) AM503/P6302???? You have to be joking - i could get slide rule accuracy with a VOM! 4) Who said that there would be 15V to begin with? a) The zener drops some voltage. b) If the meter *was* on !any! current scale, the voltage drop would be 100mV full scale. c) If the meter is on (any) voltage scale, the current terminal would appear to be either open or "high" non-linear resistance (do not have meter yet - still accumulating $ for it). d) It seems that if one programs it, then it does not autorange (if it does that at all). Not clear on that aspect of the meter; i have seen no information concerning autoranging - only autozero which can be turned off.

Robert Baer wrote in news:IgSVh.3208$ snipped-for-privacy@newsread2.news.pas.earthlink.net:

You never mentioned what resolution you were seeking for your measurement. (accuracy and resolution are different things)

Are you designing some sort of precision V reference? temperature will have an effect on your zener.

Current limiter driven by *15v* is what you said;since if the circuit opens(by the relay),even briefly,the limiter will try to raise the current by raising the VOLTAGE to it's max,that is where you get the 15V from.

(I learned that the hard way,got a 500V shock from a current source dialed down to zero current,an old meter calibrator)

--
Jim Yanik
jyanik
at
kua.net

Crude description of circuit: +15V, then current limiter to zener to be tested, then 100 ohm for current sample; top of zener to voltmeter. Currents to be forced: 10uA to 560uA in log steps for a total of 8 currents. Relay switches DVM: meter voltage line to either top of zener ("V" mode) or open (top of zener switched to dummy 10Meg = "I" mode); current line to either open (bottom of zener switched to dummy 100 ohm = "V" mode) or bottom of zener to meter current line. So far,it seems the best program order is (fixed width font): relays | V | V | I | V | V

-------+---+---+---+---+--- DVM | V | I | I | I | V

So the absolute maximum voltage at the DVM current input(s) would happen when one half of the relay is (still) in the voltage mode, the other half switched over to current mode, and the DVM in the voltage mode. That would be 15V minus about 1.7V current limiter drop minus zener voltage (seems will be in the region of 8V), or about 7V worst case. Since the contacts are being moved by the same armature, it would seem that both arms would move about the same time and one would not remain in the NC position while the other arm was moved to the NO position. So i think that i have taken reasonable precautions, but $1000 out of my pocket is a major expense and i would like to protect that investment (hence the posting of the Q).

I have not taken precise measurements or used a scope, but it seems these relays actuate in about 5mSec and bounce for about 1mSec. The relays i use are Axicom V23105-A5003-A201 with 12V coil driven from +15V and 270 ohm series resistor (that series combo has a diode snubber across it). I do have some NEC MR62-12UKSRY relays that i could use instead (used a DIP socket).

I think you need 3 relays, one to go across the current sense path (perhaps with a small series resistor to match meter loading), one to switch the leads to the current path for measure measurement, and one to switch the meter to the zener voltage measurement. Sequencing becomes obvious and relay operating speed is almost a non-issue this way. I did a lot of things like this 20 to 30 years ago.

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 JosephKK
 Gegen dummheit kampfen die Gotter Selbst, vergebens.  
  --Schiller

I will draw that up and consider it. Thanks.