Products > Test Equipment
UT61E drift and recalibration
Nuno_pt:
cuban8,
Nice catch, do that or only replace the 2K pot with an more stable one, but one with 5% tolerance and 10ppm/C° will cost about the same as the DMM.
So increasing / adding / replacing R16 for another more precise and lowering / replacing VR1, should give more precise and stable readings in Vdc.
cuban8:
Actually, tolerance is basically irrelevant for both VR1 and R16 - you're trimming this resistance anyway. Tempco is more important, but since VR1 and R16 form a voltage divider with R15, this resistor also needs to be changed for a better one if you really want to improve things.
Nuno_pt:
Ok, since we are one this, the schematic that I've list VR1 = 1K and R 15 / 16 = 9K09, we already know that VR1 is 2K, so the best is to take out the resistances and measure them and then replace both of them, for new values if you change the VR1 value, I think that Vdc will've better stability and accuracy with this mod.
Now I'm looking at the capacitance, for what I can see on the ES51922 data, R1k (pin 49) and R9k (pin 48) require an precise 1k and 9k resistors, they have 909k (R17) and 9k09 (R19) on the same schematic, and R18 (1k) is this in series / parallel with the other two, for better accuracy should this three be remove and place some precise ones with low % and low ppm/C°?
If you change that would it be better to change also VR3 (10k pot), R33 (100k) and R36 (200k x 4)?
Does VR3 adjust all ranges of capacitance or just one?
EDIT:For adding Capacitance below.
From what I can see on the ES51922 data, page 11 parag. 1.5:
C1 is adjust by itself for 22.000nF range (this should be an precise cap, % and ppm/C°), for more precise readings and stability.
VR3 adjust only 220.00nF = That is C2
10k adjust 2.2000nF = That is C3 (On my schematic there is one 100k resistor between C2 and C3, connected to pin 9 = BUFFH)
The ranges from 22.000µF to 220.00mF are all adjust by the ratio of the pins R9k/R1k, (R17,18,19 network) the more precise this network more precise the ridings on this ranges.
Is my reading correct on this?
stephenlm324:
Although fitting the LT1790 improved the temp stability it did not cure the problem. I have stuck a thermocouple on the PCB under the main chip and tested with a 2V reference over 18 to 28 degrees C. Without the LT1790 the display count changes by 10, doing a good impression of a thermometer. After the LT1790 was fitted this dropped to a count of 6 over the same temp change.
To get rid of the last of the temp drift I have resorted to a thermistor (Vishay NTCLG100E2104JB Farnell 116-4823) in series with 1Mohm across R16 (9.09k on the schematic but measured at 7.55k). This gives a count of 2 drift at the moment, I need to fiddle more, dropping the 1M to 947k should do the trick. This compensation could be used without the LT1790 but the series resistor would need to be 780k ohm (calculated) to get more compensation, probably cheaper as well. This will of course mess up the calibration but no problem if the horrible 2k pot is still in.
Temp stability of the resistors in the potential divider should not be an issue, so long as they are all the same coefficient they will go up and down together with temp, and the voltage should stay the same.
Big problem is that 2k multiturn, the data sheet for this type of pot states 1% error with vibration which is 20ohm. I replaced with a 50ohm and a couple of resistors in series (there wasn't one standard value so used two). Vibration susceptibility now 0.5ohm and much easier to adjust. To figure out what resistor/s are required desolder the pot, measure the resistance with another meter and subtract 25ohm so that it's in the middle of the travel on 50ohm replacement.
The good news is that the resistance range does not drift significantly with temp trimming R34 will not help.
Thanks to Nuno_pt for spotting the mistake, now corrected.
Nuno_pt:
Setephenlm384,
Thanks for the follow up on the update about the temp stability, seem that we are starting to get somewhere even without external Vref, the big must in here is always replace the 2k trimpot for one more stable / lower value, to get the precision and stability.
Like you point out if we can get it stable at 0 / 1 count at the 18° to 28°C temp range that will prove that this meter could sold from factory without being more expensive, just by trimming better the components.
Could you leave the meter on in that temp range for about 24h or more and see what is the drift after that time with external Vref and without only with the resistors arrangement, one thing that I read about the references it's that you've to burn it for about 300 hours or more to make them age, then redo the calibration.
As for the resistance I don't see nothing to improve the ranges that uses R34 as an divider.
I'm looking now at the capacitance side, like I describe in the early post, after looking an little better, R19 is not present, only R17 (9k09) and R18 (1k).
On the 1M and the thermistor in series shouldn't be R16 instead of R1.
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