| Products > Test Equipment |
| UT61E drift and recalibration |
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| Nuno_pt:
Another Chinese Vref mod see here ( http://www.crystalradio.cn/thread-398876-1-1.html ). Would be nice if someone made the translation. |
| markman:
Translated: http://translate.google.com/translate?sl=auto&tl=en&js=n&prev=_t&hl=en&ie=UTF-8&u=http%3A%2F%2Fwww.crystalradio.cn%2Fthread-398876-1-1.html I think the LT1790 might still be a better match with specs maybe slightly worse but current consumption also less. So... If one substitutes all the key resistors and adds a reference, then this might become a stable meter over time?? Maybe while we're add it, we add some circuitry so it can do low-voltage current measurements? Is there anything significantly better for $60 + $25 in resistors + $10 in reference parts? Does anyone have a schematic for this meter? |
| Nuno_pt:
Hi markman, The schematic that I've is in attach, is the one that is on this forum, but be careful that this DMM has suffer many revisions. R16 is 7.55k will on schematic is 9k09, the other is R19 is not there and so one. About the low current measurement is an problem in all multimeters, some more than others, and is problem from the shunt resistor, so there is nothing that we can do about. Only attenuate by Dave's µCurrent. |
| stephenlm324:
I have finished fiddling my temp compensation. The meter now reads correct from 18 to 33 deg C briefly displaying +1 count. I suspect it is the main chip that is temp sensitive, certainly the on chip Vref is off so it's a good bet that the rest of the chip is off. Anyone considering adding the external Vref might consider using LT1790BCS6-1.25 which has a typical temp coefficient of 12ppm they are £2.70 GBP from Farnell, half what I paid for the top spec and I still had to add the compensation. I have seen no sign of any long term drift. All of the heating and cooling I have done would accelerate this but it always goes back once the temperature settles. Any reports of long term instability should be put down to vibration and temp changes as these are having a much bigger effect. |
| Nuno_pt:
Stephenlm384, Can you post some pictures and the schematic of your mod, so the mod will be better understand by all, (1 picture worth 1000 words). Glad that you got it stable at ~1 count, that is pretty impressive, now we've a stable DMM from 18 to 33°C. Now that we've the stability problem solve, maybe we can address the calibration on the capacitance side, if that will be worthy of the time, what you think? I've just found this from another Chinese mod, they are calibrating the resistance side, with trimmers. See link ( http://bbs1.38hot.net/thread-6945-1-1.html ) |
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