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| AC RMS DMM tests |
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| mawyatt:
Somewhere/how we've damaged the device. Was going to take some measurement late yesterday and getting significant errors when using the Squarewave. A scope trace showed the squarewave hosed up, so first thought was blown output P & NMOS. These were replaced, same result, then replaced CD4060, same result. Did more troubleshooting and found the ADR 4550 gave proper 5 volts when in DC mode, but output dropped during AC mode. Figured P and/or NMOS was still bad, and drawing too much current and pulling the ref down, so replaced both again with same result. Then figured the ADR4550 couldn't supply the current of the CD4060 (~10ma at 4MHz clock), so added a bypass resistor from U1 8 V regulator feeding the ref to the ref output to help supply additional current. This helped but didn't achieve a precise 5 volt ref output. We didn't have another ADR4550 handy so used a ADR435 (same pinout) which has a higher output capability, and problem solved!! We must have momentary applied an over voltage to one of the exposed Banana jacks and blew the MOS drive device in the ADR4550 output, it could supply an almost no-load condition for DC measurements, but no additional current under AC mode. If we ever do a PCB respin will add some small series protection resistors and diodes. We updated the schematic and everything is back to working altho we've lost the couple years burn-in of the ADR4550 reference :-\ A couple measurements with a new (few months) KS34465A revels everything is back in order!! VDC 5.0007415V 250Hz Squarewave VRMS (SD) 2.5000447V Anyway, updates in case anyone is considering one of these. Best, |
| mawyatt:
Updated measurements 4/15/24, setting for 250Hz Squarewave. @ 42% RH 26.3C Model Ref DCV SW DCV Avg SW ACV (RMS) KS34465A Grey (new) 5.0008727 2.5004551 2.499903 KS34465A Grey (~1.5y) 5.0008817 2.5004559 2.500101 DMM6500 5.000880 2.500445 2.499920 ~25y old Fluke 87 5.00 2.500 2.506 KS U1233A 4.999 2.499 2.509 Few hours later @ 41% RH 25.6C Only 1/2 to 1hr Warmup KS34465A Tan 5.0008538 2.500445 2.500140 HP34401A 5.00080 --- 2.49970 AG34401A 5.00080 --- 2.50004 SDM3065X 5.00081 2.50039 2.49800 The 34401As Square Wave DCV Avg bounced around some, needed some averaging so not recorded. Edit: Added UT210E 4/17 4.99 2.49 2.45 Additional results added: Fluke 77 5.00 2.497 2.706 (2.4363 RMS eqv)* DHO814 5.0282 2.5178 2.5086 SDS814X 5.0135 2.5036 2.4926 * Average Responded AC which (pi/2)/rt(2) times RMS, so RMS is 0.9003163*reading. Anyway, seems the new ADR435 reference (see post above) is settling in, and all the meters agreeing within reason. Also the questionable "Square Wave" seems to produce acceptable results with all our DMMs, handheld and lab bench types, analog or digital RMS, so will continue to rely on this for AC RMS sanity checks :-+ Of course, as always YMMV. Best, |
| Majorassburn:
Those are terrific results from a wide variety of DMM's. You really have that nailed as proof of the DC square wave as viable and accurate for AC range DMM checking. Of course, you are doing it much more professionally than I have done with my eBay device. I used a CD4047 as the basis for my quick checker only because I had dozens of them laying around. Here's a link if you're interested. https://www.ebay.com/itm/285781922404 Please keep updating your test results because you have some of the most popular DMM's at your lab and it's nice to know that the AC functions are displaying within their specs, albeit at very low voltages. One of my future experiments will be to boost the DC square wave voltage to see if your great results hold on higher AC ranges. |
| alm:
I think it would be interesting to test on lower end DMMs, like average responding meters, or meters with just 50-400 Hz bandwidth. These are all pretty high end meters. |
| mawyatt:
Lowest end DMM we have is UT210E, added see above. Best, |
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