Poll on Accurate Resistance vs Capacitance Measuring Circuit - Page 1
Forum section: electronics / beginners

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 14, 2011, 06:37:29 pm
content: I have a little bit issue or confusion on my Uni-T DMM, which one should i trust more? resistant measurement? or capacitance measurement? so this poll can give me some clue. hope everybody vote. Thanx.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: April 14, 2011, 06:52:41 pm
content: Well the " measuring circuit " confused me a bit ..Would you describe my own new " Resistor decade box " as " measuring circuit " ?Second question:Are we talking about reference parts ?

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 14, 2011, 07:12:54 pm
content: your decade box is not measuring circuit to my definition. i mean the circuit inside a DMM or LC meter. and DMM with R and C measurement. anything that read out R and C value.

author: Zad
author_rank: Super Contributor
timestamp: April 14, 2011, 07:17:11 pm
content: I would tend to trust the resistance ranges more, check your user manual for accuracy. Doing proper capacitance testing is a real pain in the posterior, not least because the value changes with the voltage across it.

author: sacherjj
author_rank: Frequent Contributor
timestamp: April 14, 2011, 08:20:52 pm
content: It just seems like a 4 wire resistance measurement is much easier to be accurate than a capacitance measurement. (Two wires providing current for voltage drop. Two wires reading voltage across resistor without that current flowing in them to reduce the error of voltage drop across the probe wires.)With capacitance, you have a harder time with the probes acting partially as capacitors or inductors.

author: saturation
author_rank: Super Contributor
timestamp: April 14, 2011, 08:53:49 pm
content: Here's an interesting variation on a DIY capacitance meter I posted on eevblog using a DSO and a function generator. In this design the main difference is the formula as the end point is 1/2 the starting Voltage:http://www.edn.com/article/511051-Circuit_measures_capacitance_or_inductance.php

author: Unknown
author_rank: Unknown
timestamp: April 14, 2011, 09:30:41 pm
content: Just looking at typical specs for a similarly priced resistance meter (DMM) and capacitance meter should provide the answer. You could also look at how expensive the top-off-the-range resistance/capacitance meters perform, my guess is that the resistance meters will be much better. Most good multimeters with a capacitance range have much worse specs for capacitance compared to resistance, although you could argue thatTheFluke UK calibration servicecan measure resistances with an uncertainty of 2ppm, but capacitance only with an uncertainty of 200ppm.Resistance measurement is a much simpler process, it's just DC voltage and/or current, which are much easier to accurately determine than AC voltages. It also helps that resistors are much closer to ideal than capacitors. For example, resistors do exhibit some voltage coefficient at high voltages, but some ceramic capacitors (like Z5U) can easily lose half of their capacitance if some DC voltage is applied. This makes getting an accurate value much harder. It's also rare to see capacitors with a tolerance better than a few percent, and even the specialty precision ones are only 0.1%. 1% resistors are dirt cheap, and 0.1% are also easily available, and they go much lower once you get into the specialty precision stuff.Precision capacitors also tend to have a very low value, which makes parasitic capacitances much more significant. I don't think you'll find many 1uF 0.1% capacitors.

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 16, 2011, 06:55:47 am
content: @zad: my DMM indicate the same accuracy 1.5%@sacherjj: yeah yeah! 4 wires, how come i forgot that?@saturation: the equations in your link seems familiar to me. how do we accurately monitor (measure) that Vi/Vo is exactly a ratio of some exact value like sqr(3), sqr(2) etc?@alm: thanx for detailed explanation. i have more confidence now.hmm i can see capacitance measurement can price as much as a very good digital oscilloscope...http://cgi.ebay.com.my/HP-Agilent-4286A-RF-LCR-Meter-1-MHz-1-GHz-w-Test-Head-/110451551903?pt=LH_DefaultDomain_0&hash=item19b76cee9f

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: April 16, 2011, 12:47:18 pm
content: I can not understand , the point of your inquiry .But I can speak about my DMM's , and the story is sort .The Fluke 28II freshest and top of the cream , cannot measure small ceramic capacitors.Measuring anything else like resistance or volts , its no issue.The 20 years old medium priced DMM , it can measure small ceramics and medium capacitors .But in anything above 1Mohm , it looks that measures totally wrong and completely out of specs.Now I have a good reason to never trash my oldie medium priced DMM , because it does what the Fluke does not.And now the conclusion, you collect and keep 2-3 instruments that its one has an strong point .

author: Unknown
author_rank: Unknown
timestamp: April 16, 2011, 02:22:56 pm
content: Yes, the capacitance range on DMMs sucks, which is why people buy dedicated LCR meters.

author: tekfan
author_rank: Frequent Contributor
timestamp: April 16, 2011, 04:41:38 pm
content: If you really want good resistance measurment, 4 wire is the only way. Even this technique has some flaws. Two wires provide a constant current across the resistor (the easy part). It doesn't matter how long they are because the circuit inside the meter is always supplying the same current. Then the other two wires sense the voltage on the resistor. The same circuit is used for this as for measuring DC volts. So if the input impedance of the voltmeter isn't very high >1 Gohm you can get seriously inaccurate readings in the higher ranges.Same goes for LCR meters. 4 wire technique is a must in LCR meters.

author: saturation
author_rank: Super Contributor
timestamp: April 16, 2011, 07:33:57 pm
content: I'm not sure that handheld LCR meters are any more accurate than a good DMM with capacitance support from brands like Agilent or Fluke. I've personally tested many and am completing a performance test of the 1252a and 87V meters, and they are on par. Alas, I've never used those super expensive ones to act as comparison. But dedicated handheld LCR meters aren't any better than 1%, which at least the 87V or the 1252a are too. Where LCR meters excel is the added calculations done like quality factor, ESR etc., and particularly easier uH measurements.As tekfan says, calculating capacitance to any better resolution requires far more advance techniques, and such meters suddenly jump from $150 DMM types, to $21,000+ types.In the 20+ years I've owned Fluke 80 series DMM, they changed the method of calculating capacitance, the early one were just RC time constants using old board timer and constant current; its described in the service manuals of the generation 1 and 2 Flukes. Model 3 and up use a method now common today even in lesser brand DMM and LCR meters, but where they differ between brand names and no-names is the quality of the implementation. But none I've seen have gotten >1% at best for accuracy. if done right.

author: Unknown
author_rank: Unknown
timestamp: April 16, 2011, 08:07:22 pm
content: Quote from: saturation on April 16, 2011, 07:33:57 pmThe Agilent U1701A (the only handheld 'LCR' meter they're still selling) has a max. resolution of 0.1pF, and an accuracy of 0.5% for the 100nF-1000uF ranges. The Agilent U1253B has a max. resoultion of 1pF and an accuracy of 1% from 10nF to 10mF, so the dedicated meter is slightly better, especially for low values. This ignores inductance measurement, which usually sucks on those DMMs that offer it.Quote from: saturation on April 16, 2011, 07:33:57 pmBetter resolution, definitely, you'll probably require a special fixture to even get a usable 1pF resolution. Getting a better accuracy for higher values (=less worry about parasitics) for much less than $10k should be possible. Accurate DC voltages, resistors and frequency measurement/generation are all easy and cheap. But if you're buying new and need an off-the-shelf solution, you'll have to spend the big bucks.

author: saturation
author_rank: Super Contributor
timestamp: April 16, 2011, 08:29:09 pm
content: Thanks alm! 0.5%, for $158. Its not on all the ranges, but its better than the DMM's which are 1% throughout.

author: Wim_L
author_rank: Regular Contributor
timestamp: April 16, 2011, 10:54:48 pm
content: Quote from: saturation on April 16, 2011, 07:33:57 pmOf course. Those instruments are only for rough checks. If you really need precision, then you need precision at the intended operating frequency, not just at the one or two preselected measurement frequencies of the handhelds. Which takes you to the very expensive impedance analysers that allow for freely selecting the measurement frequency, or that sweep their entire frequency range and give you a nice bode plot with real and complex (or magnitude/phase) components as a function of frequency.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: April 17, 2011, 12:49:31 pm
content: Quote from: Wim_L on April 16, 2011, 10:54:48 pmThe only time that I ever needed precision in capacitors ,was when I needed to build an stereo preamplifier .And so I think that the percentage of the technicians, who will have an such need ,so to get something as an very special tool , will be very low.

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 17, 2011, 01:37:17 pm
content: Kiri! can you build this?http://cgi.ebay.com.my/IET-Labs-General-Radio-1413-Precision-Decade-Capacitor-/380210305115?pt=LH_DefaultDomain_0&hash=item58864cd85ba precision capacitor decade box! look at the price!i think they put good weight of gold inside.

author: sacherjj
author_rank: Frequent Contributor
timestamp: April 17, 2011, 04:39:20 pm
content: Quote from: Mechatrommer on April 17, 2011, 01:37:17 pm60 caps in there. Must be $50/cap precision... Wow.

author: Unknown
author_rank: Unknown
timestamp: April 17, 2011, 06:25:21 pm
content: Haven't looked at this specific seller, but lots of 'test equipment vendors' on eBay price their equipment at about 20x market price, in the hope that once in a blue moon someone comes along that's crazy/desperate enough to buy it. For example, if you have a test procedure that uses a certain piece of equipment, acquiring it for a lot of money may still be cheaper than changing the procedure and retraining staff. Looking at the price of sold items is a much better indication than looking at asking prices.But 0.05% basic accuracy is quite impressive, so I don't expect these to be $0.01 Z5U capacitors either.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: April 17, 2011, 11:11:58 pm
content: Quote from: Mechatrommer on April 17, 2011, 01:37:17 pmwell I was ready to make something similar ,but at the end i decide to not go for it.The reason is simple , I have no use for such an instrument.I would need one, only if I would had take seriously the idea of starting my own business ,about multimeter's repairs & calibration.

author: sacherjj
author_rank: Frequent Contributor
timestamp: April 18, 2011, 04:40:18 am
content: Quote from: Kiriakos-GR on April 17, 2011, 11:11:58 pmI found a substitution box with the most common values to be good enough for just circuit hacking. And apparently about $5k cheaper too.

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 18, 2011, 05:00:33 am
content: Quote from: sacherjj on April 18, 2011, 04:40:18 amif you can provide the link. it will be helpfull