What type and amplitude signal does dmm use for L, C and R measurements ? - Page 1
Forum section: electronics / testgear

author: pfm
author_rank: Regular Contributor
timestamp: January 07, 2021, 10:00:40 pm
content: I need to measure L and R of a delicate component. It is actually a turntable cartridge which typically contains tiny coils that possibly have extremely low power handling capability. Before I put the leads of a dmm to them I wanted to check what am I subjecting the coils to.When a dmm measures L (inductance) what type of signal does it use ? What frequency ? and most important what amplitude ? I'd hate to roast the coils of a fine cartridge.Same question for R. Does it use DC ? AC ? whatamplitude?All my questions are for basic mass market consumer grade dmm only, nothing fancy like a $$$ desktop test equipment.Thanks.

author: Martin72
author_rank: Super Contributor
timestamp: January 07, 2021, 10:28:40 pm
content: Hi,Oh, it´s a wide range....Some are using a frequency signal only for inductance and for C and R dc, some use for every three ac signal, some got a amplitude of only 2Vpp, some 12Vpp....Most of the cheaper LCR-DMMs using a frequency not higher than 1Khz, the better ones 100Hz/1Khz/10Khz depending on the ranges...There are no specifiactions in general, you have to look at the datasheet if avaible or measure it before using it.For example, I did measure the output of a cheap LCR-DMM (Vici DM4070), it uses ac-signal only for inductance and it use 2 frequencies, see pics.Because of it´s low output-voltage, maybe the ATLAS LCR40 could be a suitable one.

author: pfm
author_rank: Regular Contributor
timestamp: January 07, 2021, 10:58:11 pm
content: Thanks for your reply Martin.Any chance you know how much low is the output voltage - just a rough approx is fine too.I should have mentioned that I would be reluctant to test with more than a few mV into the cartridge. The output of the cartridge is typically 5mv with a headroom of say maybe upto 20-25mv for hot(loud) audio signals on the record. The point being that the coils are probably able to handle very low volts only.So I am quite afraid of sending more than maybe 50mv max to the cartridge. Do you think there might be any dmm that uses such low amplitude for L test ?edit: ok I see its 4Vpp in your pics.

author: Martin72
author_rank: Super Contributor
timestamp: January 07, 2021, 11:24:22 pm
content: Hi,One of the lowest voltage I´ve "seen" was on a Mastech LCR-Tweezer, as it got this as a special mode, for measuring inbetween circuits (afaik 100mV or lower).I must confess, I don´t know much about Moving Coil Systems, as Phono are not my favorite source...What DC-resistance moving coils are usually got ?Edit:Mastech 8911, it uses 100mVrms

author: ledtester
author_rank: Super Contributor
timestamp: January 07, 2021, 11:54:52 pm
content: QuoteMost consumer grade dmm's don't measure L. Do you have a specific meter in mind? (I guess I'm talking about multi-meters. Other's, called LCR meters, do measure inductance, but they're not really a "consumer" item like multi-meters.)As for capacitance, the site lygte-info.dk has reviews of several multi-meters and a lot of them contain a graph of the waveform used in capacitance measurement:https://lygte-info.dk/info/indexDMMReviews%20UK.html

author: DaJMasta
author_rank: Super Contributor
timestamp: January 08, 2021, 12:05:17 am
content: An example from the high end: the Hioki 3532-50 will go up to 5Vpp output level and as low as a 10mV output level. Resistance can be determined by an LCR meter with an AC stimulus (and this is required for inductance, though capacitance can also be done with a switched in DC source), but resistance is usually done in a DMM with a fixed current source rather than voltage, and it varies with the range. This can be as much as a few mA for very low resistance measurements, but for high resistance ranges can be a uA or less for very large resistances on specialized meters.Lower stimulus will work fine, except that you run the risk of running into the natural noise floor of your instrument, so to maximize dynamic range of the measurement, usually the manufacturer will use as high as they think is safe to get away with, so that even sensitive components are unaffected by the measurement (and in some situations this can be heating effects causing drift in the measurement).

author: bob91343
author_rank: Super Contributor
timestamp: January 08, 2021, 12:17:40 am
content: If your DMM is autorange, don't use it. I have successfully measured phono cartridges with an ordinary VOM. On the high ranges there is very little current and on the low range the cartridge has enough resistance to limit the current in most cases. Best to measure with a quick touch so as not to allow time for coil heating.I have never burned out a cartridge. I was in the stereo repair business for many years.

author: pfm
author_rank: Regular Contributor
timestamp: January 08, 2021, 02:04:29 am
content: Quote from: Martin72 on January 07, 2021, 11:24:22 pmThat seems interesting, I will look into it.Typical inductance is 500-700 milliH , and typical resistance is 600-1000 ohms.Quote from: bob91343 on January 08, 2021, 12:17:40 amIts autoranging but I can preselect and lock a range before measuring. Would that be ok or still not advisable ?

author: bob91343
author_rank: Super Contributor
timestamp: January 08, 2021, 02:31:42 am
content: Yes manual range selection is better. Otherwise during the automatic range selection you don't know what the transient conditions would be. Start with the highest range and go lower until you see a reading good enough, rather than go to the lowest range which puts the most current through the unknown.

author: pfm
author_rank: Regular Contributor
timestamp: January 08, 2021, 06:40:16 pm
content: Quote from: Martin72 on January 07, 2021, 11:24:22 pmI ordered the 8911. I dont think there would be anything that could have lower test voltage than .1V.Besides its versatility I liked how detailed their manual was on the test frequencies, voltage, and test method -http://www.mastech-group.com/download_s.php?id=234Thank you!

author: 25 CPS
author_rank: Frequent Contributor
timestamp: January 08, 2021, 07:03:32 pm
content: Quote from: pfm on January 08, 2021, 06:40:16 pmIt might be worth doing a test run with taking measurements on a junk cartridge you don't care about if you have one on hand or can get one for dirt cheap just to get the measurement technique down and to be sure the LCR tester you go with isn't going to damage the cartridge that you care about.When you measure the cartridge, see if you can do it in a quiet room and listen carefully.I suspect there's a good chance you might hear it throw off the 1 kHz test signal as audio. Backfeeding the coils with the signal from the LCR meter will cause motor effect in the cartridge due to the presence of the magnet and put the cantilever and stylus assembly into motion and move air that you might hear as sound while you're taking measurements.

author: pfm
author_rank: Regular Contributor
timestamp: January 08, 2021, 07:16:12 pm
content: yes thats what I plan on doing. I have a small train of junk cartridges- ok thats on overexaggeration.No way I am putting it to my Stanton 881S first.I usually remove the stylus before messing around with the cart, but you might be right, it could very well emit the sound.

author: Martin72
author_rank: Super Contributor
timestamp: January 08, 2021, 10:16:56 pm
content: Hi pfm,You know this list ?https://aaanalog.de/media/downloads/TA-Daten.pdf

author: 2N3055
author_rank: Super Contributor
timestamp: January 08, 2021, 10:42:28 pm
content: Just a note, my LCR Pro1 (tweezers) can select amplitude for test..5 test frequencies – 100Hz, 120Hz, 1kHz, 10kHz, 100kHz3 test voltages – 0.2Vrms, 0.5Vrms, 1.0VrmsI have old Genrad 1657 and UNI-T 612 (that is working surprisingly good, to be honest..).Since I got that tweezer, they get very little use...

author: pfm
author_rank: Regular Contributor
timestamp: January 08, 2021, 11:26:45 pm
content: Quote from: Martin72 on January 08, 2021, 10:16:56 pmI think I may have seen it long back. It doesnt have L/R specs for a lot of common/popular cartridges.For testing old cartridges you do need to measure, as sometimes they drift or develop imbalance between coils.Thanks for sharing it though.