wrong Input impedance of latest  dmms? - Page 1
Forum section: electronics / testgear

author: 001
author_rank: Super Contributor
timestamp: September 07, 2018, 11:46:47 am
content: Somebody tellshttps://forum.banggood.com/forum-topic-400344.htmlWTF

author: MK14
author_rank: Super Contributor
timestamp: September 07, 2018, 12:10:41 pm
content: In general, ICs when powered off, present a diode drop to ground and the power rails. This is often a side effect of how ICs are made (unintended stray PN junctions all over the place, depends on technology used to make the IC as well) and intentionally, usually for ESD protection.Since most DMMs use a few volts in resistance checking mode, these diode drops (0.6/0.7 volts), plus resistors all over the place. Are probably creating that "strange" meter reading of around 1.1 million ohms (the picture is too blurry for me to be sure).I.e. It is probably a perfectly normal effect for switched off (unbiased) electronics.

author: threephase
author_rank: Regular Contributor
timestamp: September 07, 2018, 12:23:26 pm
content: Wouldn't you want the meter to be completely open circuit when it is switched off? i.e all electronic components should be out of circuit.I guess the concern would be what would happen if you were measuring a voltage and turned the instrument off prior to disconnecting the leads, what current would be drawn? Probably bad practice, but it does happen.They allude that with 20V applied, 10mA flowed. If it is linear, then it shouldn't be too much.Kind regards

author: HKJ
author_rank: Super Contributor
timestamp: September 07, 2018, 03:10:37 pm
content: Quote from: 001 on September 07, 2018, 11:46:47 amI sort of doubt that. The first meter I found was the AN8009 and it do not have it:2kOhm sounds like a PTC and some resistance on the board.

author: HKJ
author_rank: Super Contributor
timestamp: September 07, 2018, 03:34:31 pm
content: I was too fast, now I found the AN8008 and it has switched the PTC in in off mode.AN8002:AN8008:

author: Mechatrommer
author_rank: Super Contributor
timestamp: September 07, 2018, 03:38:21 pm
content: Quote from: 001 on September 07, 2018, 11:46:47 amthe lesson is...1) dont leave the meter unattended, or connected to some potential while turned off.2) otherwise... buy a not $20 meter.

author: 001
author_rank: Super Contributor
timestamp: September 07, 2018, 05:06:52 pm
content: Thanx a lot!It is horrible news

author: HKJ
author_rank: Super Contributor
timestamp: September 07, 2018, 05:29:16 pm
content: Quote from: 001 on September 07, 2018, 05:06:52 pmWhy? The normal recommendation for multimeters is to select the correct range before connecting it?The PTC (and DMM) will probably survive mains voltage. It is not worse than having the meter in ohms range.Of course I would prefer a disconnected input or at least 10Mohm, but I do not see it as a big problem. I doubt I would ever encounter it, I always turns the meter on before connecting it.

author: David Hess
author_rank: Super Contributor
timestamp: September 07, 2018, 06:05:32 pm
content: The meter's design is bad or the meter is broken. That should never happen.

author: 001
author_rank: Super Contributor
timestamp: September 07, 2018, 07:22:58 pm
content: Again,Few DMM have 1M input impedanceIs it problem?

author: MK14
author_rank: Super Contributor
timestamp: September 07, 2018, 08:07:27 pm
content: Quote from: 001 on September 07, 2018, 07:22:58 pmSuch a problem, could well be minor, compared to the (probable) lack of safety features on many very cheap multimeters. Which could lead to them exploding and injuring or killing the operator, if accidentally connected to a powerful enough, high voltage supply.I've seen teardown video(s) of that DMM, and it seems to have these crazily tiny fuse(s). Which I suspect, may not have anything like the safety of proper full sized HRC fuses.tl;drYou are worrying about one specific fault of these multimeters. Apart from the (probable) lack of real safety capabilities, they probably have many issues and limitations, compared to proper, expensive quality branded multimeters.But I haven't got any such DMM (AN8008), so could be wrong and or biased. (partial Fluke fan-boyism etc).

author: joeqsmith
author_rank: Super Contributor
timestamp: September 08, 2018, 12:10:39 am
content: Old newshttps://youtu.be/TSGLA9heboY?t=1312

author: David Hess
author_rank: Super Contributor
timestamp: September 08, 2018, 06:14:54 am
content: Quote from: 001 on September 07, 2018, 07:22:58 pmThe input impedance does not matter for low impedance measurements like batteries, power supplies, and the AC line. Even variable impedance with different ranges can be handled if documented like on old analog meters where the impedance was typically 20,000 ohms per volt.Where you can get into trouble however is medium and high impedance circuits and high voltage probes where changing the meter range alters the reading. Most or all automatic ranging handheld digital multimeters are not suitable for use with high voltage probes but for obvious reasons, this is not widely advertised or documented.