What are the advantages of a separate terminal for µA mA on a multimeter ? - Page 1
Forum section: electronics / beginners

author: tigrou
author_rank: Regular Contributor
timestamp: October 26, 2019, 01:13:08 am
content: Some multimeters have 3 inputs terminals, some (usually the more expansive ones) have 4.AFAIK the main difference is that the ones with 3 inputs have the µA mA shared with volt terminal.Thispagesays a non shared terminal is safer (but don't really explain why).

author: rsjsouza
author_rank: Super Contributor
timestamp: October 26, 2019, 01:32:40 am
content: When you are rotating the selector switch, any mistake that takes you to µA or mA will short the inputs through the built-in fuse.If you are connected to a sensitive circuit, you will damage it.If you are connected to higher power circuit, you will have a blown fuse.

author: wraper
author_rank: Unknown
timestamp: October 26, 2019, 01:42:21 am
content: Quote from: rsjsouza on October 26, 2019, 01:32:40 amLikely you will have blown meter and probably even blown traces in selector switch, making it irreparable. Fuse is not guaranteed to save the meter. Also implementations in cheap meters often are simply illusion of security.

author: malagas_on_fire
author_rank: Frequent Contributor
timestamp: October 26, 2019, 11:02:46 am
content: Quote from: tigrou on October 26, 2019, 01:13:08 amChances are that with shared uA/mA with V is that if you selected previously uA/mA, do other stuff, forget about it and probe for volts then the scenarios told by rjsouza would occur , shorting circuit and / or the fuse, since the probes are attached to the same functions, but you forgot to change the selector switch for example.With dedicated jack, you are forced to change the probe from V to uA/mA thus reducing the chances of blowing something and makes sense to have high impedance inputs with fusible inputs.

author: Gyro
author_rank: Super Contributor
timestamp: October 26, 2019, 12:10:08 pm
content: A shared mA and voltage jack guarantees that a meter doesn't have a decent CAT safety rating, no matter what you might find printed on the front.Even in low energy circuits, it will soon outweigh its purchase cost in replacement fuses, possible damage and frustration.

author: tigrou
author_rank: Regular Contributor
timestamp: October 26, 2019, 12:16:23 pm
content: Thanks all for replies.Now I understand why V DC / V AC positions on a rotary switch are usually as close as possible from OFF position on most multimeters: to prevent you accidentally going trough uA/mA while switching it off.If I understand correctly, to blow a 4 terminals multimeter while measuring volts, you need to do two mistakes :To leave one probe plug into uA/mA AND to select uA/mA measurement.

author: Gyro
author_rank: Super Contributor
timestamp: October 26, 2019, 12:28:40 pm
content: Quote from: tigrou on October 26, 2019, 12:16:23 pmNot quite - many (most) meters have the internal current shunts and fuses permanently connected to the uA/mA and A terminals. So only one mistake is needed. However it is a lot more noticeable than accidentally turning the switch through a current range. It is always good practice to quickly check which terminals the leads are plugged into before making a measurement if you have been using the current ranges (which is not often in practice).

author: rsjsouza
author_rank: Super Contributor
timestamp: October 26, 2019, 06:35:55 pm
content: The segregated inputs also allow for a bit more sofisticated implementation, where the meter is able to detect with 100% accuracy when a probe is plugged to the current inputs. If the rotary selector is put at any range other than current, the meter is able to warn the user.That beeper is usually reserved for the more upscale models, though. The cheaper models I know that feature this are Dave's BM235, Fluke 115 (and up), Keysight U123x series and the Klein MM600/700.