Why would a multimeter not have AC current ranges? - Page 1
Forum section: electronics / projects

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: June 10, 2017, 09:14:46 am
content: I have a "cheap" multimeter that I bought a long time ago when I was new to electronics. It's been a nice tool - it's no fluke 87, but I've opened it up and I know that it should be able to take a beating, and it's good enough to tell me "here be 5V" or "here be no 5V" - which is all I needed. I've been planning on getting a second-hand kiethley 2000 as a first big-boy bench meter, but thats another story.However, I noticed yesterday when testing a powersupply that it did not have a AC current measurement range. This suprised me - It can do AC voltage (no RMS, but I can live with that for now) but no AC current. I found this very strange, since it can do DC current. All current topologies I know of tend to just use a shunt resistor to measure current - a current meter being a voltmeter + 1 resistor. As the resistors are clearly there (since it does DC current), and it can do AC voltage, is there any "sensible" reason why they wouldn't implement an AC current mode? (asin, is there a lot more complexity to it than I'm suggesting)?

author: bktemp
author_rank: Super Contributor
timestamp: June 10, 2017, 09:57:40 am
content: The smallest AC voltage range is probably 20V or so?The DC->AC converter is a simple peak detector using a single diode.Rectifying the low level AC current signal requires an additional amplifier or an ideal rectifier using an opamp.But that is an additional ic meaning additional costs. Therefore the AC range is often omitted on the lowest cost multimeters.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 10, 2017, 10:00:16 am
content: Some of the very cheap meters only have higher voltage AC ranges, as this makes the rectifier slightly simpler. Though this is not a big deal today anymore. It was a problem with the old analog ones - there no AC current was quite common.Another point might be safety. If a meter does not have an AC current range, chances are lower that users would use it in current ranges at mains voltage. In theory and by the law the meter should still be safe - but less likely to be tested with a deadly outcome. So it might be a good thing not to have the AC current range on a not so safe meter.

author: oldway
author_rank: Super Contributor
timestamp: June 10, 2017, 11:18:55 am
content: For testing a power supply without PFC, a not true rms ac current meter is almost useless as current of such power supply is highly distorted....It has high peak value and only two measuring values have some interest: peak and RMS current.For safety reasons, I highly recomend to measure mains ac current with a true RMS current clamp, not with a multimeter.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: June 10, 2017, 11:50:47 am
content: Quote from: oldway on June 10, 2017, 11:18:55 amUnfortunatly those are not something I have access to. I was also not interested in measuring the power - I just wanted to have an idea of how much current it was drawing to make sure I understood the system correctly. I was interested in it drawing 1A instead of 100mA.Quote from: bktemp on June 10, 2017, 09:57:40 amAh, that explains a lot. I didn't realize that it made sense untill I looked at the meter and saw that this is indeed the case! Thanks!

author: TimFox
author_rank: Super Contributor
timestamp: June 10, 2017, 03:13:06 pm
content: On a normal DMM, the "AC current" (redundant) range is done using a low-value resistor and measuring the AC voltage across it with the AC voltmeter function. The separate connection to that resistor must be fused, since an accidental connection of the low resistance across a normal voltage would dissipate too much power. These components add cost to what is supposed to be a cheap meter, which you don't need to determine "here be 5V" or "here be no 5V" of your original requirement.For the time being, you can measure AC current by using an external low-value power resistor, but it is very dangerous to connect such a thing to the mains AC line. A better option might be to buy a cheap current transformer (some are available on eBay) where you pass an insulated single wire through the hole and measure the secondary voltage across the specified resistor (typically 100 ohms).

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: June 10, 2017, 04:02:27 pm
content: Quote from: TimFox on June 10, 2017, 03:13:06 pmHow is this any different from DC current? (lets not get into the is-it-redundant-or-not-debate). The device can measure 10A in DC hence it has a resistor that can dissipate that much power. It already has a HRC fuse on the current range.

author: P90
author_rank: Frequent Contributor
timestamp: June 10, 2017, 04:11:29 pm
content: The cheap meters don't have AC current ranges simply for cost savings... extra components and circuitry needed to be added to the basic multimeter COB,

author: CopperCone
author_rank: Super Contributor
timestamp: June 10, 2017, 05:54:09 pm
content: there needs to be a smiley on eevblog that looks like a money crazed face with dollar signs for eyes, like this $_$a picture says a million words after all.

author: DBecker
author_rank: Frequent Contributor
timestamp: June 10, 2017, 07:13:19 pm
content: I'll leap into the issue: "AC current" is not redundant.Being pedantic risks being precisely wrong.While "AC" might have started as an acronym for "Alternating Current", it has long since taken on the meaning "cycling polarity". Similarly DC actually means "unchanging polarity".Essentially every use of today follows these meanings, and it's really awkward to use the archaic meaning. You might as well try to convince the world that they are using the word "truck" wrong.

author: Dave
author_rank: Super Contributor
timestamp: June 11, 2017, 02:43:49 pm
content: Quote from: TheUnnamedNewbie on June 10, 2017, 04:02:27 pmWe are talking about cheap multimeters here. You're lucky if you get a ceramic one instead of a glass fuse in there, but you will never see an HRC in a cheapo.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: June 11, 2017, 03:10:01 pm
content: Quote from: Dave on June 11, 2017, 02:43:49 pmI'm talking aboutmymeter which does seem to have HRC fuses.

author: CopperCone
author_rank: Super Contributor
timestamp: June 11, 2017, 04:35:12 pm
content: if you watch the latest video by mikeselectricstuff about the microwave you will see why HRC fuses are a good idea. So you can actually replace the fuse when it blows.