multimeter use

[gerryw]

Many many years ago I was told that if you have an unknown voltage at a pair of terminals (don't know if it's ac or dc) with an analogue multimeter, then you should start by "testing" for dc first (on the highest range) then, if no dc voltage is indicated, switch to ac (on the highest range). Continue down the ranges till an accurate reading can be taken. Can anyone confirm this procedure for me, and even better tell me why?

Thanks in anticipation

gerryw

 

[dknguyen]

Can't tell you why the meter is this way, but it makes sense if the meter would be damaged, since AC has zero DC (no damage if measuring DC) and DC can measure DC (duh!). So if you measured DC first you can't go wrong. AC on the other hand might be damaged if you try and measure DC with it, so you use DC (which doesn't get damaged either way) to check that there is zero DC before you try and measure AC.

Starting at the highest range makes sense too of course since that's the range least likely to be damaged and you work your way down to increase sensitivity.

But I've never needed to do the "DC before AC" this since I usually know what's on the line before I measure it.

 

[Nigel Goodwin]

Can't tell you why the meter is this way, but it makes sense if the meter would be damaged, since AC has zero DC (no damage if measuring DC) and DC can measure DC (duh!). So if you measured DC first you can't go wrong. AC on the other hand might be damaged if you try and measure DC with it, so you use DC (which doesn't get damaged either way) to check that there is zero DC before you try and measure AC.

I would disagree with the reasoning, there's no damage in either case.

The reason for checking on DC first is simply that on DC it won't read an AC voltage, but the majority of meters will read DC on AC ranges (but incorrectly). So if you tried AC first, you would still have to try DC afterwards to confirm if it was AC or DC.

 

[Dean Huster]

I agree with Nigel. Consider the fact that 99% or more of times you already know if the point where you'll be measuring will be AC or DC. The trick is to always begin on the highest range and work your way down. And for the best accuracy and precision, select the lowest range that will provide the greatest meter deflection without going overrange.

Dean

 

[eblc1388]

It is possible there is a few DC volts riding on an large AC voltage or the voltage is a distorted AC waveform.

If on DC range the meter is giving 5% of its FSD, then what would normal people do?

They step down the meter measuring range, and damage their meter.

So, I would measure using AC range first.

 

[Dean Huster]

Actually, the entire discussion for me is moot. The FIRST test instrument I grab if I don't know what's there is my oscilloscope. It doesn't lie, it isn't "blind". The problem here is that most beginning folks only own a meter. If a DMM, the discussion is moot as far a meter damage, as any decent meter is pretty robust.

If you have a massive AC voltage riding on a small DC level, you would most likely already know that your circuit would be producing that and you would set your meter accordingly. A huge AC voltage riding on a small DC component is not all that common as opposed to a small AC voltage riding on a large DC voltage (pick any audio amplifier, especially a vacuum tube/valve amplifier).

Yes, going into a high AC low DC situation like that totally blind would warrant AC first. But there's no way you can measure the DC component on an analog meter. Most good analog meters (Simpson 260, Triplett 630) are set up for an "output" mode with a DC blocking capacitor in series for measuring AC voltages in the presence of large DC levels. None are set up to block AC for measuring DC.

Dean

 

[Nigel Goodwin]

Most good analog meters (Simpson 260, Triplett 630) are set up for an "output" mode with a DC blocking capacitor in series for measuring AC voltages in the presence of large DC levels.

You mean an incredibly small number of meter have DC blocking capacitors on their AC ranges?, the vast majority don't.

But as suggested all along, it's really pretty rare that you don't know if you're measuring AC or DC - I can't think of an occasion where it's ever occurred?.

 

[Hero999]

I don't know about the meter you use but mine always reads 0V when measuring AC on the DC setting and DC on the AC setting.

 

[Dean Huster]

No, Nigel. I'm referring to an alternate input jack marked "OUTPUT" that you use on the ACV ranges that inserts a blocking cap in series with the probe. On the normal ACV ranges where you use the V/OHMS input jack, there is a direct connection to the meter with no blocking cap. The Simpson 260, Triplett 630 and other meters (actually, all four of my analog meters, none of which are the same brand) have this "OUTPUT" jack. It's intended for measuring AC voltages in the plate or collector circuits of audio amplifiers, blocking the DC voltage and allowing the intended AC voltage to pass to the meter. One has to be careful, still beginning on the highest range, as the charging of this cap by a high DC voltage can give the meter movement a pretty good WHACK! if you begin on a low range.

Dean

 

[Leftyretro]

It is important that newcomers to electronics are made to understand the very real dangers that they hold in their hands, in the form of a multimeter, analog or digital.

I fear too many training programs and instructors don't put as much effort into this kind of safety training as they do in the normal theory and practical subjects they cover. Maybe some do, I fear too many don't. Possibly it's because some or even many instructors lack real world field experience.

While modern digital DVMs are somewhat safer then the older analog meters, some beep at you if you select volts mode but have the meter leads in the amps mode (however it doesn't prevent damage to the circuit or possibly the meter if you ignore or don't understand why it's beeping at them), a real dangerous situation that is done all to often by too many unaware,unthinking or untrained people.

I have seen the results of several close calls at the refinery I worked at for 27 years. One electrician used a DVM rated at what 1KV max? to measure a 2.3KV switchgear and had the meter explode in his hands, and it was not the first exploding meter I had heard about there over the years.

I can't tell you how many analog meters would get sent to our central shop from field techs that had burned out range resistors, meter movements completely destroyed, etc. We had a fair sized cardboard box filled with such damaged Tripplet multmeters! Dropped from 40ft I can understand, but meters with flame damage

I've also seen many pieces of expensive equipment damaged due to miss use of test equipment and their test leads. I've also seen many wrong diagnosis done while troubleshooting because the tech didn't understand or carefully read the mode and scale that the DVM was in. One complained of bad source power that only read 60vac instead of the required 120vac, turns out that he would engage the frequency mode after selecting AC volt mode, duh...... Too many 'trust' the numbers displayed on a DVM, without applying common and educated thought to if it was a realistic value or that they might not be reading the range correctly. Auto-ranging is a convient feature but I've seen it fool or confuse many a persons too many times.

We do no better service when posting back to newcomers about their meter questions. Never assume they have been trained, told or even read the instructions about the proper use of their meters.

Modern test equipment are wonderful instruments, I love the instrumentation field, but when the instrument is smarter then the person using it there is always a change of damage and injury.

Lefty

 

[RetiredHAL]

I have a Fluke and a Triplett with 600 V DC and 600 AC jacks. I have had this meter for about 15 years. I love the Fluke for its ability to give me accurate readouts. But I will only use the Triplett meter for trusting myself to "touch" something. I am showing my age but the Triplett feels like an "AVO" multimeter in a smaller package.
I always have some idea of what I am trying to test or measure (why else would I have the meter out?)
One aspect that is frequently overlooked are the safety of test leads (cracked at the entrances to probes).
Always use the Ohms Scale first to make sure that the leads and probes can measure a short circuit before you measure voltage.

RetiredHAL

 

[gerryw]

Hi Guys, Looks like I've stirred up a bit of a hornets nest with my question. I agree that more than 99% of the time I know wether I'm measuring ac or dc. It is because this is such a rare event that I've forgotten the "rule", and asked the question. In my work, I've needed an oscilloscope even more rarely than I've needed the answer to this question and it's testament to the good training I had way back then, that the guidance was given in the first place.

In my work, I use my trusty old AVO8 about once a month, a DMM 2 or 3 times a week and a probe tester many times a day. I was presented with the issue of having to determine the supply voltage to a piece of kit that had no documentation. This is what started this ball rolling, and as my memory has defeated me with regard to the "rule", I thought I'd throw the question to a wider audience to get a difinitive answer as none of my colleagues had ever even heard of the rule.

I now know what supply I have for the piece of kit, but wanted to ask the question.

If you don't use it - you lose it (and there are times I wonder if I ever had it).

Regards Gerry W
It would be nice if we could get a consensus on this though.

 

[Dean Huster]

Lefty: "I fear too many training programs and instructors don't put as much effort into this kind of safety training as they do in the normal theory and practical subjects they cover. Maybe some do, I fear too many don't. Possibly it's because some or even many instructors lack real world field experience."

Dean: Electronics hobbyist since 1961, U.S. Navy CTM PMEL (test equipment repair and calibration) six years, Tektronix bench tech six years, electronics instructor (secondary and post-secondary vocational and college) since 1982. If you saw all the liability lawsuits potentially facing vocational instructors, you'd wonder why any of us teach. A parent will sue of Johnny got a splinter or if Johnny got a "D", even if Johnny is a screw-up. Every course I teach always involves safety. Any subject taught where a student has the potential to be hurt is prefaced with safety. They have to pass safety tests and demonstrate safe work practices. They get ejected from the classroom for deliberate safety violations. I think you're assuming the worst about the average instructor.

With "exploding" meters, the problem is that folks are trying to save a buck or two by buying their meters from Radio Shack or some cheap Asian source. These meters are not "category" rated and they drag them into work to test high-energy circuits involving high-voltage, high-current industrial power systems. Unless a meter is rated for at least CAT III or better yet, CAT IV, a technician has no business using it outside the confines of consumer electronics, regardless of how experienced he or she is ("he-she" is another instructor's liability issue these days).

Well, sorry about the move off-topic, gerryw. My contribution to the "consensus" is that in general, a person should know in general what to expect at a certain point in a circuit. If a person only had knowledge of computer repair, connecting a meter to some random point in an x-ray machine would be stupid. A person should be familiar enough with the category of equipment being worked on to have a good idea of what to expect, AC, DC or quantity at a certain point in a circuit and will set the meter function and range accordingly.

Dean

 

[Nigel Goodwin]

he or she is ("he-she" is another instructor's liability issue these days).

Do you get many female students?, certainly in my experience females in electronics are VERY rare indeed - The Sharp Electronics Engineer Of The Year competition used to have a seperate ladies section, so we always had two ladies in the finals. To be more exact, it was always the SAME two ladies, one for Microwave Ovens (good looking woman, about my age), and one for TV/VCR (VERY good looking young woman!!! ) - while neither would probably get through to the final without the seperate womens entry, they were both very knowledgeable and competent engineers.

I agree about the he-she rubbish, I can't stand the 'political stupidity' forced on us these days.

Did you know you're not allowed to fly an English flag on public buildings in England, you can fly Welsh flags, Scottish flags, even Irish flags - but apparently flying an English flag would be 'racist'?

 

[Dean Huster]

Nigel, it would be political and career disaster if we were to treat male and female students differently. Having separate contests would invite so many lawsuits it would make your head spin.

I've had my share of female electronics students. Most were on "worker relocation programs" where factories closed down and the students were retraining for other professions. I did have one secondary student who was a cheerleader in high school and came to us in her junior and senior years, completing the two-year program. She went to work for Tektronix in their Dallas (Texas) Service Center. Another was a little petite thing who was the cutest thing to ever slice through air (well, you, too, were being politically incorrect with all the descriptions, Nigel) who went to work for Anheuseiser-Busch at their Houston (Texas) brewery and then came back to our school to teach the Instrumentation and Control program. I can't say that gender had much effect on the ability to perform in electronics. There was a typical male-to-female ratio of 20:1, so it APPEARED that the guys had more on the ball when in reality, ratio taken in to account, the girls probably were better. There was the one really cute, petite blonde in our evening courses who was often braless and wearing an open armhole, large "aperature" knit sweater that left absolutely nothing to the imagination. You couldn't not notice. And when you read the poorly-written rule books, you couldn't say anything, for the rules said the "underwear should not be exposed outside the clothing". It said nothing about a requirement to wear underwear in the first place!

Teaching has its moments!

Dean

 

[Nigel Goodwin]

Nigel, it would be political and career disaster if we were to treat male and female students differently. Having separate contests would invite so many lawsuits it would make your head spin.

Men and women ARE different, it's stupid (but that's politicians for you) to pretend they are the same. You have seperate men and womens events in pretty well all sporting activities?, although generally the mens events are often open to women as well (such as snooker). But the different physical makeup of men means that women can't compete against men in most events. Using the snooker example again, there's no physical reason why women can't compete with the men, and there are some amazing female players about - but they aren't good enough to ever get through to the televised stages of any event. To make matters even worse, womens snooker has pretty well lost all it's sponsership, and all the top female players have moved to the USA to play pool instead!.

As for treating male and female students differently, don't your students have seperate toilets and showers? - if you weren't treating them differently they would be communial (however, lets not go there!).

BTW, while I'm saying men and women are DIFFERENT, I'm not suggesting either is BETTER - both have advantages and disadvantages. I think it's fairly common knowledge that girls generally do better at school for example - but if you want rocks lifting or breaking, it's best to pick a boy.

 

[Dean Huster]

And there are always exceptions to rules, too. I've had lots of post-secondary ("adult") female students who were going to school to get their "Mrs. degree". You had to watch out for some of those, for they were predators. For example, one summer a female student came up to me wearing a flowing, white cotton dress and she was beet-red from laying out in the sun too long. She said, "My sunburn hurt so bad that I had to wear this dress instead of jeans and I can't even wear any underwear. What me to show you?" One head said "Heck yeah!" but the smarter head prevailed and said, "No, thank you," and put my body on the move to another student's bench.

I find it interesting though that you're comparing physical events for the male/female challenge instead of mental events, which electronics (and chess, Scrabble, cribbage, Canasta, etc.) is. I don't find that gender ever affects performance in this area as long as the person, male or female, has the right attitude and reason for being in school.

Dean

 

[Nigel Goodwin]

I find it interesting though that you're comparing physical events for the male/female challenge instead of mental events, which electronics (and chess, Scrabble, cribbage, Canasta, etc.) is. I don't find that gender ever affects performance in this area as long as the person, male or female, has the right attitude and reason for being in school.

That's why I mentioned snooker as an example, as physical strength isn't required - there's no reason to distinquish between them for 'mental' events. But even at that there's not that many events where women compete evenly with men?.

My point was simply that male and female aren't the same, and rules insisting they are make no sense.

As for physical strength, my daughter is 17 years old, small and slim, yet is probably stronger than 90% of the boys in her school - and could probably out fight even more than that?. It's all a matter of training and skill, up to a certain point - then the male physical attributes surpass the female ones.

 

[Hero999]

My point was simply that male and female aren't the same, and rules insisting they are make no sense.

I agree, I think that's one of the largest problems in our society at the moment, there's too much talk of so called equal opportunities and treating everyone the sam which fails to note that everyone is different.

As for physical strength, my daughter is 17 years old, small and slim, yet is probably stronger than 90% of the boys in her school - and could probably out fight even more than that?. It's all a matter of training and skill, up to a certain point - then the male physical attributes surpass the female ones.

Not only physical training but most of the boys won't reach their full physical strength until they're in their 20s.

 

[RetiredHAL]

In my work, I use my trusty old AVO8 about once a month

Hi gerryw,
Guess what I found in my old "Naval Electrical Pocket Book"
published in 1957 by "Her Majesty's Stationary Office" on the inside cover.
Cheers
RH
Yes it is still a very good meter.!!