Question about resistance range in digital multimeter - Page 1
Forum section: electronics / beginners

author: bshi02
author_rank: Regular Contributor
timestamp: April 12, 2024, 02:32:45 am
content: I saw below youtube video which shows how to examine STR-w6754 switching regulator.It seems that He use UT33C digital multimeter in this youtube video. and He set resistance range to 20k ohm.In this youtube video, He placed red probe to pin number3(GND) of STR-w6754 and placed black probe to other pins.And It display 3.69 to 5.47 in UT33C DMM.As I don't have any DMM which has resistance range of 20k ohm, alternatively I make use of analogue multimeter which has 10 k ohm resistance range.I placed red probe to pin number3(GND) of STR-w6754 and placed black probe to other pins.When I placed black probe to pin1(Drain) and pin4(VCC) My analogue meter's needle won't move at all.and I placed black probe to pin 5(SS/OPL) and pin 7(OCP/BD) , My analogue meter's needle move near between 1 ohm and 2 ohm.When I placed black probe to pin 6(FB) My analogue meter's needle move only one third of total scale(between 30 ohm and 50 ohm).Is it possible to compare resistance value from UT33C DMM(20k ohm range) to analogue meter(10k ohm range) in order to examine the faulty of STR-W6754?

author: IanB
author_rank: Super Contributor
timestamp: April 12, 2024, 02:50:48 am
content: If your analog meter does not move, it either means there is an open circuit (very high resistance), or the battery is missing or dead.If the circuit has a low resistance, the needle will move all the way to the right.You should test your analog meter by touching the probes together and make sure the needle moves right round the dial. Then you should move the ohms adjustment wheel until the needle is on zero (at the extreme end of the scale). Then you should try to measure some known resistors to make sure the meter is measuring properly. You can do this test with each ohms range the meter has.If the needle does not move, make sure the meter has a fresh battery installed.

author: bshi02
author_rank: Regular Contributor
timestamp: April 12, 2024, 03:09:25 am
content: I ALWAYS move the ohms adjustment wheel until the needle is on zero before I measure ohm with analogue meter.And I already tried to measure some known resistors and I confirmed that analogue meter REALLY display correct ohm value for known resistor.and I replaced battery with new one but displaying ohm value is the same with previous measurement.

author: IanB
author_rank: Super Contributor
timestamp: April 12, 2024, 03:45:02 am
content: If you have tested your meter and you know it works, then what it measures is what it measures.If the reading is not what you expect, then most likely something is wrong with the device being measured.

author: Kim Christensen
author_rank: Super Contributor
timestamp: April 12, 2024, 03:52:48 am
content: Be aware than testing semiconductors with an ohm meter will give you different results with different meters. This is due to the different test currents and voltages that the meter outputs.I assume you did these measurements with the STR-w6754 out of circuit just like in the video.A near short (1-2ohms) between pins 5&7, when measured out of circuit, definitely sounds like a bad IC.

author: bshi02
author_rank: Regular Contributor
timestamp: April 12, 2024, 04:10:59 am
content: I measure ohm value of STR-W6754 out of circuit,As I set 10k ohm resistance range in analogue meter 1-2 ohm actually means 10k-20k ohm.and I always placed red probe to pin 3(GND),and I placed black probe to pin1,pin3,pin4,pin5,pin6,pin7(pin2 is NC) each in turn.So resistance between pin 3&5 is 10k-20k ohm,and pin 3&7 is also 10k-20k ohm.But resistance between pin 1&3 is opened,and pin 3&4 is also opened.I wonder whether opened resistance between pin3(GND)&pin1(DRAIN) and pin3(GND)&pin4(VCC) is judged to be bad IC or not.

author: Kim Christensen
author_rank: Super Contributor
timestamp: April 12, 2024, 04:16:06 am
content: Quote from: bshi02 on April 12, 2024, 04:10:59 amI don't think you can make a judgement using this data. The IC could be good or bad. No way to really tell from ohm readings unless you had measured a near dead short between some pins which would indicate an obvious fault.

author: 807
author_rank: Frequent Contributor
timestamp: April 12, 2024, 12:05:15 pm
content: Remember that analogue meters have the voltage polarity reversed when in resistance settings compared to a digital meter.If you were testing a diode for forward continuity for example, you would connect the black lead to the anode, not the red lead as you would when using a digital meter.

author: Gyro
author_rank: Super Contributor
timestamp: April 12, 2024, 12:19:25 pm
content: It's not just analogue meters that have voltage polarity reversed on resistance settings, the UT61E (and presumably some others) have reverse polarity on resistance ranges too but correct on the Diode test setting.You wouldn't be able to use the [Edit: DMM] resistance range for semiconductor junction testing because the open circuit voltage is too low, but it's caught me out a couple of times on other things.

author: BeBuLamar
author_rank: Super Contributor
timestamp: April 12, 2024, 12:30:09 pm
content: Which analog meter do you have? I will check my Simpson 260 and 270 as well as the Radio Shack unit. I don't think the polarity is reversed but the voltage it put out would be different from say a Fluke 87 or something like that. I don't know what the chip does but the voltage may be too low to forward bias a diode or transitor etc..

author: CaptDon
author_rank: Super Contributor
timestamp: April 12, 2024, 01:03:36 pm
content: SOME analog meters may have a negative voltage on the probe referenced to the ground lead, But not all. Also be aware, many DMM meters on the lower scales will pass a current regulated 1ma. (.001 amps) across the circuit being tested and actually measure the resulting voltage. Example, .001 amps applied across 100 ohms = .100 volts. Drop the decimal point and the reading is a correct '100'. Many DMM's use this standard of measurement and readings could be similar across different makes and models. Some DMM's provide enough open circuit voltage in the 'ohms' position to destroy logic level FET's and even some logic devices!! With analog it is a total crap shoot!!! I have an old analog triplett meter that uses both a 1.5 volt battery and a 22.5 volt battery for the different ohms scales. Everyone had their own way of doing things. An exaggerated statement here, Take 10 different DMM's and measure the base to emitter junction of a transistor in the ohms scale and you're are likely to see two similar sets of readings. Try 10 different analog meters and you may get six different readings with only two pairs of meters being similar. Testing solid state circuits with an ohm meter and publishing results is meaningless unless your meter works exactly as Mr. Video's meter did.

author: 807
author_rank: Frequent Contributor
timestamp: April 12, 2024, 01:23:46 pm
content: Quote from: BeBuLamar on April 12, 2024, 12:30:09 pmOf all the many analogue meters I've had since the early 1970's, all of them had the positive voltage on the COM black lead. Of course, that's not to say that every single meter was like that, but whenever any magazines or books had articles about testing semiconductors, they would always tell you to put the black lead on the anode, or put the black lead on the base of an NPN transistor to check for forward conductivity.2 of my old analogue meters have survived. The one I have in front of me now (Hung Chang HC-3030S) uses a 1.5v AA for the X1, X10, & X1k Ohms ranges & 9v PP3 for X10k range. Diode junctions can be tested on the lower 3 ranges.Agree about the different voltages from the meters. The old analogue meter way of checking the junctions was merely an indication to show for shorts/OC junctions. The actual resistance reading was meaningless.

author: BeBuLamar
author_rank: Super Contributor
timestamp: April 12, 2024, 01:35:43 pm
content: I have 3 analog meters. The Simpson 260 series 2 and Simpson 270 series 5 are not reversed. Com is negative. The Radio Shack Micronta 22-220 is reversed. I have several DMM's but since they are all Fluke I assume they are the same so I test only the 87V and it's not reversed either.The voltage from the DMM varies because it's a current source rather than voltage source. The voltage from the Micronta is 1.5V all ranges. Both Simpson voltages are 1.5V for all ranges except for the highest range and the 260-2 has 6.5V and the 270-5 is 9V

author: CaptDon
author_rank: Super Contributor
timestamp: April 12, 2024, 01:37:43 pm
content: Another RADICAL difference in older analog meters is the 10,000 ohms per volt or the real craptastic 1000 ohms per volt meters vs. the V.T.V.M. VOM's or the later FET-VOM's. The active component meters behave far differently measuring solid state devices than the simple 'battery and a meter' el-cheapo's! I had an old Philmore meter with 4 color scales on it. I loved it even though it was of the 10,000 ohms per volt style. Sadly, it was lent to a friend who blew it up. That model was no longer available and the one he bought for me as a replacement was never as good as the one he toasted.

author: Le_Bassiste
author_rank: Frequent Contributor
timestamp: April 12, 2024, 02:06:03 pm
content: Quote from: CaptDon on April 12, 2024, 01:03:36 pmgood point. and that's also true for the PHILIPS PM2519 DMM in OHMS mode.

author: 807
author_rank: Frequent Contributor
timestamp: April 13, 2024, 11:54:30 am
content: Quote from: BeBuLamar on April 12, 2024, 01:35:43 pmHadn't heard of Simpson before. Just Googled them. They go back a few years.It's ironic that there's a mistake in the schematic for the 260. It shows the -ve of the battery going to the positive terminal, which is obviously wrong, as it shows the positive of the battery connected directly to the negative side of the meter movement.Also, I see that the 270 has resersable polarity in resistance settings with reference to the positive terminal.Another analogue meter I forgot about was the AVO 8 I used at work. They have positive voltage on the common terminal.

author: wasedadoc
author_rank: Super Contributor
timestamp: April 13, 2024, 03:39:33 pm
content: An analogue meter needs additional switchery to avoid the polarity swap on resistance ranges which have zero Ohms at the other end of the scale from zero voltage and current.Think about it. A passive analogue meter basically measures current. Current going in to the red terminal moves the needle away from zero. When using the internal battery on resistance ranges, current from the battery still needs to go in to the red terminal. If current is going in to red it must be coming out of black. Hence black more positive than red. If not, there must be means to reverse the current through the coil of the meter movement.

author: BeBuLamar
author_rank: Super Contributor
timestamp: April 13, 2024, 03:59:23 pm
content: Quote from: 807 on April 13, 2024, 11:54:30 amThe Simpson 260 and 270 are still available new. However, the 260 is series 8 now and my series 2 was made sometimes in the 50's. I don't know how old my 270 series 5 is because it's still the current version. I thought Simpson was so well known that everyone know them. When I started out back in the late 70's the Simpson was the go to meter. I couldn't afford one then.

author: IanB
author_rank: Super Contributor
timestamp: April 13, 2024, 04:21:22 pm
content: Quote from: BeBuLamar on April 13, 2024, 03:59:23 pmSimpson would be well known in the USA. In the UK, people would be more familiar with AVO. In fact, up until the 1970's, AVO was synonymous with analog multimeter, just like Hoover was synonymous with vacuum cleaner.

author: BeBuLamar
author_rank: Super Contributor
timestamp: April 13, 2024, 05:24:17 pm
content: I heard of AVO and have never seen any of them in person but I think the model 8 is very nice. I like the meter movement.

author: CaptDon
author_rank: Super Contributor
timestamp: April 13, 2024, 07:34:16 pm
content: Comical reply from wasedadoc, since vacuum tubes prove current flows from the negative cathode to the positive anode. Current Does Not flow from positive to negative.

author: wasedadoc
author_rank: Super Contributor
timestamp: April 13, 2024, 09:37:46 pm
content: Quote from: CaptDon on April 13, 2024, 07:34:16 pmCaptDon needs to check the facts. Electrons flow from the negative cathode to the positive anode but by convention current is deemed to flow in the opposite direction.Put "current flow and electron flow direction" into a search engine.

author: 807
author_rank: Frequent Contributor
timestamp: April 15, 2024, 10:51:18 am
content: This is the manual for the Simpson 260 series 2. The Ohm meter diagram is on page numbers 24 & 25. It looks to me like the battery should have been shown the other way round, with the positive end of the battery connected to the positive terminal. As shown, the positive end of the battery connects straight to the negative terminal of the uA meter. That can't be right.We Know that in resistance ranges the positive terminal of this meter actually has a positive voltage on it, further indicating that the diagram is wrong.Unless, of course, the convention of battery polarity has changed over the decades?

author: 807
author_rank: Frequent Contributor
timestamp: April 15, 2024, 10:59:57 am
content: Quote from: BeBuLamar on April 13, 2024, 03:59:23 pmAs IanB says, Simpson was probably more well known in USA. The workhorse in UK was the AVO meter. Big, heavy & built like a tank. Very distinctive with its 2 large switches. The overload cutout worked well too. I "tried it out" many times

author: BeBuLamar
author_rank: Super Contributor
timestamp: April 15, 2024, 11:18:20 am
content: Quote from: 807 on April 15, 2024, 10:51:18 amYes I checked and the positive probe has positve voltage on it in resistance range.