Strange dmm reading - Page 1
Forum section: electronics / beginners

author: Bingobelle1975
author_rank: Newbie
timestamp: October 21, 2024, 09:12:15 pm
content: Hi allI have a very strange reading that i do not understand.The dmm i have is a fluke 87v serie III in like new condition. I also have 3 vintage yo fong 1100A from about 1985 plus minus a couple of years. All 3 is NOS and where still in theres package wrapped in the plastic bags so never used intill a couple of days ago .The components im messing around with is some kind of old breadboard education kit, properly late 70's or early 80's. The components are plugged to the breadboard with mini banana plugs.So, if i for example measure dc or ac current over a resistor or several resistors in series all four dmm show same resultat and also it is correct according to ohms law. For example a 100 ohm resistor and 21 volt dc or 16.8 volt ac they show the same miliamps more or less.The ac 16.8 volt is directly from the transformer included in the education kit and the 21 volt dc is taken from the same transformer but using a rectifier and a capacitor on 2200 micro farad in parallel after the rectifier.Now to the strange part. I did a timer circuit to light up a lightbulb according to the included text book and it works perfect.It is build up with two tic 126 tyristors and a 2n 4871 ujt and of course some resistors and capacitors.This circuit draw some idle current since one of the tic 126 is always open even if the light is of. I wanted to see how much so i connected a dmm in series with the output from the transformer and then to the rectifier W06.All three yo fong dmm measure about 51 miliamps in idle and 92 miliamps when bulb is on.The fluke shows about 94 miliamps in idle and about 150 miliamps when bulb is lit.What is happening? All four dmm shows samer readings when measure ampere going throw one or several resistors, regardless if i use ac directly from the transformer or dc via the rectifier. But as soon as i try to measure the timer circuit ( the ac current before the rectifier) the fluke shows the double amount of idle current .Should i trust the fluke or the others! One expensive fluke or three vintage Taiwan dmm?It blows my mind.Same if i put the fluke in serie with the others. The fo fong dmm shows about 51 mA and fluke about 94 mA. But on resistors on series or a single resistor regardless of ac or the rectifier dc they show same and also correct when checking to ohms law.Is there a explanation?ThanksChristian.

author: TimFox
author_rank: Super Contributor
timestamp: October 21, 2024, 09:28:42 pm
content: Does your Fluke measure the rms value of the AC current, including the DC component?The cheap meters probably measure the mean absolute value without the DC component.

author: indeterminate
author_rank: Regular Contributor
timestamp: October 21, 2024, 09:31:12 pm
content: It is to do with the shape of the waveform.with a tyristors in circuit you no longer have a sinewave and the fluke will be displaying the RMS value of the current while the yo fong will be displaying sumthing like the average currenta bit about thyristorshttps://www.electronics-tutorials.ws/power/thyristor.html

author: Bingobelle1975
author_rank: Newbie
timestamp: October 21, 2024, 09:37:01 pm
content: HiNot sure to be honest. I have been thinking that the circuit somehow fools the cheap dmm but the current im measure is before the dc components. It is directly on the output from the transformer so ac. And also they show same readings when only using resistors regardless of it is ac directly from transformer or dc taken after the rectifier. The same rectifier circuit that powers the delay time circuit.

author: The Soulman
author_rank: Super Contributor
timestamp: October 21, 2024, 09:47:34 pm
content: Well, current is the same trough out the circuit.So there can be some dc current as well, different multi meters handle that in different ways.Do you have some sort of current probe for your oscilloscope?

author: Bingobelle1975
author_rank: Newbie
timestamp: October 21, 2024, 09:58:10 pm
content: Thanks!Read the link you gave me but have to admitt that that is way over my understanding for the moment. Im just a electrician that just started to try learning electronic to be able to do some repairs on vintage receivers in the future.Anyway, is the short answer that the tyristors do some stuff with the wave form of the current even before the rectified dc part of the circuit?If i had been measure "inside" the circuit in the dc part i would have assumed this but i did not think that it would affect the ac before the rectifier.So if i understand correct, the tyristors alter also the ac current?Thanks!

author: TimFox
author_rank: Super Contributor
timestamp: October 21, 2024, 10:03:31 pm
content: Note that the actual RMS value for a waveform with a DC component is quite different from the RMS value of only the AC component.Many RMS voltmeters measure the voltage after an input capacitor that removes DC (and attenuates frequencies below maybe 10 Hz). Most DMMs measure the voltage across a “shunt” resistor to determine the current, and may have that input capacitor after the shunt.“Average-responding” meters respond to the mean absolute value, but are calibrated for the RMS value of a sine wave with that mean. They will show a much different value for a square wave (for example) with the same RMS value.

author: Bingobelle1975
author_rank: Newbie
timestamp: October 21, 2024, 10:06:11 pm
content: I think i only have standard probes for my scope. You can choose 1x or 10x with a small switch ( dampening i guess if measure high voltage) but did not even know that there is such thing as current probes. So much to learn.

author: indeterminate
author_rank: Regular Contributor
timestamp: October 21, 2024, 10:19:21 pm
content: Quote from: Bingobelle1975 on October 21, 2024, 09:58:10 pmYesThink of current like water flowing through a pipeThe Transformer needs to supply the current before the rectifier for it to be present in the circuit after the rectifier

author: Bingobelle1975
author_rank: Newbie
timestamp: October 21, 2024, 10:32:57 pm
content: Thanks all for yours explaining. I do not get it all right now but now i know that my fluke is not broken. I was a bit worried since it where 3 against 1:)Now i know the current wave form will be altered even in the ac part and that less designed dmm will have a harder time to read the values correct .I guess i should trust my fluke more than the yo fong ones.I also guess that "inside" the dc part of the circuit if taking a reading there it will be less different measures between the fluke and the others since that is dc and not altered ac wave forms.Will try that tomorrow.

author: TimFox
author_rank: Super Contributor
timestamp: October 21, 2024, 10:50:03 pm
content: For complicated waveforms, messier than sine or square waves, a simple AC measurement (either RMS or mean absolute value) is rarely useful, unless you need to know the heat in a resistor, or properly described random noise.An oscilloscope is more useful in complicated cases.

author: iMo
author_rank: Super Contributor
timestamp: October 21, 2024, 11:18:05 pm
content: The OP has the ammeters wired between the transformer and the rectifier, where there is the filtering 2200uF capacitor after the rectifier. So the meters show some AC and the thyristors mess with DC only.PS: based on his schematics the thyristors toggles with DC supply because there is the capacitor 2.2uF between their anodes so the negative pulses on the respective anode switches the thyristor off (based on the UJT oscillator pulses into the one gate).

author: Bingobelle1975
author_rank: Newbie
timestamp: October 22, 2024, 03:40:42 pm
content: Would like to ask one more question.The 0,1 mF capacitor between ujt and the thyristor to the right, what good is that for? I bypassed that capacitor and the circuit works just as good with or without it.Also, how can the dc current puls from ujt go through the capacitor and Ignite the thyristor? I thought capacitor stopped dc current, in other words how can the right positive side of the capacitor transfer that to the left side of the capacitor to Ignite thyristor?

author: cybermaus
author_rank: Frequent Contributor
timestamp: October 22, 2024, 03:52:24 pm
content: Not going to way in the question asked, but I just want to comment on your marvelous vintage setup in the photoVintage scopeVintage DMMand absolute toppervintage 100-in-1 high quality socketed experimenting set.I had to make due with one of those cardboard boxes with springs.

author: Bingobelle1975
author_rank: Newbie
timestamp: October 22, 2024, 05:16:30 pm
content: Plz enjoy attached picture. I think the calculator is the oldest of the stuff. It is from 1971.

author: cybermaus
author_rank: Frequent Contributor
timestamp: October 22, 2024, 06:26:00 pm
content: Exceptional. I feel like I am 14 again.Except I did not have such nice stuff!The fluke is a bit of a dissonant.