Electrician needs Rescue  _ Tsunami of square waves ahead !!   - Page 1
Forum section: electronics / beginners

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: May 15, 2011, 02:51:53 pm
content: I will get to the point ..I own one UNI-T DMM ( UT30D year 2001)On it , haves one fixed square wave output at 50 Hz with 100K Ohm resistance output.My own (Japanese frequency counter 0-150MHz 4 digits) Leader , measured the frequency at 00.52 = 52 Hz.And also the Fluke 28II , confirmed the frequency at 51.87 Hz ( better resolution) .I also measured the duty cycle of this signal , and was at 50% .The UNI-T User's manual says additionally :1) The output voltage range will be over 3V when it is loaded 1M Ohm.2) Use square wave output to repair audio equipmentSo far so good .And Now the problem description.If i try to measure this square wave as voltage,on the DC 600mV ( special range on the Fluke 28II)I get as output 1.20 mVAt min/max/Peaki get :Max = +29.3 mVMin = - 26.2 mVAnd my questions :1) How can I get the 3V output from it as reading ?2) Only oscilloscopes can do that ? ( Read the wave form voltage )3) How much effects the measurement, the impedance of the DMM it self. ?And also, I had accidentally discovered, that the Low-Pass filter on the Fluke causes an extra burden ( load) ,when it becomes active , that makes the DMM unstable at taking readings of Frequency and Duty cycle,at those small in voltage signals.I got accurate readings in both Frequency and Duty cycle , but the LCD reading playing like ON-OFF ON-OFF.Example per second in Hz: 50...17...50....17 , and the same happens with the duty cycle.With out the Low Pass filter , everything looks normal , and stable.I get 7.4 mV ACWith the Low Pass drops to 5.2 mV ACLooks that I found the hot spot about the sensitivity of it ..

author: bilko
author_rank: Frequent Contributor
timestamp: May 15, 2011, 02:59:22 pm
content: Do you get the same readings on AC Voltage selection ?Are you using a true RMS meter ?

author: metalphreak
author_rank: Frequent Contributor
timestamp: May 15, 2011, 03:00:25 pm
content: The 100k internal resistance is in series with whatever you connect at the output terminals. If you connect relatively low (to the 100k res) resistance, most of the square wave output voltage will drop over the 100k resistor, so you'll get a low voltage reading. If you connect a 1M ohm load then a majority of the voltage will drop over it instead of the 100k (basic voltage divider rules apply).

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: May 15, 2011, 03:35:04 pm
content: Quote from: yachtronics on May 15, 2011, 02:59:22 pmNice question ...a)By setting the DMM at the specific mV DC range ( max600 mV DC)I get an output of 0.3 mV ( yes it is dropped from before )At min/max/Peak mode:Max= +29.3 mVMin= - 26.2 mVb) On the standardDCrange0-6 Vrange .I do not get any voltage ..But at min/max/Peak mode:Max= + 0.091 VMin= - 0.080 Vc) At the only one, AC V input ( with lowest range the600mV AC) .I get about 8mV ACAt min/max/Peak mode:Max= + 0.085 V ( +85 mV)Min= - 0.086 V ( - 87 mV )Well even if it struggles to make the measurement , it looks like a True RMS ..

author: vk6zgo
author_rank: Super Contributor
timestamp: May 15, 2011, 03:53:29 pm
content: I'm a bit confused by the values you quote.Does this device put out a 3v p-p squarewave if you check it with an oscilloscope?If you use a 1x probe,you should see 3v if that is the spec,10x will probably not be much different.Does the most negative (or most positive) part of your square wave appear at zero volts on the scope? If so,you have a real square wave.A DMM should read something less than 3v dc when placed across this output----If there was any justice in the world,it would be 1.5v dc.It should certainly not be around the figures you quoted,unless the signal is really a clipped ac waveform.If it is the latter,you would get a better reading on the ac range.VK6ZGO.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: May 15, 2011, 05:07:06 pm
content: I do not have an Oscilloscope .This function on the UNI-T, called as :Signal injection Function.( it looks that also others DMM's have this too )I did Google about it , and found and other people that measured similar low output voltages ,other than the named specs ..Some said that it is 400mW output at 47Kohm .It supposed that you can inject this wave :1) on sound amplifiers2) As signal continuity tester.3) To feed with signal one PWM fan, so to spin,the 50% duty cycle on it = the motor will turn at half speed = good for testing.I have verify so far that this square wave oscillator ,It is at 52Hz + 50% Duty cycle .What I am looking now is the voltage output of it ..According to some information that I have ,if this Signal injector can offer an true 50Hz ( square wave ) at 3V output,it could be used also as calibrating source for multimeters.The 3V haves an great importance, as anything lower than that ,technically it is considered as low output ,that partially effects the 100% accuracy.Either way , when an multimeter haves 0-600mV range + TRMS ,it should display properly, voltage + frequency and data cycle ,even from the first mV until the 600mV . ( at square waves )

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 15, 2011, 07:02:50 pm
content: you need an oscilloscope to be certain how the signal looks like. and if your ac rms dmm cannot satisfactory measure it, you can build full bridge rectifier to bring the negative pulse into positive. i'm suspecting the signal is centered at 0V (similar to pure sine) from your min max reading.

author: Unknown
author_rank: Unknown
timestamp: May 15, 2011, 07:42:01 pm
content: Quote from: Mechatrommer on May 15, 2011, 07:02:50 pmAgreed.Quote from: Mechatrommer on May 15, 2011, 07:02:50 pmHow do you think AC measurement in DMMs works? A standard bridge rectifier consisting of four diodes won't work well though, due to the diode forward voltage drops. About 1V will be subtracted from the voltage, and voltages below 1V won't register at all.

author: RayJones
author_rank: Frequent Contributor
timestamp: May 15, 2011, 08:00:49 pm
content: I'm a bit perplexed why people are worrying about "relative to ground".If the signal is coming from a DMM, surely it is floating from ground, and you can apply it however you see fit?As for the original question I'd expect to read somewhere about 1.5Vrms into 1Mohm if it is indeed a 3V p-p square wave.This should halve when sent into 100Kohm.Given it is supposedly 50Hz, bandwidth of the meter's AC range should not be a factor.So unless the reading DMM has an unusually low AC input impedance you either have a faulty source signal, or a dodgy DMM to read with.An oscilloscope is the only way to be certain.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: May 15, 2011, 08:35:42 pm
content: I do not know what to say , my own brain showsOL, on the LCD on my head..I tried to use sorter coaxed leads , so to eliminate any interference.Replaced the battery with new .No gain at all , or very small.The only things that I can say as confirmed ,Is that the Low Pass filter it does cause extra burden on the weak signals in volts.Also , there is a good stability about the frequency 52Hz and the 50% Duty cycle,on this small DMM.I have an better signal generator , but I have no control about duty cycle.And so with out the little UNI-T I would be unable, even to tell how the duty cycle looks like , on the Fluke.Shafri is correct about the signal, that is centered at 0V ,that's why I was trying to measure at Min-Max Peak .In this scenario ( square wave) , by measuring in any other mode ,you get mostly atmospheric charge than true voltage,In the beginning I had the two DMM connected by their leads = 2.5 meters long antennaThe oscillator on the DMM it is very weak ,and the locked frequency of 50Hz , an truly stupid idea , it is identical with the Mains network !!My opinion is that it should be designed to be at the acoustic spectrum 200KHz - 1000KHzAny way it is the second time , that I explored the potentials of this oscillator ,and proved that I wasted my time on it.Yes , all of you , you are correct ,I have to add on my wish list one true capable signal generator,and one oscilloscope , if I need to fight with the waves , and win .

author: vk6zgo
author_rank: Super Contributor
timestamp: May 16, 2011, 03:11:15 am
content: Quote from: RayJones on May 15, 2011, 08:00:49 pmIf the signal is AC coupled, or is originally a peak clipped AC signal the signal will be symmetrically distributed either side of zero volts.In other words, to the DMM,it is a distorted AC signal,so will not read correcttly on the DC range.As a Signal Injection option,it probably is AC coupled,as you don't want to upset the operation of the stages of the device you are checking.There is nothing wrong with the idea of a bridge rectifier,as long as you allow for the 1.2 volts forward drop of the two series diodes.VK6ZGO

author: RayJones
author_rank: Frequent Contributor
timestamp: May 16, 2011, 07:50:31 am
content: The UT30D is a handheld battery powered unit.It is not ground referenced, nor mains powered so clipped AC mains are irrelevant, and any square wave produced is purely relative to itself.The measurement of the waveform is purely relative to the + and - output leads, no "balanced about ground" etc ever comes into it.

author: mechelec
author_rank: Contributor
timestamp: May 16, 2011, 10:02:26 am
content: Would not the reference be between the two terminals of the output.Then an "AC" signal would have relative positive voltage at one terminal for half the cycle and a negative voltage for the other half.If it is a "DC" or PWM signal the voltage between the terminals will be 3V for half the cycle and 0V for the other half.If you are reading 1.2 mV on a DC range then the output is clearly "AC" i.e. if you measure between the terminals it will be +3V for half the cycle and -3V for the other half.If you measure this with a true RMS meter on AC volts you should have a reading of 3V as the RMS value of a square wave will theoretically equal the peak value.

author: RayJones
author_rank: Frequent Contributor
timestamp: May 16, 2011, 10:25:56 am
content: Yes, a 6V peak to peak 50% square wave would be 3V RMS.If it was biased to be 0 to 6V it would still be 3V RMS.Likewise a 0V to 3V "DC" signal would be 1.5V RMS.No magic, just simple integral calculus.A cheapo meter which is not true RMS will lie through it's teeth, and using a DC range measure an AC signal whatever its relative levels are is of no benefit to obtaining the answer.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: May 16, 2011, 10:38:21 am
content: The problem could be fake specs , about this Signal injection Function .I visited today two local friends of my, who they do TV-set repairs , and they do have oscilloscopes.Both of them had forgot how to use them, about checking waveforms .And they use them only, to test components.I have one more friend of my, that he could possibly help,about testing the UNI-T DMM with the oscilloscope ,but I will know more about it in few hours. ( If he will accept to help )

author: mechelec
author_rank: Contributor
timestamp: May 16, 2011, 10:48:05 am
content: Not entirely true.A 0 to 6V 50% PWM signal will display 3V RMS on most meters using an AC RMS range.However this is in fact the average value not the RMS.If you have a meter that can read AC + DC RMS the true RMS value of this signal is 1/sqrt(2) * pk = 4.24 VIf you have a 0 to 1V signal at 50% you can work out the RMS value quite simply:First square the value which will be exactly the same signal i.e. a 1V square wave.Take the mean which is of course 0.5V if the duty cycle is 50%.And then sqrt which will be sqrt(1/2) = 1/sqrt(2).

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 16, 2011, 12:21:11 pm
content: whats wrong with my dmm?

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: May 16, 2011, 12:48:44 pm
content: QuoteWell, before you smash it with a hammer ..Try the same measurements, with 3.0V Peak to Peak. ( if possible )

author: mechelec
author_rank: Contributor
timestamp: May 16, 2011, 01:01:43 pm
content: Nothing.That agrees exactly with what you would expect.Apart from the 5% error those values are consistent with the theory.Your AC + DC measurement of 1.35V ~= 1.414V or 2/sqrt(2) (+/-5%) which is the expected true RMS value.and the AC value of 0.95V = 1V -5% which is the expected average value.The result that is a little strange is the reading of 0.95 V on DC range.It is clear that the meter responds by averaging the value on the DC range if it is changing rapidly enough i.e. it has a low pass filter effect which will show only the DC compnent of the waveform.

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 16, 2011, 01:43:36 pm
content: same row same column for 3Vpp 50Hz signal...bias 0V (-1.5V,+1.5V) bias 1.5V (0V,3V)Vdc 0 1.424AC 1.421 1.421AC+DC 1.424 2.015

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: May 16, 2011, 04:41:14 pm
content: More news from my camp,the small UNI-T was tested with the oscilloscope .And all that it does is an very tiny and noisy waveform.There is no way to tell , that this is an true square wave signal.And so I have to stop here .

author: vk6zgo
author_rank: Super Contributor
timestamp: May 17, 2011, 07:04:07 am
content: Quote from: Kiriakos-GR on May 16, 2011, 04:41:14 pmWell,you can assume that UNI-T either lie in their teeth,or more likely, don't proof read their owner's manual,& the referenceto 6 volts just "snuck in" accidentally.As they say it is aSignal Injector, a fairly low level would be more usual than a high level.VK6ZGO

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: May 17, 2011, 11:32:21 pm
content: Quote from: Mechatrommer on May 16, 2011, 01:43:36 pmI was hopping that one more expert than me, he would offer two words as explanation,in your question.Even so, its commonly known that the high quality DMM are the ones who have better internal filters,and can accurately measure square waves , or even saw waves.There is also different techniques in use for those calculations.Some times even needed advanced functions on the DMM ,so to perform such measurements .As example is the " Peak " function on the Fluke 87V and 28II . (another maker could possibly describe it with other naming)1) If I measure my Mains its 220V ( normal AC mode )2) If I use the function Min/Max/Average I get 218/222/2203) If I use theMin/Max" Peak "function ,I get 316 V AC asMAX, and asMinI get minus 316 AC Volts .Logically this range of Fluke DMM's, they do have the potential to measure the true Peak to Peak voltage of all sort of wave forms.And on a second thought , I must feel happy that my 28II , managed to measure frequency and dusty cycle,from this low cost 10 years old UNI-T that I own .Just because the 12 years old oscilloscope, failed even to display this tiny in power square wave.I bet that after all the information's that you gathered from this thread,in the next DMM that you probably get , you will be more demanding about the functions thatit should have,before actually offer your hard earned cash for it .