Why can I measure AC on a DC power supply? - Page 1
Forum section: electronics / beginners

author: Mark Loukko
author_rank: Contributor
timestamp: August 16, 2015, 07:46:06 am
content: I’m a beginner and this is my first post so I apologize if this turns out to be a monumentally stupid question ;-)Why can I measure AC on a DC power supply? To show you want I mean I’ve setup the following test:I made my DC power supply switch between 2 and 3 volts as fast as it can. It turns out it can do this around 4.5 times a second creating a square(ish) wave. My first multimeter measures DC, the second measures AC and my oscilloscope shows the waveform.The first multimeter reads an average of about 2.5 volts DC. This makes sense since I’m changing the voltage so quickly. So far so good.The second multimeter shows an average of around 400mV AC. The multimeter displays RMS and I’m not creating a nice sinusoidal wave so OK I guess this number makes sense but why do I see any AC at all?The oscilloscope, when DC coupled, shows my square(ish) wave completely above 0 volts as expected. When AC coupled, half of my square(ish) wave is above 0 volts and the other half is below 0 volts indicating AC.How can my second multimeter and oscilloscope show AC when the source is a DC power supply? From my understanding the definition of AC is voltage or current that periodically reverses direction (polarity). The DC power supply cannot change polarity.I must be missing a fundamental AC concept here or maybe it’s just terminology.Here is a link to a short video showing my setup:https://youtu.be/uvKTb98Qt8ECheersMark

author: IanB
author_rank: Super Contributor
timestamp: August 16, 2015, 07:52:57 am
content: If your DC voltage is changing up and down in a cyclic way you can understand this voltage as a sum of two components: a pure DC bias component and an AC component superimposed on top of it. Your multimeter and AC coupled oscilloscope are able to remove the DC bias component and show just the AC component which remains.In a more general way all periodic or cyclic waveforms can be decomposed into a sum of different individual frequencies. This is known as Fourier analysis. When one of the frequency components is DC (or 0 Hz) this is just a special case of the general situation.

author: IanB
author_rank: Super Contributor
timestamp: August 16, 2015, 07:59:28 am
content: To consider an analogy, maybe you have visited the sea shore? The sea has an average level ("sea level"), but it has waves superimposed on top of it. The sea in general has a constant level ("DC"). But when the waves encounter, for example, a rock pool, then water sloshes back and forth in and out of the pool ("AC").To take the analogy a little further, the sea level goes up and down with the tides over a period of hours. This is a low frequency "AC" component to the sea level. The waves on the other hand undulate with a period of a second or so. This is a high frequency "AC" component to the level. By Fourier analysis all the different frequencies in the water level could be separated out.

author: TheAmmoniacal
author_rank: Unknown
timestamp: August 16, 2015, 08:07:42 am
content: I share your curiosity here (yes I'm a noob/amateur/idiot). I have a few questions:1. What would happen if you add a diode on the output of the PSU? I assume nothing?2. Alternating current literally moves back and forth, while changing DC moves in one direction?3. What would a meter in AC+DC mode read?

author: IanB
author_rank: Super Contributor
timestamp: August 16, 2015, 08:38:39 am
content: Another puzzle for you. Suppose you are in a car travelling at a steady 60 mph. While you are in the car, you swing a pendulum in the direction of travel so the that pendulum bob at the bottom of its swing is travelling at 20 mph. Does the pendulum bob ever go backwards? (Is the pendulum bob still swinging back and forth, or is it always moving forwards?)

author: Richard Crowley
author_rank: Super Contributor
timestamp: August 16, 2015, 08:51:44 am
content: So your supply "switches between" (aka. "alternates") between 2 and 3 volts.And then you ask why you are measuring "Alternating Current"?You are measuring AC because you are generating AC.You happen to have an offset of 2.5V which disappears when you AC-couple it.As to why you are reading the particular numbers on your meters, that is a rather more complex matter of how those meters handle non-sine-waveforms.

author: TheAmmoniacal
author_rank: Unknown
timestamp: August 16, 2015, 08:58:37 am
content: If the AC part of this signal is "actually" swinging backwards would depend on your reference (if you are inside the car or outside it). Could you place the ground lead (reference) somewhere in this setup to eliminate the offset? (put yourself in the car to see it swing around 0?)

author: dom0
author_rank: Super Contributor
timestamp: August 16, 2015, 10:06:08 am
content: QuoteNo, the 10-50 pF input capacitance always shunts the input (and scope designers take some care that it stays constant regardless of DC or AC coupling - if it changed, you'd need to re-compensate high-Z probes). The series capacitor in AC coupling mode is usually somewhere near 50-220 nF, depending on the exact L.F. cutoff, and does not load the source (since it is connected in series).

author: Mark Loukko
author_rank: Contributor
timestamp: August 16, 2015, 03:51:36 pm
content: Hi Everyone,Thanks for everyone's feedback. I'm heading out of town right now. I will read all the responses tonight.CheersMark

author: Mark Loukko
author_rank: Contributor
timestamp: August 17, 2015, 05:32:19 am
content: Again, thanks for everyone's comments. I'm pretty sure this is just a terminology thing on my part.Quote from: IanB on August 16, 2015, 07:52:57 amQuote from: Richard Crowley on August 16, 2015, 08:51:44 amYes, I asked why I'm measuring "Alternating Current" because as far as I know the current never actually alternated or reversed its polarity.The current still flowed in one direction, just at different amplitudes.Here is another example but simpler than my previous one...Place a 1.5 volt AA battery with a 100 ohm resistor and you get .015 amps of DC current.Add another AA battery in series and we now have 3 volts and .03 amps of DC current.Take one battery away and we are back to .015 amps.My little brain tells me by removing the battery we simply reduced the voltage and current. The current never reversed or alternated its flow inside the wire. Or did it? If so can somebody please explain?CheersMark

author: Richard Crowley
author_rank: Super Contributor
timestamp: August 17, 2015, 06:13:11 am
content: "Alternating Current" does NOT imply that is is alternating around the zero reference. That may be the most common kind of AC waveform you might encounter, but the definition does not require BIPOLAR in any way.

author: IanB
author_rank: Super Contributor
timestamp: August 17, 2015, 06:51:10 am
content: Quote from: Mark Loukko on August 17, 2015, 05:32:19 amPlease reconsider the pendulum example above (post #4). Is the pendulum in the car swinging back and forth, or is it always swinging forwards, just at different speeds?

author: Psi
author_rank: Super Contributor
timestamp: August 17, 2015, 07:32:09 am
content: Thought problem for noobs....If you have 2V AC and you connect a 12V battery in series what do you have now? 2V AC or unfiltered 14V DC

author: vk6zgo
author_rank: Super Contributor
timestamp: August 17, 2015, 10:19:01 am
content: Quote from: Mark Loukko on August 17, 2015, 05:32:19 amThe multimeter doesn't care!------If thedcvoltage goes on & off.or if you could switch very quickly between your one battery & two batteries in series,the meter will quite happily see it asac.All it cares about is that there is achangein voltage ----it will see the difference between the two voltage levels.In fact,if you connect your meter,set toacvolts,across a battery,at the instant you make the connection,the meter will attempt to give you a reading of that sudden change.You can make a simpleacvolt meter by connecting a diode in series with one of the meter leads of adcvolt meter.You then only measure the average amplitude of the 1/2 cycle during which the diode is conducting.DON"T DO THIS WITH THE MAINS!-------the chances ofkilling yourself,blowing up your meter or your diode are high!"Half-wave rectifiers" as described above, tend to give some errors which can be resolved by using a "Full wave rectifier".This causes each consecutive cycle to be in the same polarity,so adcmeter can read the average of the amplitude of all half cycles.This can be,(& is,in practical devices),converted to read in RMS volts------but more on that later.This is how analog multimeters readac,but the rectification is done inside the meter.I'm not a DMM guru,but as far as I know the same thing is done in them.You could measure both 1/2 cycles individually,then combine the results with software,but that means your Analog to Digital Converter (ADC)would have to have a greater dynamic range.(maybe this is done in some meters).

author: Delta
author_rank: Super Contributor
timestamp: August 17, 2015, 10:44:03 am
content: You created an AC signal riding on a DC offset. Good question though mate, I hope the answers make sense, as this is an important fundamental to understand. 😀

author: Mark Loukko
author_rank: Contributor
timestamp: August 17, 2015, 05:36:13 pm
content: Quote from: Richard Crowley on August 17, 2015, 06:13:11 amhttps://en.wikipedia.org/wiki/Alternating_currentsays the definition of AC is "...the flow of electric charge periodically reverses direction"My understanding of AC is the green line below.where the electric charge in the conductor moves back and forth at some predefined frequency (E.g. 60Hz for wall plugs here in Canada)So in my humble opinion I think:AC = alternating current (electrons physically move back and forth in a conductor)The "AC" setting on a multimeter simply reads achangein voltage. It does not care if the voltage is going positive or negative.When the oscilloscope set to AC coupling it simply inserts a capacitor in series. The capacitor, when charged, prevents any more steady DC current from flowing. When the voltage does change the capacitor will discharge or charge allowing some current to flow again. The result shows up as "AC" on the oscilloscope. Just like the multimeter, "AC" simply shows a change in voltage.If what I'm saying is correct this explains why my multimeter was showing AC connected to my DC power supply. It was simply reading thechangein DC voltage. I thought it meant the voltage was reversing polarity. Silly me, I thought the A in AC stood for alternating.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: August 17, 2015, 05:49:07 pm
content: Don't feel bad. English is notorious for re-using words. The meaning of AC has evolved in the 100+ years since it was first applied. When originally used it really did mean alternating. As our understanding of electricity grew, and we expanded beyond pure power applications we (as a profession) chose not to invent a new and separate word. In the newer meaning the old version is a special case. It simplified the language for those who understood, but left traps for the newly introduced.

author: Mark Loukko
author_rank: Contributor
timestamp: August 17, 2015, 05:53:00 pm
content: Quote from: IanB on August 17, 2015, 06:51:10 amFrom a perspectiveinsidethe car, yes the pendulum swings back and forth at 20 mph.From a perspectiveoutsidethe car the pendulum swings back and forth but while it swings back it loses 20 mph of forward momentum. When it swings forward it gains 20 mph of momentum. Just like AC current the average momentum/voltage is 0 mph/0 volts.

author: Richard Crowley
author_rank: Super Contributor
timestamp: August 17, 2015, 06:08:40 pm
content: Quote from: Mark Loukko on August 17, 2015, 05:36:13 pmYes, I don't dispute the textbook/dictionary definition.HOWEVER:1) One could argue over the "perspective" of "reverses direction". Again, I don't read that as REQUIRING the voltage to pass through zero at any time.Any more than saying that "I am returning to home" doesn't require you to go beyond your home and then back.2) How a simple meter handles mixed AC and DC is undefined.If it is something that concerns you, then do experiments exactly as you are describing to "benchmark" how YOUR meter responds.Remember that each of them could be different.

author: Mark Loukko
author_rank: Contributor
timestamp: August 17, 2015, 06:12:58 pm
content: Quote from: CatalinaWOW on August 17, 2015, 05:49:07 pmThanks, that does make me feel betterI've learned a so much from watching Dave over the past year. He is 70% education , 20% entertainment and 10% waffling (ok that might be a bit low ;-) ). There is so much information on the internet its sometime hard to decipher the actual truth. That's why I bought my own test equipment, I wanted to see what really happens doing simple experiments.I probably should have titled this thread "What is your definition of AC?" :-)

author: Mark Loukko
author_rank: Contributor
timestamp: August 17, 2015, 06:24:02 pm
content: Quote from: vk6zgo on August 17, 2015, 10:19:01 amThank you! Your post made all the pieces fall into place for me.(well, at least until somebody shoots holes in my theory)

author: jlmoon
author_rank: Unknown
timestamp: August 17, 2015, 06:37:27 pm
content: Quote from: Psi on August 17, 2015, 07:32:09 amDepends on whether your 2V AC source is inductively or capacitively coupled to the (presumed DC source - battery)

author: Mark Loukko
author_rank: Contributor
timestamp: August 17, 2015, 07:46:27 pm
content: Quote from: Psi on August 17, 2015, 07:32:09 amI believe the 12V battery would cancel out the 2V AC and you would be left with DC oscillating between +13 and +11 volts.For me, since the voltage is no longer going below 0 volts the AC component (electrons moving back and forth) is gone and you are left with simple DC changing in voltage.The multimeter would read 2 volts "AC" simply because of the difference in voltage.Lets reverse it...12V AC with a 2V battery in series.I think you would get a sinusoidal waveform from +8 volts to -4 volts. There would still be AC but more of the waveform would be above zero volts now. If you added another 4 volts DC the waveform would be from +12 to 0 volts. No voltage is going below zero so my little brain tells me we are simply left with oscillating DC but the multimeter would read 12 volts "AC".

author: Zero999
author_rank: Super Contributor
timestamp: August 17, 2015, 07:56:44 pm
content: Quote from: Mark Loukko on August 17, 2015, 07:46:27 pmNow you're just talking semantics.It's perfectly correct to call a 2VAC power source connected in series with 12VDC, 2VAC with a 12VDC bias. In fact, even with the 12VDC bias present, the current will change direction because the cable's capacitance will still be charged and discharged as the voltage changes. Assuming conventional current flow, when the voltage increases, current will flow into the cable and when the voltages decreases, current will flow back from the cable into the power supply. This will happen irrespective of whether the 12VDC bias is present or not.

author: eilize
author_rank: Contributor
timestamp: August 18, 2015, 01:53:14 am
content: Quote from: Mark Loukko on August 17, 2015, 05:36:13 pm

author: vk6zgo
author_rank: Super Contributor
timestamp: August 18, 2015, 03:07:15 am
content: Quote from: Mark Loukko on August 17, 2015, 05:36:13 pmWiki looks very authoritative,but they sometimes get things wrong,& people can,& do, amend it..A case in point was a "Ham Radio Wiki",where they showed a Bridge rectifier & labelled it a "Kratz" rectifier.I had never heard of this name,& a search both online & in Electronics books showed no sign of him,so I amended it to read "Bridge" rectifier.It turns out that the name is really "Graetz"-----a well known name in European Electronics history.That said,I had never heard such a rectifier called by this name,either.Anyway,back to topic:-"Pulsatingdc" is a fairly archaic term,which was often used to describe the output ofdcgenerators.(All generators are really alternators,but in the dc generator,the commutator acts as a mechanical rectifier)The output waveform of adcgeneratordoesreturn to zero at the points where the (internal)acwaveform goes thru zero prior to commencing the next cycle.Another case where this term was used was in unfiltered rectifiers.When the rectifier ceases to conduct,the output waveform returns to zero.Adcvoltmeter will see these "positive going" or "negative going" half cycles as adcvoltage.(Anacvoltmeter will also give a reading as it sees them as anacvoltage).Early EEs did not have Oscilloscopes,but they knew howdcgenerators worked,so the term pulsatingdcwas a reasonable term.When amplifying devices appeared the "cat got among the pigeons"!There are very few such devices which do not work by converting the energy of adcpower supplyinto amplifiedac.Their outputs looked like anacsignal riding upon adcvoltage.(Users normally only needed the amplifiedac,so had to separate it from thedc.This usually entailed coupling capacitors,interstage coupling transformers ,etc.)"Pulsatingdc" was no longer a valid term to describe the combined signal."it looks like a duck,quacks like a duck,,,,,,," so the description of such a waveform became that of "dcwith anaccomponent".In the meantime,more theoretically inclined EEs & others,devised the concept of electrical signals being made up of a "fundamental"acsignal,plus a number of harmonics.Using this concept,it is possible to analyse any waveform.PS:- the previous poster has "hit the nail on the head"!If you add 2volts (peak)acto a +12voltdcsupply,it will change between +!4 volts (for the peak of the positive half cycle),through +12volts at the zero crossing between 1/2 cycles,& +10volts (for the peak of the negative half cycle).You can see (for this example),that the ac component passes through its positive peak where it adds to thedcvoltage,passes through zero,where thedcvoltage is that of the +12volt source alone,& then subtracts from thedcvoltage at the negative peak.With batteries,you can demonstrated that the resultant voltage of sources connected in series with reverse polarity subtract,& in the same polarity,add.All the sources have an independent existence,regardless of their series connection,as does theaccomponent of a signal.

author: Rick Law
author_rank: Super Contributor
timestamp: August 18, 2015, 05:50:44 am
content: Quote from: Mark Loukko on August 16, 2015, 07:46:06 am[boldadded in quote]Since no one yet brought this up...It is likelynotan an issue of AC verse DC. You are changing it only 4.5 times a second, by changing, I take it you mean voltage change up or down. So a full cycle takes two changes (high-to-low then low-to-high). Two change cycles occurs only 2.25 times a second. That is avery very slow wave at 2.25Hz. Even if I misunderstood and your "changing it" mean a complete hi-to-low plus low to high, that is still aslow 4.5Hz, your DDM's sampling window is very significant when it is that slow.At 4.5Hz, that is a good 222/1000 seconds per cycle (ie: 222ms). It islikely out of working range for your DMM's TRMS/RMS to workso AC reading is meaningless. (My UT61E's AC measurement is 41Hz min to about 1KHz, 2.25hz does not even come close.)With DC reading, 222ms is a long time. Averaging just 10 full cycles will take 2.22 seconds and most DMM report at least once a second. If say your DMM is updating three times a second, it would have just 1.5 cycles to average. Hard to average out to a good number with just one full cycle and a fraction of the next.So, you are not within the working range of your tool (DMM). A scope will give you far better measurement.

author: eilize
author_rank: Contributor
timestamp: August 18, 2015, 05:57:38 am
content: you must absolutly see the difference between a periodic signal and an alternating signal/as i said, average of an alternate signal =0if you add a dc to an alternate signal, you have (always) a periodic signal (but not anymore alternating)where the average is the dc valueso :when you switch to ac on your scope, you see only the alternate signalback on dc position, you see the periodic signal (ac+dc)on your fluke, your read the rms value of the alternating signal(it's put on alternating signal measurement)

author: Mark Loukko
author_rank: Contributor
timestamp: August 18, 2015, 05:17:19 pm
content: Thanks for the history lesson vk6zgo :-)QuoteFor those who like visual aids (like me) this is what the output voltage of a DC generator looks like:QuoteI will look this up. More bedtime reading!QuoteI graphed this in Excel. Please see the attachment. Did I get it right?QuoteI'm will have to process this for a bit :-)

author: Zero999
author_rank: Super Contributor
timestamp: August 18, 2015, 05:30:02 pm
content: Quote from: Mark Loukko on August 18, 2015, 05:17:19 pmActually that's not true. As long as the shaft is turning and it's working properly, a DC generator's output voltage never fallsbelowto zero.A DC generator will have more than two poles on its armature, normally an odd number. Suppose it has three poles on its armature, the output will look like a rectified three phase rectifier which never goes to zero.#EDIT:The output of a DC generator will look something like this:Except it will be much more noisy.

author: Mark Loukko
author_rank: Contributor
timestamp: August 18, 2015, 05:34:38 pm
content: Quote from: Rick Law on August 18, 2015, 05:50:44 amGood point about the frequency. I have the Fluke 87-5. I just checked the manual and it looks like the min frequency is 45Hz and max is 20kHz for reading AC.I knew the frequency was super super slow but that's as fast as my DC power supply could change the voltage up and down.At the time I was surprised to see anything showing on AC while connected to my DC power supply.The same signal was connected to my oscilloscope which had no problems reading the slow frequency.

author: Mark Loukko
author_rank: Contributor
timestamp: August 18, 2015, 10:54:50 pm
content: Just an FYI...For those interested, I found another thread on this forum asking the same sort of AC / DC question:https://www.eevblog.com/forum/beginners/what-really-is-ac-and-dc/On another forum I found the following... It was quite entertaining to read. After reading the second quote I believe I'm a "purist".http://boards.straightdope.com/sdmb/archive/index.php/t-494980.htmlQuoteQuote

author: vk6zgo
author_rank: Super Contributor
timestamp: August 19, 2015, 03:09:54 am
content: Quote from: Mark Loukko on August 18, 2015, 10:54:50 pmThe terms "ac voltage",& "dc voltage" may be read,for the former case,as:-"A voltage,which if connected to an external circuit would cause an alternating current to flow",& in the latter case:-"A voltage,which if connected to an external circuit would cause an direct current to flow".As in your quote,EEs are sloppy---but not only EEs,the general public do the same all the time.An "American" (actually a citizen of the United States of America) may put "Gas" (Gasoline) in his "Auto"(Automobile).From context,we know that he isn't literally putting "GAS" into it.An Englishman may put the same stuff in his "Motor"(Automobile).From context.we know he isn't pouring fuel into the oil filler cap of his engine,or into the guts of an Electric Motor.Getting back to Electronics,even something that a "purist" may use to describe a current,say :-"A dc current"is like this,as it really means literally"a direct current current"Such "sloppy" terms,(which have been round for a century or so),& the concept of an "ac component" are useful,as they make it possible to describe circuit operation which it would be horribly unwieldy to describe if we used strict interpretations of definitions.Another way of addressing things is to think of dc as ac of 0Hz frequency.It we had the time,& enough capability,we could synthesize a signal which is the same in all respects as youroriginal on/off PSU switching,by the combination of purely ac signals.