Why is this happening? - Page 1
Forum section: electronics / beginners

author: gargamel_220
author_rank: Newbie
timestamp: November 23, 2014, 12:03:11 am
content: Hello to all,i am new to electronics so any help will be appreciated.I have a transformer with output of 15.2V as my DMM says when on 200 ACV range. As i need DC i use full wave rectifier and here comes confusion. When i measure the output of rectifier at 200 DCV range i get 24V, then i notice i get 62V if i touch one of the leeds (metal part) and 13.6V if i touch booth ends. With different DMM i get 13.4V, 16V, 13.6V (same testing order with same 200 DCV range). First DMM is cheap UNI-T UT33B and the secend also some cheap one.So i am interested to find out why do i get those resaults, how does DMM works, why my finger/fingers is changing the resault, is my DMM broken, am I doing it wrong?

author: janengelbrecht
author_rank: Regular Contributor
timestamp: November 23, 2014, 12:15:47 am
content: Dont think 200ACV is the standard...230V is the common rms voltage.You mentioned a bridge rectifier ? And an elec. capacitor after that i presume to get rid of the ripple ?Then the RMS voltage should be secondary voltage on transformator multiplied with the squareroot of 2 and then you subtract 2 diode drops around 1.4V. That is your voltage on the capacitor.If your meter measures weird voltages on the capacitor.....i would try another multimeter and see if the "problem" still is thereIf it is I would like to SEE the circuit and how you meassure voltagesI need data so to speakNo cap ? No clean DC

author: IanB
author_rank: Super Contributor
timestamp: November 23, 2014, 12:32:54 am
content: Quote from: gargamel_220 on November 23, 2014, 12:03:11 amThe output from a bridge rectifier is not "DC". It is full-wave rectified AC.Where your meter is concerned, "DC" is supposed to be a smooth, steady voltage, such as you get from a battery. This is not what comes out of a bridge rectifier.The short answer is your meter is getting confused by what it is seeing, so it does the best it can with confusing input data.The long answer is too long to explain here.However, if you rectify 15 V AC with a full wave rectifier, you will get "15 V DC", near enough. (Perhaps 14 V DC in reality, but that is "near enough".)

author: rs20
author_rank: Super Contributor
timestamp: November 23, 2014, 12:48:19 am
content: Quote from: IanB on November 23, 2014, 12:32:54 amAlthough, the moment you add even a little smoothing cap, you will see the peak AC voltage across the cap, not the RMS. So 15 V (RMS) AC in will become sqrt(2) * 15 - diode drop = 20.5V DC. Until you put a load on it, of course.

author: gargamel_220
author_rank: Newbie
timestamp: November 23, 2014, 11:10:09 am
content: Hello,Thanks for the replies.Quote from: janengelbrecht on November 23, 2014, 12:15:47 amOn my primay side of transformer 240V rms, secundary 15,2V rms. By 200ACV i meant range on my DMM. 15,2V rms goes to bridge rectifier and no cap on output.Quote from: rs20 on November 23, 2014, 12:48:19 amI believe i have tried with the cap and i did get something like 20.5V so all of that is ok.So far i gather that i cant measure DC voltage unless it is constant nor AC unless it is pure sinusoid. So in my case i canonly calculatethe output. Correct me if that is not the case!I did the measurement again. Regarding pure sinusoid i get on both DMM-s 240v and 15.2V (transformer pri/sec). On bridge rectifier output with no cap on DC range on DMM a) i got 23.5V and DMM b) 13.5V. On AC range DMM a) 35V and DMM b) 28.7V.So i wonder how DMM gets to that resaults, how dose it measure or what and what does it calculate to get to that values. If complicated to explain you can point me to where i can find that.Is there simple explanation on why i get different resaults when i touch one of the leads or both?? Measurements in attachment. And why i got similar resaults on DMM b) and not DMM a). Do they measure/calculate differently.

author: Refrigerator
author_rank: Super Contributor
timestamp: November 23, 2014, 11:32:51 am
content: The human body is nothing else but a big sack of electrolyte, touch two different metals and you'll get voltage (ultra low current of course), since DMMs are very high input impedance they pick that voltage right up.That's why you get that high voltage when you touch the leads.

author: rs20
author_rank: Super Contributor
timestamp: November 23, 2014, 11:52:41 am
content: Quote from: Refrigerator on November 23, 2014, 11:32:51 amElectrochemical effects will never give you more than 5V worth, no matter how low the current draw.

author: gargamel_220
author_rank: Newbie
timestamp: November 29, 2014, 09:47:14 pm
content: Hello,still a mistery here...By my calculation V(dc)=0.9V(rms); 0.9x15.2=13.68 ---- so by that DMM b) is ok when measuring od DC range, and DMM a) only when i put my fingers on the points that i measure, WHY??Do i act as some sort of capacitance or resistance in parallel or something?Is my DMM a) broken?Help!!

author: IanB
author_rank: Super Contributor
timestamp: November 29, 2014, 10:31:57 pm
content: I think it is because you don't have a load on the bridge rectifier. Diodes only work as diodes if they have current flowing through them. If you take the AC output of a transformer and feed it through a bridge rectifier with the DC side of the rectifier an open circuit then you will get very unpredictable voltages. Basically the diodes in the rectifier can't work properly (can't switch on and off) without some current flowing through them to power their operation.If you put a small load on the DC side of the rectifier (for example a few hundred ohm resistor) or a small indicator lamp, then I think you will start to see predictable and stable voltages.

author: Circlotron
author_rank: Super Contributor
timestamp: November 29, 2014, 11:00:15 pm
content: There is some capacitance between the primary and secondary transformer windings. I'd say this is passing a small current through the transformer and only gets rectified when one side of the bridge rectifier gets grounded somewhat by touching it. As a test, ground one of the secondary wires before it gets to the bridge and try your experiment again. Or try it with a lower impedance analog meter.

author: IanB
author_rank: Super Contributor
timestamp: November 29, 2014, 11:08:10 pm
content: I've done the experiment with diodes myself. If you leave one end of a diode floating it's like it's blowing in the wind. You won't measure any consistent or stable voltage on it. You have to put a load on the diode so current flows through it, then the voltage will become predictable.

author: mstevens
author_rank: Regular Contributor
timestamp: November 29, 2014, 11:47:43 pm
content: Quote from: IanB on November 23, 2014, 12:32:54 amAFAIK this is called "DC"... AC refers to the sinusoid crossing the x-axis, i.e. V<0 or more correctly I<0 since the C in D/AC stands for current.

author: rolycat
author_rank: Super Contributor
timestamp: November 30, 2014, 12:47:05 am
content: Quote from: mstevens on November 29, 2014, 11:47:43 pmWebster's definitions are:QuoteandQuoteWhich suggests that "full wave rectified AC" is neither DC nor AC.In my unlearned opinion IanB's description is thus correct if potentially confusing, just as the DC output of a power supplycouldbe described as "rectified, smoothed and regulated AC". Quondam AC.

author: IanB
author_rank: Super Contributor
timestamp: November 30, 2014, 01:05:24 am
content: Quote from: rolycat on November 30, 2014, 12:47:05 amIt could in fact be considered as the sum of two components, a DC component with an AC component superimposed on top of it. A true RMS meter with an AC+DC function would be able to show both voltages separately.QuoteIndeed, I should apologize for the confusion. I initially thought the meter's DC averaging algorithms were getting thrown off by the shape of the waveform, but on reflection I changed my mind. I think it is much more likely the problem is the lack of a load on the rectifier. I remembered being puzzled by the same thing when I tried to probe the output of a bridge rectifier on a scope. The scope showed crazy traces until I put a load on the rectifier and allowed some current to flow.The explanation is as follows:- A diode is a switch, and switches require power to change state- Power is the product of voltage difference and current- If the rectifier is open circuit, there is no current flowing through it- If there is no current, then power = voltage x zero = zero -- there is no power- If there is no power, the diodes cannot switch and change state- If the diodes cannot change state, there will be no rectification- If there is no rectification the measured voltage will not be stable or consistent

author: Shock
author_rank: Super Contributor
timestamp: November 30, 2014, 01:40:52 am
content: Further to IanB's post the further the signal deviates from steady DC voltage the harder it is for the meter to interpret and display the voltage. As this occurs digitally it entirely depends on the design of the multimeter.It sounds like your really asking how you can measure this properly. Well you don't use a cheap multimeter to measure unfiltered DC, instead you would use a tool capable of interpreting the waveform such as an oscilloscope.When designing circuits for the first time many people use circuit simulator software, these allow you to check and adjust your circuit quickly and many allow you to see waveforms.Here is a neat little "real time"java based (runs in a browser) circuit simulator with a built in oscilloscope that is very easy to learn for beginners that you can experiment with, such as to see the effects of AC to DC rectification and filtering.http://www.falstad.com/circuit/

author: mstevens
author_rank: Regular Contributor
timestamp: November 30, 2014, 01:43:13 am
content: Quote from: rolycat on November 30, 2014, 12:47:05 amI'm not trying to start a flame here just trying to educate as most really get this concept wrong. Fortunately for me Mr. Webster was/is not an EE and I do have a BSEE and soon to have a MSEE.Let's see if I can give an example say you have voltage sign wave that has been rectified. So the first 180 degrees of the wave is above zero and then it repeats above zero at pi radians or 180 degrees. ... I am going to simplify (quantize) this wave and describe it as such: 0V, 2V, 4V, 6V, 8V, 10V, 8V, 6V, 4V, 2V, repeat.Obviously, now say this supply is connected to a 2ohm load, the current through the load would be: 0A, 1A, 2A, 3A, 4A, 5A, 4A, 3A, 2A, 1A, repeat.Note the current is NEVER, negative. Sure the current changes, it goes up and down but it never goes negative. The definition of alternating current would be 0, 1, 2,3,2,1,0,-1,-2,-3,-2,-1,0. ... Do you see? If the current does not change sign (direction), it is considered DC. I've had this question on an exam more than once, I assume professors put it on exams because many people don't take the time to learn what the term AC and DC really mean. It is a fundamental all Electrical Engineers should know but all learning environments are not created equally.Again, not trying to flame, just trying to help.

author: IanB
author_rank: Super Contributor
timestamp: November 30, 2014, 01:54:12 am
content: Quote from: mstevens on November 30, 2014, 01:43:13 amThat's very true, but the question then arises, what is the measure of that varying DC voltage? Two possibilities are a filtered average voltage, or a true RMS voltage. Depending on the meter and the measurement technique, the answer to "What is the voltage?" will vary.The nice thing about steady DC voltages is that the answer to the question is unambiguous. If you don't have a steady voltage, life gets more complicated.

author: mstevens
author_rank: Regular Contributor
timestamp: November 30, 2014, 02:23:53 am
content: Quote from: IanB on November 30, 2014, 01:54:12 amIt really depends on the meter.http://support.fluke.com/find-sales/Download/Asset/1260898_6116_ENG_M_W.PDFmay help with understanding what a meter reads. I would have tried to help the original poster but I just don't understand "touching" the probes. One should not be touching any probe when taking measurements.I will say this though, I don't think the OP's meter is broken.

author: IanB
author_rank: Super Contributor
timestamp: November 30, 2014, 02:32:16 am
content: If you try to measure the voltage on the output of a "floating" bridge rectifier, then whatever voltage you see will be influenced by stray capacitance in the system. If you touch the probes during the measurement then your body will add to the stray capacitance and the voltage will change. (Touching the probes would happen if you are holding the probes against the rectifier leads to make good contact.)As soon as you add a moderate resistive load to the rectifier output, then the output voltage will be controlled by that resistance and the effect of stray capacitance will be eliminated.

author: rolycat
author_rank: Super Contributor
timestamp: November 30, 2014, 02:37:06 am
content: Quote from: mstevens on November 30, 2014, 01:43:13 amThere is frequently a large disparity between the answers one is asked to provide in academic examinations and those which are useful to working engineers.The description of DC you provide isadefinition, notthedefinition. If you regard DC and AC as antithetical it may be technically correct, but I would submit that in practice it is not useful to characterise a sine wave signal which happens to have a DC offset slightly greater than its peak amplitude as 'DC'.

author: c4757p
author_rank: Super Contributor
timestamp: November 30, 2014, 02:44:09 am
content: Common sense: if you have a waveform that is notobviouslyeither DC or AC,describe itinstead of arguing about terminology.

author: mstevens
author_rank: Regular Contributor
timestamp: November 30, 2014, 04:54:32 am
content: Quote from: rolycat on November 30, 2014, 02:37:06 amIn law, history, humanities, etc. there may be multiple definitions to terms. Generally, in STEM definitions are absolute, there are not different definitions to a term. A meter is a meter, a watt is a watt, a joule is a joule. Sure there are constituents e.g. 1W=1V*1ohm=(1A)^2*1ohm=(1V)^2/1ohm... that is definition of a watt. I challenge you to find any reference to a definition of Alternating Current (AC) that does not stipulate that the current must change direction.Just because a current decreases in value does not mean that it changes direction. Unless you want to redefine "changes direction. If I am driving north at 60miles/hour, reducing my speed to 10 miles and hour does not mean I change direction. If stop (0 mi/hr) and then start traveling north again does not mean I change direction. In STEM, there is no ambiguity as to what "changing direction" means.Technically, the waveform out of a full wave rectifier is DC with some Voltage peak-peak ripple. You can pick up any power electronics text to confirm these definitions.I'm pretty sure I gavethedefinition of AC/DC. I would stand corrected if you would provide the/any proper reference.

author: rolycat
author_rank: Super Contributor
timestamp: November 30, 2014, 10:48:53 am
content: Quote from: mstevens on November 30, 2014, 04:54:32 amUnfortunately there are multiple definitions to terms in STEM as well. In signal processing the DC component of a signal is defined as its mean value; it has no periodic element. Your definition of DC as 'any waveform in which current does not change direction' makes no sense in that context.QuoteI did not and do not take issue with that definition of alternating current as a standalone phenomenon. However, a signal may have an AC component which does not cause the current to change direction.QuoteI have already given a proper reference: the Merriam-Webster dictionary definition of DC.As demonstrated above, the term has different meanings in different contexts, even within STEM.

author: mstevens
author_rank: Regular Contributor
timestamp: December 01, 2014, 03:00:18 am
content: Quote from: rolycat on November 30, 2014, 10:48:53 amNo disrespect, but may I ask what credentials you have to speak on this subject? Are you an electrical engineer or have an electrical engineering or other STEM degree?

author: IanB
author_rank: Super Contributor
timestamp: December 01, 2014, 03:30:06 am
content: In technology circles, descriptions and terminology are all about precision: stating exactly what is meant without ambiguity or possibility of misinterpretation. Under these circumstances a steady unvarying voltage is different from a varying voltage such as from a full wave rectifier, even if the net heating effect of either is the same. Physicists and engineers will therefore be precise about which is which whenever this matters. A fluctuating DC voltage will be described as a "DC voltage with an AC component superimposed" or an "AC voltage with a DC offset". Any rectified voltage may be loosely described as "DC", but the precise term DC is reserved for voltages with no frequency components above 0 Hz. (You would likely be very upset if your DC lab power supply had ripple on it, and would reject any argument of "But it is DC! The voltage doesn't reverse direction!")

author: mstevens
author_rank: Regular Contributor
timestamp: December 01, 2014, 04:40:34 am
content: Quote from: IanB on December 01, 2014, 03:30:06 amI believe it is bad for to not teach beginners correctly. I did not make the definitions, I'm just giving them to you. Hmmm... thought experiment directed to you IanB... Say you have a .1 mVpp AC signal with a 12V DC offset is that AC or DC? Say you have a 1 uVPP AC signal with a 12V DC offset is that AC or DC? ... My point... when does the AC quit becoming AC and is just a perturbation to the DC?I agree with you that you hear the terms an AC signal with a DC offset, or a DC voltage with some AC ripple; but, considering the strict, accurate definitions of AC and DC, those are misnomers. One should refrain from using them with beginners because they can be potentially confusing.A rectified (full or half) sinusoid is DC; it may not be and ideal DC source but it is well defined unambiguous DC. I'm typing on a bad keyboard now so I won't do it. But, find a post the definition of DC. Then explain to me the ambiguity of how a full or half wave rectified sinusoid does not adhere to the definition.Have you noticed that all DC power supplies specify "ripple" and not AC on the DC voltage? They all have as you put it, some AC component on the DC voltage. I don't think you could find any datasheet/specs on ANY DC that would refer to that AC component as AC and not as "ripple."The fact that were having this discussion shows the need for proper education. Per my statements to rolycat there is no ambiguity in the definitions of AC and DC; physics does not work like that. In Physics, definitions are ALL well defined and unambiguous; they must be or we don't have science.

author: Zero999
author_rank: Super Contributor
timestamp: December 01, 2014, 03:19:28 pm
content: Why are you lot bothering to argue about this?The correct way of dealing with this is to post a drawing. There's no point in arguing about definitions and semantics. The original poster probably got bored pretty quickly and hasn't returned.

author: AG6QR
author_rank: Frequent Contributor
timestamp: December 01, 2014, 06:22:23 pm
content: According to the dictionary definition, AC is defined as alternatingcurrentnot voltage.So if you want to be pedantic about the dictionary definition, it's always incorrect to use the term "VAC" or "Volts Alternating Current". Current is measured in amps, not volts. What you see displayed on a typical oscilloscope screen is a voltage, not a current, and if it's alternating, then it should be called alternating voltage. Furthermore, multimeters should not use "AC" and "DC" to denote settings that are designed to measure voltages.A mains outlet may be providing current flow in only one direction, even while it's providing voltage that alternates. Would that be called AC or DC? A battery may be providing a steady voltage, but the current may alternate. Is that AC or DC? Or do you need to introduce terms like "AV/DV" for alternating voltage and direct voltage?This is a rathole from which there is no escape, I'm afraid. The pedants lost when we started to use the term AC to refer to voltage.My point is that the terms "AC" and "DC" don't always mean what the dictionary says. In practice, they usually refer to voltages, not currents, though sometimes voltages and currents both alternate. And the term "AC" often refers to voltages which are not constant, but which have a periodic waveform, sometimes regardless of whether that waveform crosses the zero line.I agree wholeheartedly with C4757p: If you have a waveform that is not obviously either DC or AC, describe it instead of arguing about terminology.

author: dannyf
author_rank: Super Contributor
timestamp: December 03, 2014, 03:03:33 am
content: QuoteDMMs have very high input impedance and low capacitance. With no load (resistance or capacitance) after the rectifier, the reading is really undefined - dependent on the meters used.With much lower input impedance, analog meters will provide better consistency.High input impedance also makes the whole circuit suspect to static.

author: Falcon69
author_rank: Super Contributor
timestamp: December 03, 2014, 03:15:47 am
content: Quote from: mstevens on November 30, 2014, 04:54:32 amBut, if you were driving west at 1,675 km/hour (the speed of the rotation of earth), and slow down below that, say, 1670km/hour, then technically, you WOULD be changing direction at a speed of 5km/hour. Even though you would not physically know it.