DC Duty Cycle - Page 1
Forum section: electronics / testgear

author: autotech1975
author_rank: Contributor
timestamp: February 16, 2016, 10:18:48 pm
content: Hello I am new here and was wondering does anyone know of a multimeter that does DC volts duty cycle and doesn't cost over $100. I use this feature in automotive repairs and have come to find most meters only measure the duty cycle on ac volts. Thanks for any info.Don

author: langwadt
author_rank: Super Contributor
timestamp: February 16, 2016, 10:23:57 pm
content: if it's DC the duty cycle is 100%

author: autotech1975
author_rank: Contributor
timestamp: February 16, 2016, 10:41:56 pm
content: for example I have a square wave say 0 to 12 volts dc , pulse width modulated , I want to see the duty cycle. this is not 100 percent .

author: macboy
author_rank: Super Contributor
timestamp: February 17, 2016, 12:24:52 am
content: To be pedantic, a PWM signal is not truly DC. You can use the AC range duty cycle measurement and it should tell you the correct result (unless the meter is severely flawed or broken). Some meters also give min and max readings on the AC range, so you could then see that the min is close to 0 V and the max is close to ~ 12 V. The actual AC volts reading will depend on the duty cycle (and peak voltage) at any given time and will not be so useful.

author: kd5iku
author_rank: Newbie
timestamp: February 17, 2016, 12:38:36 am
content: Duty cycle is an AC term, as macboy said below. Pulse width modulated signals are AC because the voltage changes, or alternates between one state and another. DC is, well, direct current, not alternating.Any meter that does AC duty cycle should work for your application. There is actually already a thread on thishttps://www.eevblog.com/forum/testgear/searching-for-a-basic-multimeter-with-max-hold-dutycycle-etc/Hope that helps!

author: autotech1975
author_rank: Contributor
timestamp: February 17, 2016, 02:21:26 am
content: ok I see. I was thinking anything with a posotive voltage was DC, my bad. I learned something new today

author: rdl
author_rank: Super Contributor
timestamp: February 17, 2016, 02:58:38 am
content: Alternating current (AC), is an electric current in which the flow of electric charge periodically reverses direction, whereas in direct current (DC, also dc), the flow of electric charge is only in one direction.PWM DC changes the amplitude of the voltage, it doesn't change direction. It isn't AC.

author: kd5iku
author_rank: Newbie
timestamp: February 17, 2016, 06:46:30 am
content: Quote from: rdl on February 17, 2016, 02:58:38 amHuh... I never thought about it like that... By that definition then, the output of a class B amplifier wouldn't be AC either. Well shoot dang! What about an offset sine wave then? I seem to recall my textbooks saying that was an AC signal. Heck, you use AC voltage on your meter to measure the RMS of a square wave.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: February 17, 2016, 09:02:37 am
content: A "square wave" is different from a pulse train. Certainly a "square wave" is AC since it has both positive and negative peaks. A signal that alternates between Ov and, say, +12 V is a DC pulse train. It's not AC, it's not _continuous_ DC but you can call it DC pulses if you like. It has a duty cycle, which is the proportion of the total period that the signal is High. "Duty Cycle" is a term that can apply to both AC signals and DC pulse trains. A good DMM like a Fluke 83 or 87 can report duty cycles and frequencies for both AC and DC ranges.Offset sine waves... yes, that's a gnarly issue. If the sine wave is so offset that it does not cross zero, then how could it be "AC" ? It would not produce an alternating current direction in a load. Yet many engineers and texts will call it that. AC with DC offset greater than the peak AC value... it gets kind of clumsy. A sinusoidally varying DC voltage is not AC, strictly speaking. But if you have, say, a DC output of a power supply sitting up at, say 40 volts, but it has a ripple on top of it of, say, 1 volt... and you look at it with an AC-coupled scope channel.... you see an AC signal that represents the ripple with the DC value filtered out. Is this truly "AC" in the sense of alternating current? Even though it now crosses the zero baseline symmetrically? Obviously some components/circuits will respond to the AC part as if it were "really" AC and ignore the DC part. So even though in a strict sense a sinusoidally (or other waveform) varying DC signal may not be "AC" it still might "act" like AC in some cases.Some of this stuff can really be over-thought.The OP wants to measure the duty cycle of a DC pulse train (probably what we used to call "Dwell", but in terms of percent rather than angle.) If the AC "percent" setting on his meter gives the correct answer... then use it. Trust... but verify.

author: Rerouter
author_rank: Super Contributor
timestamp: February 17, 2016, 09:22:27 am
content: the duty cycle measurement of the generic multi meters sold at jaycar electronics have worked fine for me, for PWM only tested at about 5Khz myself though.Did make my own tool that could do it to 500Khz, but its accuracy wasn't that crash hot past 200Khz, (ran out of timer ticks) this was literally an arduino, 2 op amps and a few passives, and i've shared that circuit on this forum a few times now,

author: Performa01
author_rank: Super Contributor
timestamp: February 17, 2016, 10:21:49 am
content: The 'A' in AC stands for 'alternating', not 'alternating direction'.DC is strictly stationary.If a waveform is not symmetrical with regard to zero, then it is a mix of DC and AC, also known as AC with DC offset.This is the whole point of PWM. We use a lowpass filter to get rid of AC (or the filtering is inherent to the load, because it simply cannot follow fast signal variations, like incandescent lights or motors). After that, there's only the DC offset left to power the load.By changing the duty cycle, we also change the DC offset. Since it is an unidirectional signal, the DC offset is proportional to the duty cycle, which is - once again - the whole point of PWM.If the load doesn't ignore the AC portion of the PWM, hence an explicit lowpass filter is fitted, the signal for the duty cycle measurement has to be taken from before that filter, obviously.If a meter only measures duty cycle in AC-coupled mode, it could not distinguish between 0% and 100% duty cycle. So I would think that any meter that has that measurement capability will have to do it in AC+DC coupling mode. This same mode would also be advertised for the true RMS measurement (as an alternative to AC only coupling).

author: mos6502
author_rank: Frequent Contributor
timestamp: February 17, 2016, 11:28:48 am
content: https://en.wikipedia.org/wiki/Pulsating_direct_currenthttps://en.wikipedia.org/wiki/Alternating_currenthttps://en.wikipedia.org/wiki/Direct_currentDon't mix 'em up!

author: HighVoltage
author_rank: Super Contributor
timestamp: February 17, 2016, 12:04:32 pm
content: Get a Fluke 87V, it will do what you are looking to measure.

author: mos6502
author_rank: Frequent Contributor
timestamp: February 17, 2016, 12:57:59 pm
content: Actually ... every digital multimeter can measure duty cycle, you only have to know the voltage level. Since a digital multimeter shows the average of the signal, duty cycle is directly proportional to the voltage displayed. So if you have a 5V signal and your meter shows 2.5V, you know the duty cycle is 50%. 1.25V - 25%, and so on. If you don't know the voltage level, you can measure it using a Schottky diode and a capacitor.But if you want a no hassle % display, you don't need a $400 multimeter. Even the cheapest multimeters can measure duty cycle, e.g. the Uni-T UT136.

author: autotech1975
author_rank: Contributor
timestamp: February 19, 2016, 03:20:27 am
content: thanks for the help. I am use to using the scope for this usually and that was set to dc but I see now that the signals I'm looking at are pulsed dc but can be read using ac on the meter. I will verify with the scope but again thanks for the responses. I appreciate it.

author: R_G_B_
author_rank: Frequent Contributor
timestamp: February 19, 2016, 09:16:56 pm
content: What you could use is a power graphing multi metre, such as the snap on vantage mt2400.It will graph voltage, current, resistance, duty cycle, frequency, RPM, vacuum, pressure, pulse width. All these can be plotted over time set to a period of upto 16 hours.It will capture a glitch with in the min max values plotted over time.R_G_B

author: Tim F
author_rank: Regular Contributor
timestamp: February 20, 2016, 06:24:21 am
content: Quote from: rdl on February 17, 2016, 02:58:38 amIf you have a PWM signal that has a low level of 0V and a high level of some positive voltage then the signal has both an AC and DC component. If the low level is 0V and the high level is 5V and the duty cycle is 25% then the DC component is +1.25Vdc and the AC component has a low level of -1.25V and high level of +3.75V.When you use a multimeter's 'AC' duty cycle function the signal is AC-coupled. The DC component of the waveform is therefore removed. Doing this will always result in the 'low level' being <0V and the 'high level' being >0V hence the multimeter does not need to know what the high and low levels of the original signal were, it just simply gives the percentage of the time that the AC-coupled signal is positive.

author: mos6502
author_rank: Frequent Contributor
timestamp: February 20, 2016, 06:54:50 am
content: Quote from: Tim F on February 20, 2016, 06:24:21 amDuty cycle is bydefinitionTon/Toff. It's a DC source being turned on and off. If you AC couple it, it's no longer a PWM signal, because there is no on/off anymore. AC coupling makes no sense with a PWM signal.

author: wptski
author_rank: Newbie
timestamp: February 20, 2016, 01:24:25 pm
content: Below is from a Fluke 87V manual and the second is from a Fluke 120 scope looking at the pulsed 12V output from the 4WD module to the rear electro-mechanical clutch on Ford'sUnIntelligent Four Wheel Drive System back in 2009.

author: Tim F
author_rank: Regular Contributor
timestamp: February 24, 2016, 06:32:03 am
content: Quote from: mos6502 on February 20, 2016, 06:54:50 amI think you mean Ton/(Ton+Toff)And by definition, if you AC couple that signal, 0V will be somewhere in between Von/Vhigh and Voff/Vlow, thus the multimeter does not need to know what the absolute values of Von and Voff are in order to measure the duty cycle - it simply only has to measure the percentage of the time that the voltage is positive.Obviously if you are near 100% or 0% duty cycle, or your signal does not have well defined levels, rising and falling edges a multimeter may have trouble measuring the duty cycle on AC-coupling. Simply measuring the DC voltage and knowing what the 'on' and 'off' voltages are may be more accurate then.

author: sdancer75
author_rank: Frequent Contributor
timestamp: May 11, 2019, 01:09:03 pm
content: So, to really understand..... When my DMM declares "% Duty Cycle Note: Duty Cycle measurements are AC only for this meter. Measurement signals must be ‘zero crossing’ and must reach ‐0.7V for the meter to properly display Duty Cycle %. " it means that alternating voltages between 0 and +5V are outranged from my DMM?

author: Rerouter
author_rank: Super Contributor
timestamp: May 11, 2019, 01:33:11 pm
content: It sounds like that, Did not know any multimeters existed with such a poorly implemented duty cycle measurement mode, that implies its using a PNP transistor for a duty cycle buffer, and likely wont give any reading for a 0-5V signal with the ground lead referenced to ground,Lucky for you, multimeter leads can easily be reversed, so move your ground lead to a 5V rail, and it will work, as the signal then falls atleast 0.7V under the "ground" or black multimeter lead.

author: HKJ
author_rank: Super Contributor
timestamp: May 11, 2019, 04:39:02 pm
content: Quote from: sdancer75 on May 11, 2019, 01:09:03 pmThis is fairly common, use two resistors and make a 2.5V point between 5V and gnd, then you can use that for your multimeter to measure duty-cycle.There is also large difference between how good meters are at duty-cycle, some can show from 1% to 99% at 100kHz with 1Vpp, but there are many that cannot and the best meters are not the most expensive ones.

author: sdancer75
author_rank: Frequent Contributor
timestamp: May 12, 2019, 09:37:52 am
content: Quote from: Rerouter on May 11, 2019, 01:33:11 pmThanks..... it's strange because my DMM is Extech ex355 which is not a cheap Chinese DMM. At page 20 in the user's manual (http://www.extech.com/resources/EX35x_UM-en.pdf), it says exactly that.To clarify your solutions. I need for example to measure an Arduino PWM output, let's say from digital channel 6, which outputs a voltage range between 0 and +5V. Where should I put the probes to get a correct measurement of the duty cycle?Regards,

author: sdancer75
author_rank: Frequent Contributor
timestamp: May 12, 2019, 09:42:51 am
content: Quote from: HKJ on May 11, 2019, 04:39:02 pm>>use two resistors and make a 2.5V point between 5V and gndCan you give me a schematic example of how to do this? And why this solution works in my case?As I mentioned above, my DMM is Extech Ex355 which is not a cheap one...So, can you suggest a better DMM either from Extech or any other manufacturer (except Flukes which are too expensive for my budget) that have better features than mines?

author: HKJ
author_rank: Super Contributor
timestamp: May 12, 2019, 10:04:00 am
content: Quote from: sdancer75 on May 12, 2019, 09:42:51 amQuote from: sdancer75 on May 12, 2019, 09:42:51 amPrice has nothing to do with it, there are cheap meters that are very good at measuring duty cycle, but most requires a zero-crossing.Quote from: sdancer75 on May 12, 2019, 09:42:51 amI cannot remember what meters do what, but you can check my reviews here:https://lygte-info.dk/info/DMMReviews.htmlI test duty-cycle and frequency counter function, the upper limit I test is 1V & 100kHz, then I increase voltage and/or reduce frequency until I get something usable. If it says it needs a zero-crossing for frequency, it also needs it for duty cycle.Brymen meters can be used, their trigger point is above 1V, but they have some restrictions on signal level and frequency.

author: Fungus
author_rank: Super Contributor
timestamp: May 12, 2019, 11:44:00 am
content: Quote from: autotech1975 on February 16, 2016, 10:18:48 pmThere's $20 multimeters that can do that. eg. All those Anengs.

author: tggzzz
author_rank: Super Contributor
timestamp: May 12, 2019, 01:16:24 pm
content: Quote from: rdl on February 17, 2016, 02:58:38 amThe mechanism used to analyse such signals is to consider the AC and DC components separately.In this case themeanvalue is the DC component; this will vary with the duty cycle. Obviously once the concept of mean average is introduced, the period over which it is averaged has to be considered. For some purposes any period shorted than days is considered ACThe AC component is superimposed (added to) the DC component. The peak-peak value will be unchanging, but the RMS value depends on the duty cycle.

author: sdancer75
author_rank: Frequent Contributor
timestamp: May 12, 2019, 03:52:06 pm
content: Quote from: Rerouter on May 11, 2019, 01:33:11 pmYou mean black probe to +5v and the red probe to what ?? Again to +5v (when the range is between 0 and 5v)

author: Rerouter
author_rank: Super Contributor
timestamp: May 13, 2019, 01:35:28 am
content: My guess on the internals is that its not looking for true 0 crossings but acting as a comparitor for 0.7V below the black lead. So my assumption is that black lead to 5V and red lead to signal should give you a duty cycle measurement.Personally i use this multimeterhttps://www.jaycar.com.au/cat-iii-multimeter-with-temperature/p/QM1323I work automotive. So i dont have to worry about mains. But it can measure frequency and duty cycle down to 100mV pk-pk. No matter what the DC offset. This is why i would call the requirement of a fixed negative offset a bad implementation.

author: sdancer75
author_rank: Frequent Contributor
timestamp: May 13, 2019, 08:00:00 am
content: Quote from: Rerouter on May 13, 2019, 01:35:28 amThank you for your quick response.I will check it out with PWM of arudino and I will let you know. I wish it will work either as well as the solution with the two resistors HKJ recommented. Its a shame that ex355 have such restrictions !!Best Regardss

author: soldar
author_rank: Super Contributor
timestamp: May 13, 2019, 10:14:20 am
content: Quote from: autotech1975 on February 16, 2016, 10:41:56 pmRegardless of the nitpicking I understand what you want.If it is a DC rectangular signal then the duty cycle is the average value divided by the peak value.If the peak value can be read directly, say at the battery, then there you have it. If it cannot be read directly then you would need a circuit that would measure it. Do you have access to the voltage as DC and before it is chopped up? Is that voltage very stable so we can assume it is always 12 volts? Or is it something between 10 and 14? Because that changes things.If the multimeter reads average value then just divide the average by the peak and that will give you the duty cycle (between zero and one). Analog (needle) meters would measure average value but the AC scale con be printed to read sine RMS or whatever. Check to see what your meter reads. Take a 12 volt DC and chop it at 50% duty cycle. The average is 6 V. What does your meter read?If this is for occasional measuring then that would work. If it is for something frequently used and you want something quick and fast then you could easily build a meter which would directly indicate duty cycle (between zero and one) which I do not think simple multimeters would do.

author: sdancer75
author_rank: Frequent Contributor
timestamp: May 13, 2019, 01:39:41 pm
content: Quote from: soldar on May 13, 2019, 10:14:20 amThanks, man !! I will try it.Usually, I don't use the Hz/Duty cycle but recently I have some problems with the cabin air ventilation of my car (Volvo s40) and the driver uses a PWM to control the motor heat/cold adjustments. I still don't know right now if it uses a DC or AC waves. I will check it and I will let you know.

author: soldar
author_rank: Super Contributor
timestamp: May 13, 2019, 03:47:53 pm
content: If you have an analog meter then the needle has enough inertia that it should indicate average voltage. With digital meters I think it should also indicate average voltage but you might need to confirm.Note that we areassuminga good PWM signal. If you are having other problems like bad connections, low voltage, etc, then you would really need a 'scope to see what's happening.

author: sdancer75
author_rank: Frequent Contributor
timestamp: May 13, 2019, 04:19:45 pm
content: Quote from: soldar on May 13, 2019, 03:47:53 pmThanks,I don't have an analog meter.... I will check the PWM driver to get some conclusions!Regards,

author: Fungus
author_rank: Super Contributor
timestamp: May 14, 2019, 06:22:49 am
content: Quote from: sdancer75 on May 13, 2019, 01:39:41 pmThey're the same thing.

author: sdancer75
author_rank: Frequent Contributor
timestamp: May 14, 2019, 06:25:50 am
content: Quote from: Fungus on May 14, 2019, 06:22:49 amYes they are the same, I mean when I was talking about DC waves that they are not crossing 0V. Thanks !!

author: Fungus
author_rank: Super Contributor
timestamp: May 14, 2019, 07:03:50 am
content: Quote from: sdancer75 on May 14, 2019, 06:25:50 am"0V" is a very relative thing. I try to never write "0V" in the context of a circuit, I always draw the little ground symbol instead.If a measurement isn't working out because of your multimeter you can use couple of resistors to create a reference point that's at (eg.) 1V to connect the COM terminal to. Or just connect it to something else in the circuit that you know isn't at ground level, eg a LED.

author: sdancer75
author_rank: Frequent Contributor
timestamp: May 14, 2019, 03:19:10 pm
content: Quote from: Fungus on May 14, 2019, 07:03:50 amYou have right !! Thank you ! You are the best :-)