DC Duty Cycle

[autotech1975]

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

[langwadt]

if it's  DC the duty cycle is 100%

[autotech1975]

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 .

[macboy]

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.

[kd5iku]

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 this https://www.eevblog.com/forum/testgear/searching-for-a-basic-multimeter-with-max-hold-dutycycle-etc/

Hope that helps!

[autotech1975]

ok I see. I was thinking anything with a posotive voltage was DC, my bad. I learned something new today 
 

[rdl]

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.

[kd5iku]

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.

Huh... 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.

[alsetalokin4017]

 

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.

[Rerouter]

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,

[Performa01]

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).

[mos6502]

https://en.wikipedia.org/wiki/Pulsating_direct_current

https://en.wikipedia.org/wiki/Alternating_current

https://en.wikipedia.org/wiki/Direct_current

Don't mix 'em up!

[HighVoltage]

Get a Fluke 87V, it will do what you are looking to measure.

[mos6502]

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.

[autotech1975]

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.

[R_G_B_]

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

[Tim F]

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.

If 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.

[mos6502]

If 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.

Duty cycle is by definition T_on/T_off. 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.

[wptski]

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's UnIntelligent Four Wheel Drive System back in 2009.

[Tim F]

If 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.

Duty cycle is by definition T_on/T_off. 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.

I 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.

[sdancer75]

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?

[Rerouter]

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.

[HKJ]

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?

This 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.

[sdancer75]

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.

Thanks..... 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,

[sdancer75]

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?

This 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.

>>use two resistors and make a 2.5V point between 5V and gnd

Can 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?