DC Duty Cycle

[HKJ]

>>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...

Price has nothing to do with it, there are cheap meters that are very good at measuring duty cycle, but most requires a zero-crossing.

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?

I cannot remember what meters do what, but you can check my reviews here: https://lygte-info.dk/info/DMMReviews.html

I 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.

[Fungus]

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

There's $20 multimeters that can do that. eg. All those Anengs.

[tggzzz]

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.

The mechanism used to analyse such signals is to consider the AC and DC components separately.

In this case the mean value 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 AC

The 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.

[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.

You mean black probe to +5v and the red probe to what ?? Again to +5v (when the range is between 0 and 5v)

[Rerouter]

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 multimeter https://www.jaycar.com.au/cat-iii-multimeter-with-temperature/p/QM1323

I 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.

[sdancer75]

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 multimeter https://www.jaycar.com.au/cat-iii-multimeter-with-temperature/p/QM1323

I 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.

Thank 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

[soldar]

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 .

Regardless 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.

[sdancer75]

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 .

Regardless 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.

Thanks, 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.

[soldar]

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 are assuming a 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.

[sdancer75]

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 are assuming a 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.

Thanks,

I don't have an analog meter.... I will check the PWM driver to get some conclusions!

Regards,

[Fungus]

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.

They're the same thing.

[sdancer75]

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.

They're the same thing.

Yes they are the same, I mean when I was talking about DC waves that they are not crossing 0V. Thanks !!

[Fungus]

Yes they are the same, I mean when I was talking about DC waves that they are not crossing 0V. Thanks !!

"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.

[sdancer75]

Yes they are the same, I mean when I was talking about DC waves that they are not crossing 0V. Thanks !!

"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.

You have right !! Thank you ! You are the best :-)