Reading PWM with a DMM? - Page 1
Forum section: electronics / beginners

author: tommythrash
author_rank: Contributor
timestamp: March 10, 2014, 05:46:30 am
content: I install vehicle hand controls for people with disabilities so they can drive. Some of these systems interface with the vehicles electronic pedal. Most of these pedals contain a pair of potentiometers that have an output signal reading of under 1v to 5v DC. I am aware that some vehicles use a PWM signal out of the pedal. I normally just hook up my DMM and test for DC voltage on each wire of the pedal to find the output signal. I have yet to run into a vehicle with a PWM signal but need to know what I will see with my DMM when set to DC voltage. I know I would need an oscilloscope to accurately analyse the signal. I do not need to analyse it, I just need to identify that it is not a normal potentiometer and is most likely a PWM signal. All I will have is a basic DMM to test and will only tap onto the connector wires with the DMM leads while connected to the pedal and the vehicle ignition on then depress the pedal to see the varying signal. Any help would be appreciated. Thank you.

author: arcom
author_rank: Contributor
timestamp: March 10, 2014, 05:50:33 am
content: I think the quickest way would be to measure the frequency or duty cycle of the pedal signal, if you have those measurements on your DMM.

author: Rerouter
author_rank: Super Contributor
timestamp: March 10, 2014, 06:08:58 am
content: if your concerned, make up a low pass filter on a little piece of veroboard, this way the PWM is smoothed into the average DC, your multimeter shouldn't care but if your concerned this would definatly cover your bases,

author: tommythrash
author_rank: Contributor
timestamp: March 10, 2014, 06:19:57 am
content: My question is, what will a PWM signal look like on my DMM when set to VDC?

author: nerdyHippy
author_rank: Contributor
timestamp: March 10, 2014, 06:28:58 am
content: That'll depend on your multimeter. Best to test it with a known signal and find out. As Rerouter points out, you could also lowpass the signal which might give you more consistent readings.

author: pickle9000
author_rank: Super Contributor
timestamp: March 10, 2014, 06:38:59 am
content: It will depend on the meter and on the frequency. My meter is a true rms and shows frequency so on my meter I would have a pretty good indication that a pwm is present. On a meter which is not true rms the meter could range trying to lock on to a voltage, the display would not settle. If you have a true rms (on the dc) it could have the same ranging problem or show you an actual voltage.Sorry I know not much help. If you do this a great deal get or make a simple logic probe that has a H L and Pulse on it, make sure it can handle the voltages you are dealing with. It's a little old school but should do the trick.

author: pickle9000
author_rank: Super Contributor
timestamp: March 10, 2014, 06:40:51 am
content: Like this.http://www.ebay.ca/itm/EISTAR-Digital-Logic-Probe-DTL-TTL-Circuit-Tester-Pen-DC-18V-Max-/300974582913?pt=LH_DefaultDomain_0&hash=item46137bac81

author: tommythrash
author_rank: Contributor
timestamp: March 10, 2014, 06:46:54 am
content: I have a fluke 87v. The reason I am asking this is I'm going to be training others to install this equipment and they may not have the same meter or be smart enough to figure out that it might be a PWM signal. I will be telling them to check the wires on the pedal with their DMM and find the signal wires. This is easy with a regular pedal that contains potentiometers. I need to find out what they might see if it is a PWM signal. It is not common otherwise I would find one and test it myself to see what they would see. I just need to know an approximation of what the DMM would do if set to VDC and you try to read a PWM signal. The PWM should use 5vDC

author: tommythrash
author_rank: Contributor
timestamp: March 10, 2014, 06:50:24 am
content: Thank you pickle9000

author: Rerouter
author_rank: Super Contributor
timestamp: March 10, 2014, 07:08:27 am
content: Out of curiosity which vehicles have you heard use PWM?, as i professionally kit up pedal controllers and have yet to see them in Mitsubishi, Toyota, Hino or Isuzu (well there is one odd model of Mitsubishi but I call that an anomaly,as its not pwm but AC coupled data on the analog signal)equally my comment about using a low pass filter is almost exactly what will be on the ECU side of the pedal controls, and will still get you the voltages necessary to replicate with your own controller, to test for yourself hook your multimeter up to mains on DC volts, it will read very close to 0V, and this is only a 50/60Hz signal, where as any vehicle PWM would be at-least 500Hz to be reliable and not jump about from averaging, meaning as-long as its faster than mains be it a post true RMS or a $5 El-crapo cheapie meter its reading should be very close to the average and more than enough to give you the info you need,

author: tommythrash
author_rank: Contributor
timestamp: March 10, 2014, 07:27:43 am
content: I am not exactly sure what models came with PWM pedals. I did a little research and it looks like early 2000's volvos and I have heard some other euro makes used it. The interface that comes with the electronic hand controls we install have dedicated wires for interfacing with a PWM pedal both signal and separate ground aside from the signal and ground wires for the regular potentiometer type. Also the programmer can read the signal when the interface is connected and then output the PWM signal in place of the pedal. ( the system uses a pot on the steering wheel the user pushes in place of the floor accelerator pedal. the pot is wired into the systems interface and that interface it put between the floor pedal and vehicle wiring that runs to that pedal. The system interface is now programmed to output the same signal the floor pedal outputs when depressed.)

author: pickle9000
author_rank: Super Contributor
timestamp: March 10, 2014, 07:34:14 am
content: I would say that a logic probe would be a simple solution for anyone being trained.- connect ground to logic probe.- connect 5v to logic probe (this is what powers the probe)- probe suspected wire for pwm, if pulse light flashes you have located the pwm.Pretty simple / reliable.

author: darrenb
author_rank: Contributor
timestamp: March 10, 2014, 11:42:41 pm
content: I have a Fluke 87V as well and I also have a Rigol DS1102E which conveniently outputs a 3V 1kHz square wave out the front on it's test point. Effectively the same as a 3V pwm at 50% duty.Using the Fluke to measure the output here's what I get..on DC Volts it measures 1.512V.on AC Volts it measures 1.506V.on DC Volts when set to Hz mode it reads 1000.0 Hz but interestingly it only does this when connected in the correct polarity. When the leads are reversed (in straight DC mode it reads -1.512V) the Hz measurement shows 0.on DC Volts when set to % mode it reads 50.0% but again needs to be connected with the correct polarity to work.In summary you could use your DMM to detect this specific pwm signal and differentiate it from a straight voltage, but you would probably have to try it with other signals that you might find before handing it over to others to use.