PWM Output


Thread Starter


I am not familiar with PWM analog outputs, and I wonder if they are compatible with 0-10V or 4-20mA signals.
I am looking to control a frequency AC drive controller with a PLC that is providing only PWM output.
I will be gratefull for all tips and tricks and what to think about to get this working without damaging any hardware.

Hakan Ozevin

I don't think that there is a VFD which accepts PWM signal. A VFD *produces* PWM power output, but normally they look for a 0-10V or 4-20 mA signal for the speed setpoint.

Pro Engineer

This is something that I have experimented with in the past, using a PWM output you could using suitable signal conditioning obtain a 0-10Vdc reference for your VSD.
The PWM waveform allows for a variable mark to space ratio, i.e the ratio of on to off time can be made variable between 0 to 100% at whatever frequency you choose, providing this waveform is damped using a suitably calculated CR network you could then obtain a dc level for your control.

Secondly depending on the PSU you are using you may need to clamp the voltage using for example a zener stabilizer at 10 volts.

I have used this method of control in various speed control applications, it allows a reasonably good method of analogue control from an inexpensive digital source.

In the case of PLC control systems it can provide considerable cost savings due to the use of standard transistor digital ouputs against analogue output modules.

If you require further info I may be able to provide you with some examples of typical component values and suggestions for switching frquencies, the former being essentially determined by the required response time of your VSD to cahnges in load etc.

It would be great if you can provide me with further info! As I said before, all help would be VERY thankfull, and it would help me alot!
First, a PWM signal is a digital signal. And as already mentioned the trick is to make it analog by a suitable RC-network. With other words the RC-network is a low pass filter, best of a higher order to have a sharp cut-off frequency. Concerning optimal control the important question is: what is the period of the PWM signal. Because the time constant of your RC-network depends mainly on the PWM-period. The RC low pass filter acts as an additional lag element in your control loop. And as higher the time constant of your filter is all the more the dynamic of your control loop has to be reduced, mainly by reducing the amplification (proportional) factors of your controller.
I have used PWM with Mitsubushi PLC's controlling ABB ACS Drives. It is a cheap & effective way to set speed with a drive (no need for analogue cards). You have to ensure that you have a PLC with transistor outputs because of the high switching speeds (pulses) (PWM is a digital output)
You also usually have to use a pull up resistor connected in series from the output as well for the drive to recognise the signal. Screening the cable to the drive is also essential. Good Luck
The best option would be a convertor designed for this purpose, available from a variety of sources (I'd suggest a google search, e.g., something like "pwm analog"). Such a convertor typically has a time-base setting to match the cycle time of the PWM source, and some can select either voltage or current-loop output. Some drives may have this available as a control option.

Conceptually the conversion isn't very complicated, and with sufficient inspiration you could build a solution from components. If the PWM cycle time is sufficiently fast, and the driven device's analog input response sufficiently slow, you might use a capacitor in parallel with a load resistor to smooth the PWM signal to a gross analog representation. Additional buffering, filtering, and level-shifting would likely be needed, though, along with a good understanding of the source and load electronics.


Ken Irving <jkirving(AT)>
Hello Sverre, and thank you for posting using a real name! (I've started to respond to Mr or Ms Anonymous a few times lately only to realize they
aren't willing to offer up to any sort of identity, so <ploink> goes any response.)

What kind of further info do you want beyond that already offered?

In my opinion it would help immensely if folks would post an email address so that trivial or boring details might be offered and hashed over offlist, where appropriate. Perhaps it's not obvious that on this list personal email addresses are removed (from the header), so offlist responses are not an option unless the poster takes that extra step (of including an address in the message).


Ken Irving ( jkirving(AT) )
Hello Ken!
Thanks for your answer!
I think I have the info I need right now, I will do a search and try to find the equipment.
If you want to mail me my address is chan(at)

I am also a member in Experts Exchange ( and they have a good solution to give something to that person that provide you with a good answer, I miss that oportunity here!

But anyway, Thanks alot!

Marc Sinclair

I used frequency control between siemens s7-200 and danfoss before I discovered USS. the Danfoss 2800 range will accept a frequency of say
0 - 2500hz which would equate to 0-100% of the drives maximium frequency. (incidentally, these drives, and the micromaster range from siemens will talk USS which gives you total control and monitoring., no inputs or outputs used!)
Hello, can you please send any info (or link where to find it) concerning s7-200 and Danfoss VLT2800 via USS protocol.

Tnank you in advance.

Marc Sinclair


If you study the 2800 manuals you will find that the drives communicate using two protocols, danfoss FC and profidrive. Danfoss have told me that the profidrive protocol over RS485 will work with USS. The manual you need is mg28e302.pdf, It
should be available through the danfoss site, or I can send you a copy. Be warned, I tried to implement this system, and gave up because no-one at danfoss in UK, could give me technical backup. I do know of an S7-200 controlling the danfoss 5000 range using USS, so I feel sure that
you can make it work. I have designed and built many systems using USS. If I can help with s7-200 code please contact me through

regards Marc Sinclair