Convert PWM to Analog out

[Kyle Grathwol]

Does anyone know how to convert a PWM output to an analog output?

Thanks in advance for your time.

 

[CroCop]

What's the PWM coming from?
What's the analog coming from?

Details, as much as you can provide.

 

[Kyle Grathwol]

What's the PWM coming from?
What's the analog coming from?

Details, as much as you can provide.

I need an analog output for a project that I am starting. The components that I am leaning toward using only offer a PWM output. This component is a chip level plc device. This device will need to operate proportional directional control valves. The valve driver I use needs an analog input to send out a mA current to control the valves.

The chip level PLC device needs to send an analog output to the valve driver which in turn will operate the valves. The PLC device I like the best has PWM output only.

 

[jlcann0n]

0-100% duty cycle = 0-100% of current signal

 

[panic mode]

just filter it (even simplest R/C will do) and scale if needed (any signal conditioner...)

 

[Peter Nachtwey]

The obvious answer is to put a RC filter on the output. The next obvious thing is that you need to determine what the value of the resistor and capacitor. The current must be limited to less than what the analog card can supply. Once that is determined the capacitor is chosen so the time constant is long enough to smooth out the PWM signal but not so much that the response is too slow.

Kyle Grathwol, do we have all weekend to get the answer or do you need it ASAP.

What's the PWM coming from?
What's the analog coming from?

Details, as much as you can provide.

The key is to get the right information. What information is that?

 

[Kyle Grathwol]

The obvious answer is to put a RC filter on the output. The next obvious thing is that you need to determine what the value of the resistor and capacitor. The current must be limited to less than what the analog card can supply.

None of this is obvious to me as this is the first time I have been faced with this situation.

The key is to get the right information. What information is that?

The required analog signal is very low mA. It will just be a signal to a valve driver that out puts say a 0 to 1500mA current in proportion to the 0-5v analog signal it is receiving. The PLC device that I want to use can only supply a PWM output signal so I need a method to convert a PWM to analog output. Everything must be small form factor. The PLC device is chip level. Everything must fit in a very small enclosure.

Kyle Grathwol, do we have all weekend to get the answer or do you need it ASAP.

I have until January. Come the first week of January I need to have finalized the component list.

I appreciate your help very much.

 

[Peter Nachtwey]

So how fast is the PWM?
What is the voltage of the PWM?
How fast must the signal be able to go from 0 to 5 volts or from 5 volts to 0?

 

[Kyle Grathwol]

- 6 Channel 16-bit
PWMs (DACs)
- Configurable
Frequency: 35hz to 1.5Mhz

- 12 Channel 16-bit
PWMs (DACs)
- Configurable
Frequency: 35hz to 1.5Mhz

The above lists the specifications for the PWM outputs. As far as speed is concerned, it must be reasonably fast.

Here is a link to the components http://cubloc.com/product/01_01.php

I am also considering using Entertron products and they have a true analog out. I am leaning toward the Cubloc but I like the Entertron product also.

Does anyone have an opinion of either of these products?

 

[milldrone]

The required analog signal is very low mA. It will just be a signal to a valve driver that out puts say a 0 to 1500mA current in proportion to the 0-5v analog signal it is receiving. .

Kyle,

I suspect the 1500ma aspect rules out many options. You might still need a RC network on the input but perhaps something like this will work for you.
http://www.pwmcontrols.com/
Be sure to read the FAQ page and check if the speed is a problem.
http://www.pwmcontrols.com/page9.html

 

[Kyle Grathwol]

Kyle I suspect the 1500ma aspect rules out many options. You might still need a RC network on the input but perhaps something like this will work for you.
http://www.pwmcontrols.com/

Thanks Milldrone. That is a very cool device. However I would still need to have an 0-5 analog output to drive it. I am still unsure of the RC device. Is this something that I have to build or is there prebuilt devices/boards to do this?

 

[Peter Nachtwey]

OK, I understand better

Kyle Grathwol, since you haven't picked the parts yet I suggest you pick the solution with a real analog output. From an engineering stand point I think it is a crime to try to save $100 and waste $2000 woking on a kludge solution unless this solution is going to be used many times so there is a pay back.

The only reason I will go on is if others want to get a small lesson on how low pass filters are implemented in hardware. There must be an educational value and even if I do continue you should still use a real analog output.

Note to others, I have no sympathy for those that need to use a PID and yet buy PLCs without one and then what to know how write their own PID. I have no sympathy for those that use 512 line encoders when they should be using and encoder with much more resolution. Too often I see people buy cheap and then waste a lot of time implementing their cheap solution.

 

[milldrone]

The only reason I will go on is if others want to get a small lesson on how low pass filters are implemented in hardware. There must be an educational value

I'm listening with note book in hand

 

[BITS N BYTES]

A PWM output CAN be used to generate a 0-10VDC output suitable for controlling many devices, have done it many times for speed reference to AC Drives. You need to figure out the impedance of the device you are trying to control and then calculate the time constant [RC] of the circuit that is compatible with the PWM frequency. In most cases 2 resistors and a capacitor [$1 of components] is all that is needed.

BUT, you are trying to vary the current [4-20mAmp] not the voltage with the PWM output. Pretty sure this won't work!

 

[Kyle Grathwol]

Too often I see people buy cheap and then waste a lot of time implementing their cheap solution

I assure you that I am not trying to go the cheap way out.

First size is an issue. The components must be small. Second I hope to build many, many of these devices. Third they must be reliable and withstand harsh environments.

I hope I did not project an image of trying to cut costs or go the cheap way out. I want components that offer good features, flexibility at a fair price. I like the chip level Cubloc that runs ladder and BASIC concurrently. I like their small displays and HMIs. I like Entertrons analog out and customizable components. I may take another look at Devilbiss or Microchip. I have not made any final decisions yet.