Author Topic: 24V PWM 400hz to 0-10V Analog  (Read 1873 times)

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Offline witty

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24V PWM 400hz to 0-10V Analog
« on: January 05, 2016, 06:46:29 am »
Hi,

I have a 400hz PWM 24VDC I would like to read on an analog input of 0-10V (high sample rate, high input impedance).

I first thoughts were to make a voltage divider of 1K4 and 1K in series to ground to drop the voltage to 10V from 24V then use a 1uF in parallel with the 1K resistor.
I would take the output across the 1K/1uF. I have a feeling this won't work and I don't have the hardware to test yet (should arrive in a few days).

I saw some diagrams like this going the other way to amplify the 5V output signal, https://www.eevblog.com/forum/projects/0-10v-led-driver-control-with-microcontroller/

What is the most appropriate solution for this problem?

Thanks.
 

Offline Seekonk

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Re: 24V PWM 400hz to 0-10V Analog
« Reply #1 on: January 05, 2016, 07:48:03 am »
The last solution is the most correct.  To be linear the source impedance should be at least ten times less than the resistor feeding the filter capacitor.  So feeding the capacitor with 47K would likely work if the input voltage is stable.   Then use a voltage divider to get the 1-V with a divider where toe two divider resistances total at least  to get the 10V accounting for some loss and buffer with unity op amp.
 

Offline pmbrunelle

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Re: 24V PWM 400hz to 0-10V Analog
« Reply #2 on: January 05, 2016, 05:30:03 pm »
With a 1k / 1.4k voltage divider you actually have less DC output resistance (583R) that that 680R resistor...

I would do the divider and parallel capacitor if possible... seems much simpler (less parts).

However, the cutoff frequency for the values you calculated is 273 Hz; much too high. Maybe use 100 uF instead.

Also seems a little borderline for 1/4 W resistors... might be good to bump them up to 1/2 W parts.
« Last Edit: January 05, 2016, 05:36:16 pm by pmbrunelle »
 

Offline Seekonk

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Re: 24V PWM 400hz to 0-10V Analog
« Reply #3 on: January 05, 2016, 06:29:33 pm »
Just remember that the charge resistance has to be the same as the discharge resistance to the capacitor.  Something many in that other thread ignored.
 

Offline witty

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Re: 24V PWM 400hz to 0-10V Analog
« Reply #4 on: January 05, 2016, 11:08:16 pm »
Hi guys, thanks for your replies; much appreciated.
If you think the divider will work with higher rated resistors and a larger cap then that is simpler; the "push-pull" output PWM is good for 450mA according to the datasheet.
I'm not quite sure what you mean by the charge resistance has to be the same as the discharge resistance to the capacitor; my intention is attached.
 


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