# PWM to Analog Voltage

Discussion in 'General Electronics Chat' started by Miramax, Apr 15, 2009.

1. ### Miramax Thread Starter Member

Nov 12, 2006
22
0
I am looking to convert a PWM signal to to analog voltage to drive a LED 10 segment bar graph.
I found this handy number here http://www.ontrak.net/pwm.htm and it looks pretty easy, its a low-pass RC filter made with a resistor and cap.
I want to drive a LM3914 and 10 led bar graph with the analog voltage to indicate what the duty cycle is. 0 leds lit = 0% duty ~ 10 leds lit = 100% duty.
If the PWM is at 1KHz, what size resistor and cap is needed?
Sorry for the noobishness, I've been learning tons from you guys so far
Thanks again

Apr 5, 2008
15,806
2,389
Hello,

You could make a lowpass filter that has a 10 - 20 times lower frequency as the PWM signal.
So a filter between 50 Hz and 100 Hz will work for the 1KHz PWM.

Greetings,
Bertus

3. ### Miramax Thread Starter Member

Nov 12, 2006
22
0
How would one go about doing this? Same circuit just different values for the cap and resistor right?
I need to do some more studding

4. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
718
The standard Value of 22uF, replacing the 1uF, and a 47 Ohm resistor in place of the 4.7k would give around a 150Hz Lowpass Cutoff in the circuit posted above. 100 Ohm would move it down to around 100Hz.

5. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
Not so good; that'll result in high current in the resistor to charge/discharge the capacitor, and high current in the output of the device.

Go fish.

Change the cap back to 1uF (or smaller) and increase the resistance.
[eta]
The existing circuit has a low-pass of around 30Hz. If you just decreased the 4.7k resistor to somewhere between 1.5k and 2k, you'll be in the ballpark of where you want to be.

Last edited: Apr 15, 2009
6. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
718
My apologies, I forgot the application. I was thinking "output"/low impedance.

Sarge is up about 3 on me now.

7. ### SgtWookie Expert

Jul 17, 2007
22,183
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I've pulled more than my share of doozies on here.

Yes, if our OP was looking to drive a speaker or something, then they'd need a low impedance output. But when driven by a microcontroller, keeping the uC's I/O limits in mind has to be a pretty high priority.

a 1.5k resistor will give a max source/sink current of around 3.33mA; easy for it to handle even with many other I/O loads.

Nov 12, 2006
22
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9. ### italo New Member

Nov 20, 2005
205
1
if you bother to go to MAXIM.com they have all kinds of chips to drive LED. I just got two free samples to try it.

10. ### Miramax Thread Starter Member

Nov 12, 2006
22
0
Well basically I am trying to make a duty cycle meter using 10 LED's