# Need Help regarding PWM Circuit

Discussion in 'The Projects Forum' started by sagardewani, Aug 1, 2012.

1. ### sagardewani Thread Starter New Member

Aug 1, 2012
5
0
I want to build a simple PWM Circuit using Op-Amp which takes +15 -15V input and gives a square wave output 0 to +15V whose frequency should be variable and may range from few Hz to KHz/MHz and the duty cycle should also be variable.

If any one can provide any help or any schematic diagram then I would be quite thankful..

Regards..

2. ### stanman11 Member

Nov 23, 2010
230
4
And the to figure out the amp, download the data on it and look at some other already made schematics to figure out how it works.

You find the datasheet by typing in the numbers on the amplifier.

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3. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
It doesn't use an op amp, but a simple 555 and comparator. The frequency is independent of the PWM, and the PWM is capable of covering the full range (0% - 100%). It can be a voltage - PWM converter, though it is shown using a voltage divider as the input. The output can be beefed up with not too much trouble, if you are interested.

If all you need is a 0 to +15VDC output then all you need is a =15VDC power supply. Requiring a dual power supply for a circuit that outputs a single does not make sense.

LEDs, 555s, Flashers, and Light Chasers - Chapter 5, The 555 and PWM, Figure 5.3.

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4. ### Burnit0017 Member

Feb 15, 2010
101
3
Hi, this is the circuit I am using. It only has a variable duty cycle, but the same method could be used on the first 555 timer to vary the frequency. Hope this helps.

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5. ### sagardewani Thread Starter New Member

Aug 1, 2012
5
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Our teacher told us that we have to generate DC output from 0 to +15V using TL072 Op-Amp... He said input to be fed would be saw tooth wave which either u could generate by making additional circuit for it or you can use function generator available in the lab at the time of demonstration.

He further said, all you would need is TL072 op-amp, resistors, capacitors, 2 variable resistors (1 for varying frequency and other for varying duty cycle) and signal diodes. And ya he also said, if you want to generate saw tooth wave too then you will either need to use two op-amps or a dual op-amp.

Now if u can suggest me how to do it with above mentioned components I would be grateful to you..

Note: I'm a beginner in this field as I haven't even learned op-amps and therefore do desperately require someone's help to accomplish the task within due time..

Regards,
Sagar

6. ### wayneh Expert

Sep 9, 2010
12,388
3,245
Time to learn about op-amps ASAP! An op-amp can be wired to perform as a comparator, the same role as the LM393 in the schematics shown. Use the lab function generator unless/until you have time to build your own oscillator.

You'll need to find and read the TL072 datasheet (always the first step when working with an unfamiliar component). It has some properties that may affect you. For instance it can sense to the positive voltage rail but not down to the lower one. Look at the "common mode voltage" range on the datasheet. And I don't think it will output fully to both rails, either. Read the output voltage specs carefully. If you need to go to 0V out and the op-amp cannot do that with a single supply, you'll need a dual supply and some way to limit the output to >0V. You may need a power supply above 15V to be able to get 15V output. Again, just reading the datasheet will reveal the answers to these questions.

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7. ### Wendy Moderator

Mar 24, 2008
20,772
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Wookie has such a schematic kicking around, I think. Time to flag him?

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8. ### ErnieM AAC Fanatic!

Apr 24, 2011
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I am confused by your requirements. You have a PWM input and you wish a square wave output? Of variable (presumably from the input) frequency?

If it is a square wave then why is the duty cycle variable?

9. ### Wendy Moderator

Mar 24, 2008
20,772
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You need several op amps. You make a Inverting Schmitt Trigger with on, and turn that into a hysteretic oscillator, which creates a triangle wave exactly like the 555 circuit I showed.

You use the second op amp as a comparator, and it makes a voltage to PWM converter.

10. ### sagardewani Thread Starter New Member

Aug 1, 2012
5
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It was my mistake.. Actually we have to generate DC signal from 0 to +15V with variable frequency and alterable duty cycle..

You know what, even I'm confused with my teacher's requirements as he hasn't even taught us the topic "OP-AMPS"
He is still playing with the basics and expecting us to build a PWM Circuit without even knowing exactly what "PWM" is

Since the project is carrying sessional marks so we will have to do it willingly or unwillingly

@ All.. BTW, Thanks for your response guys!

11. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
Have a look at the "Opamps for Everyone design reference":
http://www.ti.com/lit/an/slod006b/slod006b.pdf

The requirement of getting into the MHz means that a TL072 will not be fast enough. The requirement of using a ±15v supply to get 0v-15v out also eliminates the TL072. You might get there using a couple of LM311 comparators, or an LM393 comparator, or perhaps an LM339 quad comparator.

Bill, I got your PM, but I don't happen to have that schematic on this computer, and I still don't have either of my towers set up. There's just been too much going on around here to get that kind of thing done.

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12. ### sagardewani Thread Starter New Member

Aug 1, 2012
5
0
Well Thanks for your response... I asked my teacher today, he told me that we have to design a simple voltage comparator circuit to generate pulses and use a signal diode to rectify the output. He said you can use any op-amp or even a 555 timer IC.. He said all he will check is the output waveform on the oscilloscope. Moreover he told that the duty cycle should be variable.. Now all I want a simplest possible schematic for the above mentioned circuit.. Any help in this regard would be highly appreciated..

13. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
I've already give you that. Two chips.