hello i have been following the thread on the guy who wants to build a 200Mhz triangle wave and i saw the part where he connected a triangle wave to the noninverting input of the comparator in my case is pin 3 of LM393 and the 50hz sine wave to the inverting input that is pin 2 to get a PWM signal and i just followed his lead.Hello,
Can you show some scope pics of the two inputs to the comparator, the non inverting and inverting terminals?
And is that scope pic you had shown of the output of the comparator or something else?
If the comparator is powered ok and working ok it will definitely work at 50Hz. You might need some hysteresis though too. Note that if your input signals swing from -8v to +8v for example, you can not connect your comparator to +5v and ground or even not to +10v and ground. The comparator would have to be connected to +10 and -10 volts as well. If you need a PWM signal that swings from 0v to +5v or from 0v to +10v, then we have to do a little level shifting.
The circuits here do not have very important supply bypass capacitors.
here is a picture of the bubba and the scope wave form.changes made to the bubba are R5= 1.5meg R6=330k all other values remain thesame and like i said i used TL084 for the mainn sine wave and then TL082 fot the buffer circuit consisting of R8 and R7 and i powered from a 12v source.i bufferd the output because at that point it has the least amplitude and lowest distortion so i read about bubba oscillators and tested it.The schematic for the Bubba oscillator is not shown. The schematic for the buffer amp for it is not shown and explained why it is needed.
The maximum allowed input (input common mode voltage range on the datasheet) to the LM393 comparator is 0V to (+12V - 1.5V=) +10.5V so the triangle wave is probably OK but we know nothing about the sinewave and its buffer amp.
The LM393 comparator output is overloaded with a 1k load so its max output current is trying to be 12mA but its datasheet shows it saturating with a 4mA or less load.
The circuits here do not have very important supply bypass capacitors.
The scope is a software one that is messing up the output waveform because its sampling frequency is much too low and maybe because it is set for an AC input instead of a DC input. The software scope has filtering that might be messing up the waveform. 0V and any mention of how many volts per line are not shown.
sorry for confusing you with that bubba picture.i thought i explained well enough,so heres the actual circuit i used to clear things up.all circuits powered from a 12V lead acid battery as you said.I think the supply for the entire circuit is a 12V lead-acid battery (13.8V) and all circuits use this supply.
I think the circuit will be a pure sinewave inverter when it is finished.
A REAL scope will show the actual output voltages of the oscillators. The output of a TL082 or TL084 opamp cannot go to 0V.
hello Sir please when you mean scope inputs do you mean after i have connected the triangle wave and the sine wave to the comparator inputs pins?Hi,
Yes there are a number of possible problems that could be here, but as i said before we need to see the two inputs to the comparator on the scope and that will clear this whole mess up.
So how about two scope pic's of the two inputs now?
Your lead acid battery is about 13.8V. A rail-to-rail opamp is usually Cmos and its inputs and output can go from 0V to 13.8V when the load current is low.1)pls can you expalain what you mean by TL082 an TL084 opamp cannot go to 0V and rail to rail opamps?
American parts distributors like Digikey have a very good search engine. I doubt that you can buy rail-to-rail opamps in Nigeria.2)f you know a list of rail to rail opamps ior where i can search i would be glad.the problem is here i was able to get only TL084 and TL082 which seeems to be common where i stay and even those who sell it dont know what an opamp is. i could not even find the MC33172 you once used in your PWM circuit to drive a motor so getting a list would mean i have a pool of options to search around for.
I have a REAL scope that works perfectly. I have never seen and have never used a software scope. I think you have your software scope set up for AC signals when it must be set for DC signals and calibrated.3)do you know of any scope software that can show peak to peak and what other comparator can i use or do i stick to the 393?
hello audioguru a thought just crosssed my mind,you said that the opamps(tl084 and tl082) i used are not rail to rail which means that there is a posibilty that the amplitude of the triangle wave and the sine waves will not be that high which will result in a low PWM signal amplitude.with a rail to rail i would be able to get amplitudes very high because i would need the amplitudes to be as high as possible and it is important for the waves(sine and triangle) to go from 0V to as high as possible to give PWM going from 0V too or am i missing something here?I think the supply for the entire circuit is a 12V lead-acid battery (13.8V) and all circuits use this supply.
I think the circuit will be a pure sinewave inverter when it is finished.
A REAL scope will show the actual output voltages of the oscillators. The output of a TL082 or TL084 opamp cannot go to 0V.
hello Sir please when you mean scope inputs do you mean after i have connected the triangle wave and the sine wave to the comparator inputs pins?
do you want to see if the signal distorts or something like that.may be a little explanation here because i posted a picture of the sine wave which i input into the comparator but maybe i dont get you.
Any way i will try to get you the scope inputs as they arrive the comparator if i get what you mean hopefully later because right now i need to watch a game of football(cant help it).
bear with me.
is it important that the sine wave and triangle start from 0V to their respective voltages in order to make the PWM signal go from 0V?The difference between using a rail-to-rail opamp with an output of 13.8V p-p is small compared to using a TL08x with an output of 10.5V p-p.
The output level from the comparator will be exactly the same!
EDIT: The voltage gain of an LM393 comparator is typically 200,000 so when the inputs have a very tiny voltage difference then the output switches completely high or very low. The output of a comparator is never at a partial voltage.
yes i understand clearly what yo mean and i will get you the scope pics but there is a small issue and it is that i use a pc scope software called visual analyzer 2011 version and i caliberated it by inputing acurate 1VAC into my sound card input so as to be able to measure wave forms in volts and peak voltages so i dont know if you know about this software and how to read the scale.Hello,
Yes i asked about two scope pics:
1. The non inverting terminal of the comparator
2. The inverting terminal of the comparator
This is after the triangle and sine has been connected.
The scope pics however have to be 'GOOD' pics, where we can see the scales clearly, especially the vertical scale. We want to be able to see the wave and be able to read off the amplitude in volts for the min, max, and middle voltage. So we'd be able to see the max of say the sine and look to the left and see the voltage written on the scope grid and should be able to read it clearly. This is very important.
You should connect the scope to the pins of the comparator too, not the socket or board.
You should also measure the positive supply getting to the comparator right on the pin.
If the voltage to the comparator is ok and the signals are within range, then it has to work unless something very simple is wrong like a bad connection. Do you understand this?
The input signals to the comparator do not need to have a large amplitude and do not need to go to ground. The comparator has a very high voltage gain of 200,000 times so the input signals only need to have the same bias voltage (half the supply voltage) so that one goes higher than the other and lower than the other. The output of the comparator is a switch that goes either completely high or very low, never at a partial voltage. See my attachment.is it important that the sine wave and triangle start from 0V to their respective voltages in order to make the PWM signal go from 0V?
why i aked is because since the output level of the comparators will be exactly thesame ,then it does not matter if i use a rail to rail.
The TL08x works fine but since the TL08x dont go all the way down to 0V will the PWM signals not go all the way down to 0V assuming the signals is lets say 10V p-p using this opamp or it does not matter the opamp because the comaprator will always output a signal going from 0V?
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