How Do Zener Diodes Impact a Triangle Wave Op-Amp Generator?

[Bassalisk]

Well another op-amp thread, while the other one is being cooked.

http://pokit.org/get/a09120280ee2f6b6e3de7a99989b8bd9.jpg To speed you up, this is a triangle wave op amp generator. It generates a triangle wave at U2.

In general I understand how the circuit works, but from the engineering point of view I have a few questions.

See those 2 Zener diodes, the ones holding the voltage at U1? Let's assume that they both, combined, hold 10 V constantly. Let's also assume that my zener diode will work with nominal current 5mA.(or can anybody give me some info about nominal zener diode current, because I might end up googling some random zener diode, which can take up to 500 mA, but most of them won't).

I was told that the maximum output current out of op amp should be 5mA too.

See that resistor Rp? Is he between potentials: Op1-satmax(lets say 15V) and zener diode 10 V) ? Is he limiting the current to that integrator?

Can anybody tell me best choices, from the engineering point of view for these resistors. I am trying to get an intuition for this, rather than spamming random numbers...

 

[technician]

If the zener diodes are 10V then they ensure that a constant voltage of +/-10V appears across the resistor R connected to the -input of OP2. This ensures that a constant current (of10/R) flows into or out of the capacitor during the charging and discharging parts of the sawtooth cycle.
OP3 inverts the sawtooth waveform and OP1 is a comparator to switch from charging to discharging of the capacitor.
Specifications for individual components such as zener diodes and op amps can be obtained from data sheets

 

[Bassalisk]

If the zener diodes are 10V then they ensure that a constant voltage of +/-10V appears across the resistor R connected to the -input of OP2. This ensures that a constant current (of10/R) flows into or out of the capacitor during the charging and discharging parts of the sawtooth cycle.
OP3 inverts the sawtooth waveform and OP1 is a comparator to switch from charging to discharging of the capacitor.
Specifications for individual components such as zener diodes and op amps can be obtained from data sheets

But what current flows out of the output of the op1?

 

[technician]

The output of OP1 will be either +15V or -15V (assuming +/-15V power supplies)
The zener diodes (of 10V) means that 5V appears across Rp.
Therefore IF the max current from an op amp is 5mA AND 5mA is the Zener diode current (data sheets needed !) then Rp will be...5V/5mA = 1000ohms

 

[technician]

You have a fair bit of design information here now. Do you plan to make this sawtooth generator?
What frequency are you hoping to generate? Have you chosen capacitor and resistor values yet?
Any more help needed?

 

[Bassalisk]

You have a fair bit of design information here now. Do you plan to make this sawtooth generator?
What frequency are you hoping to generate? Have you chosen capacitor and resistor values yet?
Any more help needed?

My assumption was right. Thank you very much kind sir. I trying to get a feeling for the values, and the sensitivity of the components.

What current will be the best to charge the capacitor, etc etc. This will be trial and error I guess.

I am planning to build a demodulator. So I need a high frequency carrier wave. Around 100kHz. I don't think this design could withstand those frequencies.

In a week or two, when my exams pass, I will do this project. Currently I got a plan to use a demodulator chip, and demodulate the signal with that carrier wave. I will test this using a function generator at my faculty. But once I find my desired frequency, I have to make some kind of carrier generator that can fit on the breadboard.

Currently these designs are all in my head, and I am trying to find the best choice for the carrier wave.

Thank you again for your help, if you have any more tips, please do tell them, I appreciate them very much.

 

[technician]

It is a pleasure to be of assistance. Get on and build one of these... it is easy enough and you learn more from doing than asking.
Do you realize that not all of the resistors shown on your diagram are essential (don't know where your circuit came from !) so you could make your project even easier.
Good luck... ask again.

 

[Bassalisk]

I currently don't know what exact components I will need, regarding the values. But all in all I am trying to find those critic points in design, that might make them fail.

The other day, I wanted to secure my voltage regulator 7809. I wanted to see, what will happen if accidently mixed up output and input.

The results were explosive. So I protected the circuit with diodes, everything is fine now. :D

 

[Averagesupernova]

Pick your resistor and capacitor values to determine what the slope is to be of the triangle wave. This is easily calculated. I'll give you a hint: Q = CV.

 

[jim hardy]

""The results were explosive. So I protected the circuit with diodes, everything is fine now. :D""

here's a line of regulators made to withstand that.. i use them now.

http://www.national.com/mpf/LM/LM2931.html#Overview

 

[Bassalisk]

I am officially out of steam. Thank you for the tip, all these posts will come in hand when I get my hands dirty!

Thank you Averegesupernova, and jim.