Oscillator Experiment Questions

[mjarus]

I already have a 1k sine generator that I built awhile ago form a kit based off of a 555.  But I really wanted to try to make one that could output multiple frequencies.  I started with a basic design I found on the net here: http://www.discovercircuits.com/Andy/WienBridgeOscillator1.pdf.  Here is where the problems began...  I decided that I really didn't need to use the LM358 so I used a 4558 instead.  I also modified the Cx and Rx values so I could sub in pots for values that I had and still preserve the frequency range that I wanted.

First 4558 I tried... oops!  + and - reversed.  Turned them back around but output was too high... checked voltages and the voltage divider was @ 8.7V... not 4.5V.  Replaced with another 4558.  Bias problem fixed.

Q1: Obvious but... Is it possible to fry a 4558 but reversing the current?  I assume yes... gee I guess should have listened closer to RG about current protection.

Next I tried another 4558.  Put it in the bread board wrong (1 slot down... no connection to pins 1 and .  Fixed that.  Still no go on the output.  Could I have fried this one too?  Dunno, got another one.  That one still did not work.  It shows 8.6V at the output... this is only supposed to be a 2V P-P circuit.  Then I replaced all resistor and cap values to the spec.  Still no go.

FINALLY I pulled my head out of my rear went to the parts bin and got an LM358.  Everything worked fine.  So the big question...

Q2: What is so different about the LM358 and 4558 that one works and the other doesn't?  I looked at the datasheets and they look the same...  I only noticed some very minor differences.  I would prefer to use an 4558 as I have about 25 of them and only one LM358.  Good thing I didn't use the LM358 first...

Lessons (re)learned:

1)  Build it to a known working first.  Modify it later.
2)  Check the power polarity two or three times.
3)  When in doubt get off your rear and see if you do have the correct part.
4)  Breadboards are a real PITA.

mjarus.

[Paul Perry (Frostwave)]

The LM358 is specifically designed for single sided power supplies, while the 4558 isn't. Sure, you can use ANY op amp in a single sided supply situation if you pay attentin to biasing.. but, I think there might be a difference there.
And another thing, while for a simple amplifier, you can usually swap aroujnd a lot of different op amps & still have the circuit working, oscillators are (in my experience) a LOT fussier. Probably because, in an oscilator, you have the inputs going from one extreme to another, so the op amp inputs are ecercised over their full range, and if anything weird is going to happen, it will.

[Steve Newton]

Could be something to do with the LM358's internal structure. As it's designed to operate happily with single ended supplies it does thing's slightly differently to other opamps and can take conditions that would make them fall over, or even die.

The original designer could have figured the "kinks" in the 358 into the design.

The version of the datasheet here - http://www.alldatasheet.com/datasheet-pdf/pdf/3067/MOTOROLA/LM358.html - also shows a wein bridge oscillator design.

Paul beat me to it while I was looking up that datasheet...

[Paul Perry (Frostwave)]

Check that pin 7 really is 1/2 of the battery voltage.
If it is, which it should be, then (at a guess) you could try making R2 bigger. A lot bigger. Worth a try.

[mjarus]

Well now I feel stupid.  I can't believe that I didn't notice that the LM358 is single sided. 

Thanks for pointing that out.  I will assume that is the problem.

I did check pin 7 when I was testing and it was slightly above 1/2 the power supply.  It was around 5V.  I was thinking... and probably much to the chips detriment... that I should also bias pin 3 to 4.5V.  Would that help anything?

Thanks for all the help!

mjarus.