Something New from Doc Sketchy

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KSS
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Re: Something New from Doc Sketchy

Post by KSS » Tue Jun 30, 2020 10:50 pm

Zener knee can be quite soft. 5V1 has best response on that parameter.

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Re: Something New from Doc Sketchy

Post by Dr. Sketch-n-Etch » Wed Jul 01, 2020 1:50 am

For clarity, I should say that I meant 1N4730 (3.9V), 1N4731 (4.3V), and 1N4732 (4.7V), and NOT 1N4370 (2.4V), 1N4371 (2.7V) and 1N4372 (3.0V). I mistyped in my original post. It's anybody's guess why they made the numbers of these zeners so confusingly similar.

I understand that the knee can be soft, but given the same resistor from 15V (3.3k), even with soft knees, the same kind of zeners should give roughly the same voltages. And, assuming a 3.9V zener, a 3.3k resistor from 15V to the zener gives a current of 3.36 mA, and while this is well below the "test" current of 20 mA, it is not an insubstantial amount of current.

I actually tested a bunch of zeners tonight (1 1N4728 (3.3V), 3 1N4730s (3.9V), 1 1N4731 (4.3V), 2 1N4732s (4.7V), 2 1N4733s (5.1V), and 1 1N5237 (8.2V)), using 8 different resistor values (220R, 470R, 1k, 2.2k, 3.3k, 4.7k, 10k, 20k) with a 15V supply. Here are the results plotted as zener voltage vs true current (in log scale):
Zeners.png
Obviously, there is a lot of spread in the 3.3, 3.9 and 4.3V zeners, and the latter two never reach their nominal voltages. The two 4.7V zeners were different kinds (with different markings), and one was very good and the other was not so good. The 5.1V zeners were both very good and very similar. The 8.2V zener was pretty good.

So, looking at how badly the 3.9V zeners performed, the weird thing is that I used 3.9V zeners in my first two frequency shifters (including the one I'm using now) and the VCO waveforms are very close to the desired 10Vpp, suggesting that the 3.9V zeners I used are generating zener voltages very close to 3.9V (probably about 3.70 to 3.75V) with 3.3k resistors and 10V sources (giving true zener currents of a little less than 2 mV). I must have just gotten very lucky. Incidentally, those were zeners I had in my stash, while the 3.9V zeners I tested tonight were ones that I bought yesterday at Lee's. This suggests that the batch that Lee's bought are all pretty crappy, but the ones I had sitting around are pretty good.

It's very enervating.
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Re: Something New from Doc Sketchy

Post by Dr. Sketch-n-Etch » Wed Jul 01, 2020 4:54 pm

So, after a lot of messing around, I've decided that I'm just going to go with 1N4733 5.1V zeners, which will generate waveforms of 12.3Vpp, except for the sine waves which will still be about 10Vpp. The slightly larger waveforms have no deleterious effect on the multipliers, so why not?
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Re: Something New from Doc Sketchy

Post by KSS » Wed Jul 01, 2020 8:51 pm

:tu: Everybody knows "louder" sells! :mrgreen: Dude, these knobs may only go to 10, but the sound goes to TWELVE!

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Re: Something New from Doc Sketchy

Post by devinw1 » Thu Jul 02, 2020 11:15 am

But.......ours go to eleven!

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Re: Something New from Doc Sketchy

Post by KSS » Thu Jul 02, 2020 7:06 pm

Pfft.. Only eleven? Them 're sad bstrds. 12 is the new 10!

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Re: Something New from Doc Sketchy

Post by Dr. Sketch-n-Etch » Fri Jul 03, 2020 12:22 am

Honestly, I don't know why I've been so adamant about all my waveforms being exactly 10Vpp. It really doesn't matter if they are 12Vpp or whatever. And 5.1V zeners are superior to all other voltages because they have the lowest temperature coefficient. They also evidently have the lowest variation in voltage with current (and this is also confirmed by a plot in The Art of Electronics -- actually, it might be 5.6V that has the absolute lowest variation). Of course, when used in a zener bridge in a Dixie or Rubicon core, the tempco of the zener actually has no effect, because if the amplitude of the waveform increases, the rate at which the triangle integration occurs also increases, so the two end up canceling each other out (the comparator threshold gets higher, but the integration goes faster, so the frequency is actually insensitive to the zener voltage). I sometimes design things right.
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Re: Something New from Doc Sketchy

Post by Dr. Sketch-n-Etch » Sun Jul 05, 2020 6:43 pm

To everyone who has requested 4U boards for the Frequency Shifter: It's going to be another week or so before I get my act together. Things got busy on other fronts. Sorry, and thank you for your patience.
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