Author Topic: Should EEVblog have circuits by members of the forum page  (Read 2567 times)

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Offline vk6zgo

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Re: Should EEVblog have circuits by members of the forum page
« Reply #25 on: December 30, 2017, 03:48:20 am »
Or, the ones who run multivibrators from 12v supplies and wonder why the frequency varies all over the place. On their own they may never figure out why. With a few pointers as to correct design principles they avoid these gotchas.


OK, ... you lost me with the multivibrators.
Apart from multivibrators being far from the most frequency stable devices at any time, why should 12v supplies cause any particular problems, if they are well regulated supplies in the first place?



Because when the supply voltage exceeds the actual reverse Vbe Zener breakdown voltage by more than about 1V, when the transistor turning on pulls the collector end of the opposite transistor's timing capacitor low, it reverse biasses the opposite transistor's Vbe junction enough to Zener it. (The extra 1V is from 0.7V forward Vbe initially on the transistor base, and 0.3V Vce_sat limiting the swing).   This makes the timing significantly sensitive to the supply voltage, and also tends to degrade the transistors - an order of magnitude reduction in hFE in under 1000 hours would not be exceptional.

(See http://downloads.hindawi.com/journals/apec/2001/053209.pdf for a report on the degradation of 2N2222 transistors by reverse Vbe Zenering)

The easy cure in a multivibrator circuit is to add a diode in series with each base.

I thought it would be something like that.
Now people know your original comment was not just a "throwaway comment".

When I first learnt about M/Vs, we made them with vacuum tubes ----usually 12AT7s! ;D
 

Offline rhb

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Re: Should EEVblog have circuits by members of the forum page
« Reply #26 on: December 30, 2017, 03:41:05 pm »
I raised the idea of a wiki for collecting T&M information and got pointed here.

How about KISS?  Add a section for summaries of long running threads where people can link to important posts.  Same structure, so no new work other than setting up a  set of subgroups to match the existing subgroups.  I simply mentioned the LCR thread as an example.  The FeelTech FY-6600 thread has a ton of really useful things like SA plots of the output and a comparison to an R&S AWG.

If someone will point me to candidate wiki software and source for the EEVblog forum I'll take a look at using the user credentials from EEVblog in the wiki.  If things are not too object obfuscated it shouldn't be a lot of work.  I've got MediaWiki documentation already, though I've not fiddled with it in a while.

I've got a cold room to move in CA, so I won't be able to look into any of this until after the middle of January.  In the mean time I shall be enjoying my brother in-law's wine.
 


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