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From: john larkin <jl@650pot.com>
Newsgroups: sci.electronics.design
Subject: Re: An actual circuit
Date: Fri, 24 May 2024 10:55:26 -0700
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On Fri, 24 May 2024 11:59:31 -0400, "Edward Rawde"
<invalid@invalid.invalid> wrote:

>"john larkin" <jl@650pot.com> wrote in message 
>news:bk815jh3skuecf1tap8o41rpgdh5kkq8o5@4ax.com...
>> On Thu, 23 May 2024 13:06:46 -0700, john larkin <jl@650pot.com> wrote:
>>
>>>On Thu, 23 May 2024 15:35:00 -0400, "Edward Rawde"
>>><invalid@invalid.invalid> wrote:
>>>
>>>>I was having a conversation with a younger person who seemed to be of the
>>>>view that to make an LED flash you would need something to decide when it
>>>>should be on or off. So that would be some kind of software or digital
>>>>system.
>...
>>>
>>
>> The classic NPN astable circuit can hang up, with both transistors
>> saturated. I wonder if he jfet circuit can hang too, with Idss
>> grounding both drains and not enough gain to oscillate out of that
>> state.
>>
>> Even when they have a hang state, luck usually kicks them off into
>> oscillation. Your source resistors and  asymmetric drain resistors
>> help it start up. Try making both drain resistors 3.3K.
>>
>> If you make the source resistors lower, it will hang up.
>>
>
>Yes I noticed both points when I was designing it.
>I wanted to have it start up by itself, preferably without a kickstart 
>capacitor.
>So I had a complicated circuit with two more diodes and a transistor in the 
>hope that I could detect the hang state and force it off balance.
>I couldn't get that to work
>Then I accidentally made R2 3,3k and R6 3.3k and I didn't see how it could 
>start so quickly with no other help.
>Eventually I noticed 3,3k which maybe LTSpice takes as 3k.
>
>If R2 and R6 are both 3.3k then LTSpice says it slowly drifts into operation 
>after 40 seconds.
>But why does it go one way and not the other?
>Is that an artefact of asymmetry in the simulation?
>Or is there some hidden asymmetry in the circuit I'm not seeing when R2 is 
>3.3k?
>

Yes, it is mysterious. Set both drain resistors to 3.3K and zoom way
up on the two source voltages for the first 20 seconds. 

LT Spice may have some hidden asymmetry. It has to do math
sequentially, maybe.

In real life, parts will be plenty asymmetric. Especially jfets.