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From: "Edward Rawde" <invalid@invalid.invalid>
Newsgroups: sci.electronics.design
Subject: Re: An actual circuit
Date: Fri, 24 May 2024 14:50:19 -0400
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"Jeroen Belleman" <jeroen@nospam.please> wrote in message 
news:v2qmeq$2eknc$1@dont-email.me...
> On 5/24/24 17:59, Edward Rawde 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?
>
> Below are a pair of astable circuits. The left one is like yours,
> with a hangup state. I start it by specifying an initial condition.
> The right one will start all by itself.
>

Thanks for that. I guess I'm biased towards components I could easily get in 
the 70s. And also through hole components I can easily build a real circuit 
with.





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