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Date: Fri, 24 May 2024 14:00:06 -0400
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Subject: Re: An actual circuit
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On 5/24/2024 1:55 PM, john larkin wrote:
> 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.
> 

There are  bunch of variants of this circuit, I call it the "Forest Mims 
Oscillator" cuz in one of his radio shack circuit books was where I 
first remember seeing it as a teenager. Variants with an inductor in Q2 
collector are often used in those cheap solar garden lights.

<https://en.wikipedia.org/wiki/Forrest_Mims#/media/File:Forrest_Mims_hand_drawn_circuit_1983.jpg>

It can be a pain to get to oscillate in LTSpice but works pretty good IRL.