Path: ...!news-out.netnews.com!postmaster.netnews.com!us11.netnews.com!not-for-mail X-Trace: DXC=IE;4V9GS^H[S_5g?QdIXk]U5[F2hIijD_7J470dMQQ7[HFjJ2MJUSTR;dFo]Kk\e]Te^mW?D9fI3];n=QkXMlA5^EP?S5IYaD:]n_i[Z4@N@IV X-Complaints-To: support@frugalusenet.com Date: Fri, 24 May 2024 14:00:06 -0400 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: An actual circuit Newsgroups: sci.electronics.design References: Content-Language: en-US From: bitrex In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Lines: 65 Message-ID: <6650d5a6$0$2363133$882e4bbb@reader.netnews.com> NNTP-Posting-Host: 127.0.0.1 X-Trace: 1716573606 reader.netnews.com 2363133 127.0.0.1:37817 Bytes: 3871 On 5/24/2024 1:55 PM, john larkin wrote: > On Fri, 24 May 2024 11:59:31 -0400, "Edward Rawde" > wrote: > >> "john larkin" wrote in message >> news:bk815jh3skuecf1tap8o41rpgdh5kkq8o5@4ax.com... >>> On Thu, 23 May 2024 13:06:46 -0700, john larkin wrote: >>> >>>> On Thu, 23 May 2024 15:35:00 -0400, "Edward Rawde" >>>> 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. It can be a pain to get to oscillate in LTSpice but works pretty good IRL.