Path: csiph.com!eternal-september.org!feeder.eternal-september.org!nntp.eternal-september.org!.POSTED!not-for-mail From: "Don" Newsgroups: sci.electronics.design Subject: A fully floating BJT-based LED current driver Date: Thu, 9 Apr 2026 09:17:04 -0000 (UTC) Organization: A noiseless patient Spider Lines: 31 Message-ID: <20260409a@crcomp.net> References: <6tltskde2lkpbo4s5g44ujar7b2tttk0k9@4ax.com> <18a493794d041b26$1$2982030$4226dc73@news.newsgroupdirect.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Injection-Date: Thu, 09 Apr 2026 09:17:05 +0000 (UTC) Injection-Info: dont-email.me; posting-host="f9b6f0edf82aae8ff144ffb288b4b976"; logging-data="238285"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX1/hEGaxXb9YmCiN7BFxQKcA" Cancel-Lock: sha1:vRBMfiWAB40UtAVKOe4MJbXmNNk= Xref: csiph.com sci.electronics.design:742846 someone wrote: > If you stand back from the microscopic current mirror circuit > interpretation, the transistor equivalent of the DIAC using Q2,4,3, > with a threshold of 2.5V or so, comes into focus. Then add in some > current limiting/ regulating resistors here and there, and you end > up with the floating current source. The BC557C has an absurdly > small Early voltage and RCE, combined with the 'C' version beta in > the range of 200-800, and start-up is a done deal. Voltage compliance > shouldn't be a serious issue for driving LEDs, but it would be nice > to see an analytic to that effect. AI hasn't penetrated deeply enough > into our freebie CADs to give us that capabilty, so we're stuck with > looking at graphs. Excellent! Your circuit suggests some silicon secrets pertinent to the 120 VAC LED COB filaments mentioned by me elsewhere: Allow me to transition this topic to 120 VAC LED COB filaments. This picture shows such specimens, with the topmost powered-on and the bottommost powered-off: The protoboard's black wire connects to 120 ACV Line while its white wire connects to Neutral. My goal is to discover the silicon secrets in such a simple filament. -- 73, Don, KB7RPU veritas _|_ liberabit | https://www.qsl.net/kb7rpu vos |