Photoelectric light controls with thermionic switches - Page 1
Forum section: electronics / general

author: strawberry
author_rank: Super Contributor
timestamp: August 26, 2023, 04:22:38 pm
content: this thing may not turn off in harsh winter

author: langwadt
author_rank: Super Contributor
timestamp: August 26, 2023, 08:37:20 pm
content: but it fails on which is the safe option, and with no electronics very rugged

author: N2IXK
author_rank: Frequent Contributor
timestamp: August 26, 2023, 08:43:15 pm
content: Thermostatic, not thermionic (unless it has a vacuum tube in it)...

author: AVGresponding
author_rank: Super Contributor
timestamp: August 27, 2023, 05:30:37 am
content: Modern ones don't use CdS photocells anyway, due to RoHS. They use photodiodes now.

author: strawberry
author_rank: Super Contributor
timestamp: August 27, 2023, 06:12:36 am
content: it could be much better with two transistor hysteresis comparator

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 27, 2023, 08:17:56 am
content: Quote from: strawberry on August 27, 2023, 06:12:36 amBlows up the instant a lightning bolt strikes nearby. Warranty please!Tim

author: langwadt
author_rank: Super Contributor
timestamp: August 27, 2023, 08:18:26 am
content: Quote from: strawberry on August 27, 2023, 06:12:36 ambetter in what way?the thermostat fail safe on, rugged, provides a long low pass and hysteresis

author: strawberry
author_rank: Super Contributor
timestamp: August 27, 2023, 11:20:39 am
content: Quote from: T3sl4co1l on August 27, 2023, 08:17:56 amshielding and with some MOVST makes those 25A 1.2kV scr'sphoto cell as power source and sensor at the same time

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 27, 2023, 01:13:37 pm
content: So it bursts into flame after ten years on the pole?Sarcasm aside, it's one of those odd design corners where you really gotta respect the simplicity and dumb brute force robustness of a mechanical solution. It solves multiple problems at once: a time constant, accurate enough response, strong switching (hysteresis) with low resistance (mechanical contact), long life, and few quirks and other downsides (couple watts idle power, undesired behavior when cold or restarting). And failsafe (better to have lights always-on than always-off). And it uses as many mechanical parts, or maybe fewer, than an electronic solution, and being stamped metal and rivets, a fraction of the cost (well, maybe, or potentially).I wonder what the transient immunity of CdS actually is; you wouldn't expect them to be very linear, at least at some point, and yeah I'm pretty sure you'll get avalanche at some point; but for a visible-width track, it's got to be pretty high... They're surprisingly linear, at least at what voltages and time scales I've tested them at, and seen applications for.I don't think there's a true non-mechanical pure-semiconductor solution that strictly (all parameters) outperforms such a device, with less than... maybe $100 of parts? Depends on how much TVS is required, and Rds(on) say using SiC MOSFETs as SSR.Maybe not so bad if some assumptions can be made about the load characteristics (surge immunity is a heck of a lot easier into an inductive load), and strict outperformance surely isn't needed in a real product, but it's interesting to illustrate just how well a mechanical solution can do.Tim

author: AVGresponding
author_rank: Super Contributor
timestamp: August 27, 2023, 03:07:12 pm
content: This is what's inside a modern one:You can see the chip in the photodiode is pretty big, it takes almost the full width of that T-5 package.

author: strawberry
author_rank: Super Contributor
timestamp: August 28, 2023, 05:37:29 pm
content: solid polymer capacitors could be better choice