-55 to +125C, fixed 12V regulator, 24V input, about 200mA+ - Page 1
Forum section: electronics / projects

author: 741
author_rank: Frequent Contributor
timestamp: December 02, 2024, 03:08:54 pm
content: HiI'm needing to re-specify a 12V regulator, used to be LM7812CT on the existing design, but the minimum temperature is 0C for this.SG7812A looks good - but only seems available from Microchip directly, (5k qty) or long lead times from Digikey etc.

author: Jon_S
author_rank: Contributor
timestamp: December 02, 2024, 05:01:30 pm
content: Quote from: 741 on December 02, 2024, 03:05:31 pmCan you qualify? The SG7812AIG shows as $176.98 each on Microchip Direct for me, it's a Hi-Rel special.How about the MC7812BTG? -40°C to 125°C rated and available from all the usual suspects at about 50p in 1-off.I would strongly suspect it will work absolutely fine. You are still well within the storage temperature, and the output voltage vs temp graph in the datasheet doesn't seem to show a hint of anything interesting starting to happen down to -40°C.

author: 741
author_rank: Frequent Contributor
timestamp: December 02, 2024, 06:26:12 pm
content: I did not realise it cost that much (!)I'll take a look at the part you mentioned, thank you.

author: rhodges
author_rank: Frequent Contributor
timestamp: December 03, 2024, 01:01:05 am
content: Aside from the startup, did you consider that dissipating 2.5 watts would keep it warm?

author: Zero999
author_rank: Super Contributor
timestamp: December 03, 2024, 09:42:56 am
content: Is this a new design or replacement?If it's a new design, use the RH117 or LM117 (an LM317 but with a wider temperature range) & a couple of resistors.Note, that once you specifiy things like extreme temperatures, you tend to get military grade devices which aren't cheap.https://www.mouser.co.uk/c/semiconductors/power-management-ics/voltage-regulators-voltage-controllers/linear-voltage-regulators/?series=LM117&srsltid=AfmBOopi4YbFvRjcJ15QsJvUJQLuezD5t6XcsJf-UHk7AVYN8DJACqq8

author: peter-h
author_rank: Super Contributor
timestamp: December 03, 2024, 05:03:00 pm
content: In most cases where you want this (milspec) temp range, you have to use parts which have thison their data sheet. Whether another one will actually work is totally irrelevant.This leads to all kinds of absolutely hilarious situations on those industries e.g. in avionics you find the crappiest servo motors in autopilot servos - because the motor comes with that spec.For a one-off one often finds milspec parts on Ebay. For production, you will have to payPower ICs are usually metal can. Some are TO3 but sometimes they are the centre-stud versions at exotic prices, often used in satellites. Most discrete parts are fine on this temp range, in plastic.If you get stuck, get a power transistor, NPN, say TIP121 (-65C to +150C), put a 14V zener at its base, 1 resistor to put a few mA through the zener, and if the not-stellar load regulation is OK, there's a legal solutionCost will be pennies. A lot of people build their own circuits like this, to get the legit temp range. I've just done precisely this (24 to 12V) for an application where there are sizeable spikes which a 78xx reg will never take.

author: SiliconWizard
author_rank: Super Contributor
timestamp: December 03, 2024, 10:18:42 pm
content: Those extended temp range parts are expensive and relatively few. You can consider the LT3086M, which is around $10-$12, expensive for a linear reg but nowhere near the $100+ mark.

author: peter-h
author_rank: Super Contributor
timestamp: December 04, 2024, 01:26:11 pm
content: I'd regard the LT3086M as way too off the wall. No way to replace it if you can't get it. But it is targeted at the milspec market allright so probably ok.

author: youngda9
author_rank: Regular Contributor
timestamp: December 04, 2024, 04:46:55 pm
content: Scroll to the bottom of the link. Digi-Key lists a bunch of drop-in replacements, many with the same base part number.https://www.digikey.com/en/products/detail/onsemi/LM7812CT/1923051#product-details-substitutesMC78M12BTG has the same temp range as the part being replaced.

author: Zero999
author_rank: Super Contributor
timestamp: December 04, 2024, 04:50:13 pm
content: Quote from: peter-h on December 03, 2024, 05:03:00 pmTesting is a possibility, but it incurs costs of its own and you need access to a suitable environmental test chamber. I've done it myself, when I worked at a defence contractor. Fortunately it had all equipment, otherwise it would have been expensive to rent one.Quote from: youngda9 on December 04, 2024, 04:46:55 pmNo it doesn't. The minimum operating temperature is −40°C, rather than −55°C specified in the original post.