78XX series voltage regulator. I'm a hurtin it? - Page 1
Forum section: electronics / projects

author: mkiijam
author_rank: Regular Contributor
timestamp: December 19, 2023, 08:13:39 pm
content: This is a strange thing, but just go with it. I have my reasons...I've found the need to solve a problem in the simplest way for a client and that is to feed an existing design, namely the power supply portion of this product, with my own DC supply. I have a SMPS feeding DC (+/-12 and +5) into the OUTPUT pins of 7812 / 7912 / 7805 and to the INPUT pin of an 3.3 Volt regulator.It works... but I find myself wondering if I'm doing something(or going to) bad to the regulators? Like, do they NOT LIKE be backfed power???I could just pull the regulators out and be done, but the client asked the mother of all mod questions: "Will I be able to use it normally, like with the power(wallwart) that it came with?" So I gotta leave it as is, if that makes sense.

author: Andy Chee
author_rank: Super Contributor
timestamp: December 19, 2023, 09:17:05 pm
content: I know that LM317 does not like backfeed, see diagram below:Notice that D1 prevents a large output capacitor (or perhaps an attached battery on the output rail) from backfeeding the LM317 when the input voltage is removed.I'm pretty sure the 78xx regulators are similarly intolerant of backfeeding.

author: janoc
author_rank: Super Contributor
timestamp: December 19, 2023, 09:24:47 pm
content: Yep, backfeeding 78xx series is a bad idea unless you install a bypass diode (or there is one in the circuit already).Quotehttps://www.ti.com/lit/ds/symlink/lm340.pdf, page 13.

author: ejeffrey
author_rank: Super Contributor
timestamp: December 19, 2023, 09:27:45 pm
content: Yes, backfeed is a problem for many regulators including the LM78XX. Many designs include the diode as shown above by Andy Chee. If that is in place, and it's OK to backfeed the input with Vout - 0.6, then it should be fine. If the original was designed with no meaningful possibility of input short circuit and/or the output capacitors are small enough, the designers might have omitted the reverse protection diode. In that case, you should provide one.

author: mkiijam
author_rank: Regular Contributor
timestamp: December 19, 2023, 10:37:57 pm
content: Okay, thanks all.So I think it will be better to get +/- 15Vdc and feed the input. That should be enough for dropout and not too much heat?The only issue now is the 5V regulator as I only can find 5V SMPS modules, not 8 or something that would be better.Question: If I feed 12V into a 7812 from a SMPS would it come out 12V? Or does it FOR SURE need a higher voltage???

author: PCB.Wiz
author_rank: Super Contributor
timestamp: December 19, 2023, 10:41:13 pm
content: Quote from: mkiijam on December 19, 2023, 10:37:57 pmA 7812 will never give 12V out with 12V in. The data should give the exact dropouts.Quote from: mkiijam on December 19, 2023, 10:37:57 pmWhat current do you need ?An alternative would be a AliExpress low cost DC-DC buck module. Some have trimpots, and some you buy eg 7.5V

author: Gyro
author_rank: Super Contributor
timestamp: December 19, 2023, 11:09:12 pm
content: Quote from: PCB.Wiz on December 19, 2023, 10:41:13 pmAn LM2940 will get pretty close. If you don't push the current and operating temperature too far you will typically achieve 11.5V from a 12V input (for and LM2940-12). They are quite well behaved in the dropout region.

author: Andy Chee
author_rank: Super Contributor
timestamp: December 19, 2023, 11:15:53 pm
content: Quote from: mkiijam on December 19, 2023, 08:13:39 pmPerhaps if you could list your specifications:Input voltage? AC or DC?Output voltage 1? Output current 1?Output voltage 2? Output current 2?etc.Without much detail it's hard to recommend a solution, aside from spitballing ideas.For example, here's a random multi-rail flyback I just came up with in an old DOS calculator:

author: magic
author_rank: Super Contributor
timestamp: December 20, 2023, 06:48:31 pm
content: Quote from: mkiijam on December 19, 2023, 08:13:39 pmIf it's only a matter of protecting regulators (and not preventing something else at the wallwart input from receiving power) then I would simply add a 1N5818 going from the addded 12V PSU to the wallwart input. Then you don't even need to provide your own 5V, because the original 7805 will take care of it.

author: mkiijam
author_rank: Regular Contributor
timestamp: December 20, 2023, 07:04:20 pm
content: Here is the full story:This device is an audio processor / DSP module. The client travels the world with it. it is subject to ALL the strange AC voltages that various venues offer him. It only has an AC output wallwart and has been blown up many times. Even when not being abused on the AC in, the device uses a 3.3V regulator that receives about 20Vdc as an input. It, for obvious reasons is mounted to a massive heatsink, but still heats the unit to glitch-city.My job is to develop something that solves both issues.1. MORE impervious to various AC conditions2. Take the insane heat dissipation away from the 3.3V Reg.My plan right now is to either strap 4004's across the regs like you guys have said or get a +/- 15, 5V SMPS(meanwell) and feed the 12 regs the 15 and strap the 5V reg with the diode. The 3.3 works great with the 5 on its input.

author: magic
author_rank: Super Contributor
timestamp: December 20, 2023, 07:11:32 pm
content: I'm lost, is the 3.3V reg fed by raw rectified DC or by the 7805?If you apply a 5V PSU to the output of 7805 and the input of 3.3V reg, note that either your PSU or the 7805 may end up supplying power to the 3.3V reg, depending on which will want to regulate a higher voltage on the 5V rail. The other one will sit idle and wait for rail voltage to drop to its own regulation threshold.

author: Andy Chee
author_rank: Super Contributor
timestamp: December 20, 2023, 07:33:00 pm
content: My idea for a solution is to use a universal 19V laptop brick to handle the wild mains AC voltage input (90-264VAC input).Then build a 19V converter to match the original input of the audio processor / DSP module. If necessary, use multiple converters to drop the 19V down to your multiple voltage rails.Standard transformer wallwarts (non switchmode) were never designed for world voltages, their output may vary depending on which country you're in.A switchmode converter can easily convert 19V to 3.3V with little/no heat.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: December 20, 2023, 07:51:53 pm
content: Quote from: mkiijam on December 20, 2023, 07:04:20 pmWhat exactly does this AC output look like ? Just two wires, or is this some multi winding transformer ?Quote from: mkiijam on December 20, 2023, 07:04:20 pmWhat else does that 20V supply ?I doubt it is as simple as 'drop the 20V', or they would already have thought of that.You can get switching regulators in 78033 style pinout packages, that are often retrofitted into designs were the 'thermal budget was optimistic'.