Simple design of a 12V to 5v supply questions - Page 1
Forum section: electronics / projects

author: _c32_
author_rank: Newbie
timestamp: September 05, 2018, 01:10:00 pm
content: While building my z80 computer i wanted to make a proper power supply for it.The circuit will be using about 2A max.i searched around and my current circuit uses a lt1085 with a bd536.So far all my experience about building power supplys was using 2 caps on a 7805 so I am a little bit afraid that something is going to go wrong and send 12V to the output.Should i try to add a dropper resistor on the input?Have i made any mistakes or is there something to improve?Also how does the pass transistor not mess up the voltage from the regulator ?does the regulator compensate by adjusting the current it takes on the input?Thanks

author: Zero999
author_rank: Super Contributor
timestamp: September 05, 2018, 01:27:25 pm
content: The pass transistor is unnecessary, because the LT1083 is already capable of 2A, with no additional components.The LT1083 is overkill because a low drop-out regulator is not required to 7V. The old LM317 is perfectly adequate for this application and will be more stable too.I'd recommend the L78S05 with a 2R >5W resistor on the input to reduce the power dissipation of the IC.https://www.jameco.com/Jameco/Products/ProdDS/889461.pdfEDIT:My previous comments assumed the 12V rail is reasonably stable.Where are you getting the 12V from? If it's a car electrical system, then a LDO isn't such a bad idea, as it will work when the battery voltage drops, when the engine starts. Some over-voltage protection will also be required because high voltages can be generated, due to a load dump.

author: _c32_
author_rank: Newbie
timestamp: September 05, 2018, 02:03:29 pm
content: i used the pass transistor to spread the heat and make it cool easierMy supply is from a wall plug so stable enough i guess.since i don't have an l78s05, will a 7805 with a pass transistor be as good?

author: SiliconWizard
author_rank: Super Contributor
timestamp: September 05, 2018, 02:07:05 pm
content: Are you strictly against a switching step-down regulator? This would be much more efficient and would generate very little heat compared to a linear approach.

author: _c32_
author_rank: Newbie
timestamp: September 05, 2018, 02:10:05 pm
content: Yes, i want to get a linear supply on this project because it looks more old school (fitting for the project) and also it's an opportunity to learn more about the subject

author: Wolfgang
author_rank: Super Contributor
timestamp: September 05, 2018, 02:17:54 pm
content: Why not try an LM723 with pass transistor ?Thats probably the most stable variety for little money.https://electronicprojectsforfun.wordpress.com/power-supplies/

author: SiliconWizard
author_rank: Super Contributor
timestamp: September 05, 2018, 02:36:49 pm
content: Quote from: _c32_ on September 05, 2018, 02:10:05 pmOk. Your call of course.For the record, actually, quite a few computers of the 70's and 80's, apart from the small home computers, already had switching power supplies. I think the Apple II did, for instance. Even the goold old BBC Micro was initially using an all linear PS but due to severe heating problems, they switched (no pun intended) to a switching PS in a later revision.And there's just as much to learn (probably even more) building a switching PS (maybe you were thinking I was suggesting to use a ready-made one, which of course would also be an option, but you can design one yourself without much difficulties for this range of current).

author: Ian.M
author_rank: Super Contributor
timestamp: September 05, 2018, 03:03:05 pm
content: If the board is valuable, (and IMHO a home-built fully featured Z80 system qualifies), to prevent blowing all the chips on it if your 5V rail looses regulation and goes high, add a crowbar circuit across the output of your 5V regulator. See T.I's TL431 shunt regulator datasheet, Figure 22. Crowbar Circuit.Set it up using a variable bench supply so it crowbars at 5.3V, and make sure your regulator circuit has short-circuit protection and isn't over 5.15V.

author: Zero999
author_rank: Super Contributor
timestamp: September 05, 2018, 03:40:14 pm
content: Quote from: _c32_ on September 05, 2018, 02:03:29 pmBut now it has no over-current protection.QuoteYes, but I repeat, it will have no over-current protection, with an LM7805+pass transistor, unless additional components are added.Since you already have the LT1085, just use it as it is, without the additional pass transistor. The 2R series resistor will reduce the power dissipation. It just needs to be rated to 4W, but I wouldn't recommend going below 5W.

author: David Hess
author_rank: Super Contributor
timestamp: September 05, 2018, 04:51:32 pm
content: I might use a couple of LM317s in parallel with operational amplifiers to enforce current sharing. Multiple devices have the advantage of spreading the heat around making cooling easier. A big pass transistor as shown is simpler though. The transistor could also be placed in series to absorb some of the voltage drop instead of amplifying the current. Or multiple regulators could be used in series.For safety, add an SCR crowbar circuit to the output and a fuse to the input. Then in the event that the output rises to a dangerous level, the SCR shorts it and blows the fuse.

author: mariush
author_rank: Super Contributor
timestamp: September 05, 2018, 07:40:49 pm
content: Considering you want to keep it linear, my advice would be to simply chain two linear regulators, so that each one will dissipate a part of the voltage drop.For example, get a LM1085 (3A LDO) and take the voltage down to ~ 8-9v, and the second LM1085 can take it down to 5v - this way at peak 2A of current, the regulators will dissipate ~ 6-8w of heat.you could also put a couple of diodes in front of the two regulators to lower the voltage by around 1.5v but I don't think it would make much difference.With LM1085 you'd have to be careful because the tab is connected to Vout so for example if you want to screw two LM1085 to a copper plate / heatsink or something like that, you'd have to insulate at least one of them.

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 05, 2018, 07:45:15 pm
content: Just get a car charger for a smartphone. They're cheap and handle all the details for you.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: September 05, 2018, 08:22:28 pm
content: I used for my Z80 devboard a LM2576: it's readily available, not too noisy and doesn't require who knows what heatsink for 12V -> 5V

author: _c32_
author_rank: Newbie
timestamp: September 05, 2018, 11:14:01 pm
content: long reply, thanks a lot everybody for helping ! I learned quite some stuff todayQuote from: Wolfgang on September 05, 2018, 02:17:54 pmThanks! That looks quite interesting , i will most likely get one and play around with it .I also like that it comes in a to-100 packageQuote from: SiliconWizard on September 05, 2018, 02:36:49 pmthat's exactly what i was thinking. I am now considering a switching supply too but i have a sound chip, video and composite encoder on 5V. Could that introduce too much noise? i guess not if i implement it correctly. idkQuote from: Ian.M on September 05, 2018, 03:03:05 pmThanks a lot! That's exactly what i was looking for about overvoltagei have some stuff that i really don't want to damageQuote from: Hero999 on September 05, 2018, 03:40:14 pmi hadn't though of that, the lm723 seems to sense current after the pass transistor which makes it a bit more appealingQuote from: Hero999 on September 05, 2018, 03:40:14 pmthat sounds like the best option for now. I'll use that and then maybe check out the lm723 or the LM2576 mentioned by HB9EVIQuote from: David Hess on September 05, 2018, 04:51:32 pmthat sounds quite interesting but it sounds complicated and i have no idea how i would go about implementing itQuote from: David Hess on September 05, 2018, 04:51:32 pmso using a constant current on base-emitter and then the collector would go to the input of the regulators ?wouldn't a resistor do a better job?Quote from: mariush on September 05, 2018, 07:40:49 pmThat sounds great, maybe i can use my lt1085 to drop to something like 7 volts and then feed that to a 78S05 or whatever else.wouldn't a dropper resistor be as good though?is it to ensure that the voltage doesn't drop too much?Quote from: mariush on September 05, 2018, 07:40:49 pmI use separate heatsinks for the regulators and transistors and they are also coated with something black that makes them non conductive but thanks, that could have ruined my dayQuote from: NiHaoMike on September 05, 2018, 07:45:15 pmthat aint no funI want to build the supply myselfQuote from: HB9EVI on September 05, 2018, 08:22:28 pmbut it doesn't look nearly as cool as the to-100 package of the lm723seriously though, that seems like a good option but it supports up to 1.5 amps. Maybe i could cut down some stuff and use thatlots of good options to choose from

author: Wolfgang
author_rank: Super Contributor
timestamp: September 05, 2018, 11:17:20 pm
content: Hi, for a computer supply its maybe good enough. I'll try this and I have ordered some of those. Thanks !

author: _c32_
author_rank: Newbie
timestamp: September 05, 2018, 11:39:00 pm
content: Quote from: Wolfgang on September 05, 2018, 11:17:20 pmif you are trying it for me, thanks but I have already tried it and it works.if you want to use this yourself, depending on the current consumption of the circuit it might be an overkill and also the current protection of the regulator is useless due to the pass transistor

author: Wolfgang
author_rank: Super Contributor
timestamp: September 05, 2018, 11:43:14 pm
content: Dont worry, I will try it out just for my own curiosity and to learn something.3A output is OK for me, the LM2576 can handle this without a pass transistor.

author: m3vuv
author_rank: Super Contributor
timestamp: September 05, 2018, 11:49:17 pm
content: could you not use an old pc desktop psu and tap off of the 5v rail,ready made cheap solution!.

author: David Hess
author_rank: Super Contributor
timestamp: September 06, 2018, 03:11:46 am
content: Quote from: _c32_ on September 05, 2018, 11:14:01 pmIt requires an additional operational amplifier so it is probably not worth the extra complication. It would be more suitable for a higher current power supply where one regulator is not enough.QuoteThis idea is similar to using two regulators in series except that the first regulator is a power transistor which divides the needed voltage drop by half. In practice it might be as simple as a power transistor, resistor, and zener diode.

author: Zero999
author_rank: Super Contributor
timestamp: September 06, 2018, 08:12:52 am
content: I don't see the need for two regulators in series/parallel, the LM723 or a pass transistor, when 2R plus the LT1085 will do. It's simple and effective. The series resistor will dissipate some of the heat, so it isn't all being dissipated in the linear regulator. The LT1085 also has safe operating area protection, current limiting and thermal protection, which solutions using pass transistors lack.The only other improvement is a switched mode power supply, which will reduce the power dissipation to a minimum. The LM2576 is a bit old and isn't the most efficient IC around, but is sufficient for the application.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: September 06, 2018, 09:46:08 am
content: If it can cost a little more, there is also the LT1171; has even some current reserve and is available in non-smd case

author: _c32_
author_rank: Newbie
timestamp: September 06, 2018, 10:29:32 pm
content: Quote from: m3vuv on September 05, 2018, 11:49:17 pmi want to make my own supply and also a pc one is big and noisyQuote from: HB9EVI on September 06, 2018, 09:46:08 ami'll check it out if i make a switching one but i think i can get away with a linear and i already have the partsMy current circuit uses just an lt1085 with a dropping resistor and a tl431 with a BTA140 triac for crowbar. I'm now wondering if i should keep the pass transistor with a switch that will enable it and disable the dropper to allow for higher current but sacrifice some features, if i use it for another project. Also i could put a temperature sensor for the cpu to read with the extra space since it's a square board.This is a test fit and some stuff are not the correct values since i don't have them yetThanks a lot everyone !

author: Zero999
author_rank: Super Contributor
timestamp: September 07, 2018, 08:20:14 am
content: To switch the pass transistor on and off, connect as per the previous schematic, but with a switch in series with the base connection.R5 and R6 are excessively low for a potential divider for the TL431. Their values could be multiplied by 10 or even 100, to save a bit of power.