NPN pass power transistors for 3.3v regulation - Page 1
Forum section: electronics / beginners

author: VEGETA
author_rank: Super Contributor
timestamp: October 10, 2020, 05:08:13 pm
content: Hello,I want to make 5v and 3.3v power regulators for Dreamcast since the original PSU is broken beyond repair.I used L200 regulator for both and it worked by itself with minimal load, but with high loads the 3.3v one will not work and will drop to 2.3v or so while the 5v is ok since it draws very little current. I used BD437 NPN pass transistor with it as a pass transistor but it keeps getting hot until it no longer works and gets burned. I have heatsinks but my drilling tool is not good enough to do the job properly... I can try digging the heatsink and mounting it on it but still I need to make sure it works first.The problem:I need around 2.8A @3.3v but L200 can only deliver up to 2A.I have these stuff:LM317 --> maybe I have one or two working.L200 ---> only have 2 of it, which are used to make the said PSU circuits.BD437 --> I have about 7, it is the only NPN with high current that I have now.bunch of resistors, caps, trim pots, etc...Now, I want to make something like this:http://www.reuk.co.uk//OtherImages/adjustable-very-high-power-supply-with-lm317-voltage-regulator.jpgWill this work, assuming I put say 5 of BD437 to share current? I mean without power resistors and so on since I only have 3 of them and they won't fit.regards,

author: bob91343
author_rank: Super Contributor
timestamp: October 10, 2020, 05:56:53 pm
content: Can you split the load and use separate regulators?What you are asking for is not simple nor straightforward. Imagine I have a power supply that puts out 1 Ampere but I need 2 Amperes. You would say to get a bigger power transformer, bigger rectifier diodes, more pass transistors, and so on. Almost a total redesign.You are trying to do something similar but there is no way to climb into the IC and make it do more.

author: VEGETA
author_rank: Super Contributor
timestamp: October 10, 2020, 06:02:59 pm
content: QuoteThen perhaps you misunderstood me.I will have a 12v DC cheap laptop power supple as the input for this device. Now all I want is to have a simple design that can deliver say 3 Amps, nothing more.Notice also that my mentioned L200 design actually worked but lacked the capability of higher current. And when I put on the NPN pass it didn't hold it and got burned due to high amps with no heatsink.All I want now is to see it working, then I will cut off power (before transistors heat) and will put all my time to just drilling the heatsink stuff.

author: VEGETA
author_rank: Super Contributor
timestamp: October 10, 2020, 06:13:18 pm
content: I've got a simple idea of using an LM317 to output the 5v, so that is gonna be the 5v rail.Then use a potentiometer to get 3.3v from it, and feed it to LM358 which acts as base driver for the NPN pass transistors' base pins. By doing this I can hook any DC voltage such as 12-20v or so (just 12 will be fine), then I get 5v from LM317 no matter what... after that the resistor divider will always give me 3.3v that I need.Unfortunately I don't have other ICs now, and won't be a short time to get ones due to COVID and stuff. So I have to work with what I have.

author: magic
author_rank: Super Contributor
timestamp: October 10, 2020, 07:02:05 pm
content: Quote from: VEGETA on October 10, 2020, 05:08:13 pmNot gonna happen without a heatsink. TO-126 and TO-220 are only good for 1~2W each in air. Go calculate how much heat you are going to burn going from 12V to 3.3V.If you have a suitable inductor (APFC coil from ATX PSU maybe) and power diodes there are some hacks to turn those linear regs into SMPS.You could try to hack the 12V brick to output 5V and then regulate 3.3V from that to dramatically reduce power dissipation.

author: VEGETA
author_rank: Super Contributor
timestamp: October 10, 2020, 09:27:08 pm
content: Quote from: magic on October 10, 2020, 07:02:05 pmWhat if I had a good enough heatsink? will the circuit work and regulate properly?I don't have any inductors and so on, only mentioned parts. Not even sure LM317 actually works xD.I still have one of these:https://images-na.ssl-images-amazon.com/images/I/71ecltR6HdL._AC_SL1300_.jpgSo I think of getting the regulators on it (after making sure they work), then use them with LM358 to feed the NPNs bases.I have linked my suggestion in attachments. Sorry for the blur, my phone camera is bad.

author: wraper
author_rank: Unknown
timestamp: October 10, 2020, 09:36:14 pm
content: Quote from: VEGETA on October 10, 2020, 06:02:59 pmWill not happen. to make 3.3V from 12V supply, you need to drop 8.7V. At 3A current it means you need to dissipate 26W of power. Which means beefy pass element and a very big heatsink (~700-1000 cm3or larger volume with passive cooling or smaller with active cooling). My suggestion is just buy SMPS with 3.3V and 5V output voltages or at least get 5V SMPS PSU and then drop 5V to 3.3V if you are desperate to make linear PSU.

author: VEGETA
author_rank: Super Contributor
timestamp: October 10, 2020, 09:58:02 pm
content: Quote from: wraper on October 10, 2020, 09:36:14 pmI have 100x50x10 mm heatsink with fins.I have one of these aliexpress tinystep down converter moduleswhich are useless really, its output current is just about 1.7 amps.I ordered a replacement PSU but it will take time to arrive, so i thought of doing it myself.I have a lab psu that I made using buck modules, theoretically could go up to 4 amps but I doubt it. I could set it as 5v then take it to power the 3.3v pass linear stuff.

author: wraper
author_rank: Unknown
timestamp: October 10, 2020, 10:04:40 pm
content: Quote from: VEGETA on October 10, 2020, 09:58:02 pmIt's like 15 times too small for passive cooling. BTW I meant volume, not surface since I used calculator for volume. Typed wrong.

author: wraper
author_rank: Unknown
timestamp: October 10, 2020, 10:08:09 pm
content: Quote from: VEGETA on October 10, 2020, 09:58:02 pmDivide current by 4 and it probably will not fail with magic smoke escaping.

author: VEGETA
author_rank: Super Contributor
timestamp: October 10, 2020, 10:12:44 pm
content: I don't know if you are a bless or something, but when you said what you said I had an urge to go through the man cave to search for something useful and I found this:https://i.ebayimg.com/images/g/um8AAOSwrFtaO4ze/s-l1600.jpgIt is 30W buck-boost converter! I am confident it should be enough for 3.3v @ 2.8 amps max load right!? I will feed it 5v then take the same 5v and get it to the 5v rail.Do you think it is enough? assuming 2.8 amps for 3.3v and 0.42 amps for 5v rail? both still a lot under 30 watts.

author: wraper
author_rank: Unknown
timestamp: October 10, 2020, 10:16:01 pm
content: Quote from: VEGETA on October 10, 2020, 10:12:44 pmDunno if it's rating is true. But at least it looks that it could be possible.

author: VEGETA
author_rank: Super Contributor
timestamp: October 10, 2020, 10:41:38 pm
content: Quote from: wraper on October 10, 2020, 10:16:01 pmI tested it on 1.2 R load and doesn't exceed 2 amps at 5v and 12v inputs... this seems its own rated current anyway. things got bad now as there is no other options.this is my only choice:https://mikroelectron.com/Product/Buck-boost-DC-DC-Power-Supply-Module-XL6009/it is via local company, dunno if this one is enough.

author: wraper
author_rank: Unknown
timestamp: October 10, 2020, 10:43:44 pm
content: Quote from: VEGETA on October 10, 2020, 10:41:38 pm5/1.2 = 4.16A, it probably goes into protection.

author: VEGETA
author_rank: Super Contributor
timestamp: October 10, 2020, 10:52:37 pm
content: Quote from: wraper on October 10, 2020, 10:43:44 pmI had it set to 3.3v and 3v output, still same result. the 5v was the input, so does 12v. while the 5 and 12v input supply can get to 3 amps when set to 3.3v alone.