LM723 adjustable bench PSU not working

[Dannyx]

Re: LM723 adjustable bench PSU not working

A single 2N3055 will be good for 2A continuous, no more

Why is that exactly ? Because of the eventual heat buildup ? If I have a 20v input and I want a 1v output at 2A, according to your formula, that would give me 38w - is that correct ? If so, it seems way below what a 2n3055 can dissipate. I'm assuming you suggested such a low power to keep it in the "safe" zone with the minimal heatsinking.

[Th3_uN1Qu3]

Re: LM723 adjustable bench PSU not working

Based on the tiny heatsinks usually put inside these power supplies. You put a bigger heatsink on, it will do more current safely.

It is advisable to keep transistor case temperature under 60C. If it burns your finger, you need a bigger heatsink.

[eccerr0r]

Re: LM723 adjustable bench PSU not working

Also note that heatsinks will saturate with heat too, so you likely need a fan for small heatsinks. Larger heatsinks may be able to passively dissipate the heat.

[Dannyx]

Re: LM723 adjustable bench PSU not working

Based on the tiny heatsinks usually put inside these power supplies. You put a bigger heatsink on, it will do more current safely.

It is advisable to keep transistor case temperature under 60C. If it burns your finger, you need a bigger heatsink.

So basically I CAN go higher in current as long as I manage to cool the thing....makes sense. A fan should definitely be used too IMO...I need this info to hopefully build my own from scratch one day. LONG ago, I put together a very crude design of how I imagined it should look based on Dave's design I saw in one of his videos. That one used a simple plain ol' op-amp (well not really, he eventually resorted to using an LT3080, but that's friggin' expensive), but now I got another IC I can use, this LM723 guy which is purposely built for this and I've got a couple lying around. I thought it'd be easier to upgrade this one, because it's fairly stable. The only thing it lacks is current: can't go higher than 1.5a MAX, so upgrade the transformer, tweak the current limit and it will hopefully do what I imagine.

[Th3_uN1Qu3]

Re: LM723 adjustable bench PSU not working

I have a 2A one at work. The weak point in it is the bridge rectifier. When it died i found that it had been swapped by someone else before me.

The one i put in isn't quite optimal either, but it has been holding up so far (about 8 months IIRC). But the Chinese 2N3055 at the back runs pretty hot too. When it's been continuously running at over 1A for more than 1 hour it starts smelling funky.

A UPS transformer as the base for a lab power supply should make for a pretty solid setup if you size the other components adequately as well. The LM723 is very old tech (think 1980s) but it is simple and straightforward to use.

[Dannyx]

Re: LM723 adjustable bench PSU not working

Mine starts to reek of burnt stuff when I leave it on under high load also The 2n3055 burnt out A LOT in the past (I believe this is the third one I swap in), so to stop that from happening (or at least delay the unavoidable) I added a CPU heatsink with a fan to it. The thing that last failed now was the transformer itself: small and undersized and got extremely hot...took almost an hour to cool down enough to touch it. The rectifier hasn't failed so far and it also looks pretty beefy. It's not 4 diodes, it's a proper single-piece (don't know how you call those) part and looks like something I scavenged from TV PSUs.

[Dannyx]

Re: LM723 adjustable bench PSU not working

Really quick, here are some shots of the transformer I'm planning to use...not terribly huge, but still "beefy". Scale is CM (duuh)

[stj]

Re: LM723 adjustable bench PSU not working

just use a module.
this is the top one, but there is a table of versions on the page.

https://www.banggood.com/DP50V15A-DP...p-1072236.html

[Dannyx]

Re: LM723 adjustable bench PSU not working

just use a module.
this is the top one, but there is a table of versions on the page.

https://www.banggood.com/DP50V15A-DP...p-1072236.html

I feel like something's missing in your post...you mean like a step-down module or buck converter or whatever ? If so, I think there should be a link in there

[eccerr0r]

Re: LM723 adjustable bench PSU not working

no, I think he meant don't bother designing a wasteful linear design and depend on the chinese to get a switching solution without needing to do anything...

[stj]

Re: LM723 adjustable bench PSU not working

no, you supply the dc, it does the rest.
so your transformer + rectifier & cap.(or a laptop supply??)
then a module with the apropriate max current.
they do make ones that have boost converters btw.

[eccerr0r]

Re: LM723 adjustable bench PSU not working

Yep that seems about sums it up, no more design needed, just wire up top level stuff and make things that are just like everyone else's ...

[stj]

Re: LM723 adjustable bench PSU not working

it works - spend your time making stuff, not the tools to make stuff

[eccerr0r]

Re: LM723 adjustable bench PSU not working

It can also be said... know your tools... instead of having a black box.

[stj]

Re: LM723 adjustable bench PSU not working

black boxes matter!
so do translucent ones with leds inside - all from china!

[Dannyx]

Re: LM723 adjustable bench PSU not working

The one you linked to looks interesting as heck - would like to have a go on it and it's DEFINITELY beyond anything I'd be able to design I've seen things like that before around here also, though less fancy. Of course, I could just buy a pre-made circuit like that, but to me half the fun is building stuff...or at least TRYING to I'll definitely keep it as an option though. Cheers.

[sam_sam_sam]

Re: LM723 adjustable bench PSU not working

www.banggood.com

How good is Banggood about getting the stuff that you order can you trust there site

[Th3_uN1Qu3]

Re: LM723 adjustable bench PSU not working

I have seen a lot of popular youtubers ordering from there. It is at least as trustable as AliExpress.

[Dannyx]

Re: LM723 adjustable bench PSU not working

Would definitely like to get the DC-DC supply STJ linked to....not terribly expensive and looks fairly professional....stick an UPS transformer on it and I'd have myself a decent PSU. Some of my colleagues order stuff from Ebay and Aliexpress all the friggin' time, albeit very cheap stuff, like miscellaneous tablet and phone components, so I'm not sure at what value customs tax is added...or VAT...or some stuff like that...anyway SOMETHING extra is added if the parcel's value is higher than X. Will have to do some research. If this thing costs 36 dollars, I don't know if that's the final price I'll actually pay or I'll also pay a tax twice that, at which point it's probably not worth it...:|

[eccerr0r]

Re: LM723 adjustable bench PSU not working

Sometimes I wonder, the chinese selling unfinished stuff which are "virtually" finished to other countries as "parts" ... because the whole unit if it were "finished" has a tax on it and could not sell as many due to higher price.

I know this happened to laser diodes, not because it was a trade protection tax but rather FDA approval because unprotected high powered laser pointers are illegal in the US, but if you buy the parts to make a high power laser pointer, that's your prerogative. Except that the high power laser diode can easily be incorporated into a design and you still can fry your eyes with it...

Oh well, yeah this is off topic, alas I see all this 'dumping' going on and people snapping them up...