Gotcha. Just haven’t seen that design before. Functionally, I’m not exactly sure what happens 
If the regulator fails and outputs 10V, how does the diode on the regulator help exactly? The crowbar circuits I have looked at online (and that have been discussed a bit on this thread) are based upon a voltage max that triggers the crowbar - how does this design differ (since I don’t see a trigger)?
Gotcha. Just haven’t seen that design before. Functionally, I’m not exactly sure what happens
As i said, i have to look into a crowbar design. So that’s not included into the current schematic. So there is no OVP or OVC protection atm. Just protection for the regulator and for latchup.
The latchup I think I get (looks similar to a voltage clamp, though how those work is still a bit alien to me). But how does the diode protect the regulator exactly? I’ve just never seen that and it seems like it might not be a bad idea to implement which is why I’m curious
Someone correct me if i am wrong. But incase of a latchup. The parasitic structure (eq to thyristor) can change the polarity. So the GND is changed to a -DC and the +DC is changed to GND.
For the other diode have a look at this datasheet and scroll to page 11.
Aha! That’s super helpful, thanks! It sounds like it’s only required if there’s large caps on the other side, though that’s certainly possible (you can use the 2200uF power cap from the MB-6582 in a custom PSU design, for instance). So, yeah, that seems like a good idea
I assume I need a diode for a negative regulator as well? I did some poking around on National’s website and did not see a diode used on the 79xx datasheet, though I suspect it’d be basically the same thing?
Hi. Did u see the design i posted in the chat?
I assume u dont talk about the rectifier diods…
There is no other diods in the designs appart from the zener diode. The fourth one that looks like a double diod is a SCR. it is the shorting component in the crowbar. The zenerdiode is a 6.5v and it works this way;
When the voltage reaches 6.5 volt the scr will turn and thus opens connection to ground and shorts the psu long enough for fuse to blow.
But the crowbar depends on current limiting, such as a fuse or a fuse in combination with current limit transistor.
Even if a crowbar is defined as a overvoltage protection, voltage and current has its relationship…
Hi. Did u see the design i posted in the chat?
No, although it would be great if you could post it here. That way those that might be searching for an answer to these questions in the future will have a place to look
I think he means this one:
If you adapt this to 2 separate windings then you need to add the diodes again.
edit by nILS: Added a link to the picture’s source 
This design is from the wiki, where you can find some explanation as well. Sadly I ran outta time to brush up the other designs as well. It’s back on my ToDo list now.
No, this isnt the design I posted - I thought I had posted it in the forum too, but I might have forgot But I’ll make a new version of the schems tonight and post it later on since new information has surfaced since I made that design.
I will post as soon as I have made the new version, and you guys can tell me what you think.
My design has no -12v rail yet. I haven’t incorporated it yet because I have to keep my head straight when doing the design… I figured that if I do the positive side first, the -12v rail would be pretty easy to do afterwards.
Many thanks goes out to nILS and SmashTV for helping me out being my “tutors” in designing the psu - you guys really pointed me in the right direction :D.
The one thing that sets me back now are xformers. I have none and I’m struggling a bit to deciding what to use. I think I will decide on a design tonight and order in the part so I can finally start making a real prototype, and stress-test it a bit to see if it actually works.
About crowbars:
There is a thing with all simple current limiting circuits - as you mentioned above in some post - they are vulnerable to false triggering on startup. I think the main reason to this is large capacitors on the power rails. They charge up really quick, and when they demand some power at startup to get fully charged that can actually trigger the crowbar and the fuse will blow. I guess if the fuse doesn’t blow you will get a hickup from the psu turning on / off / on / off and so on. The solution might be to use not too big caps in the psu and also set the crowbar’s zener to a reasonable voltage so that it doesn’t trigger the scr or thyristor too soon.
The diodes for latchup is interesting part I haven’t looked into yet… I will read some more :).
I’ll be back later on with my new version
See you.
I am curious what you have come up with. So looking forward to seeing your schem.
Hi
As it turned out i had too little time to work on the schem this saturday, so i will do some work tonight and monday evening. I recon i will have a schem ready then for you to see.
I have thought of a couple of new things to add, ie led indicators for normal op and protection.
Ill see if theres a simple way to do it. Afterwards i would need some serious evaluation of the schem from people who knows this stuff, so if anyone feel like they can verify the design and suggest changes, speak up.
Hi
As it turned out i had too little time to work on the schem this saturday, so i will do some work tonight and monday evening. I recon i will have a schem ready then for you to see.
I have thought of a couple of new things to add, ie led indicators for normal op and protection.
Ill see if theres a simple way to do it. Afterwards i would need some serious evaluation of the schem from people who knows this stuff, so if anyone feel like they can verify the design and suggest changes, speak up.
Ok, well i am working on my schem atm. Now wondering if instead of just using a 78S05 it would be a good idea to combine it with a transistor something like a tip2955. That way it would have a little more room to breath. And better power dispensation. Also struggling to add the MC3423 to my eagle library. :pinch:
This is a good idea, and is very similar to my design. But current limiting / crowbar and fuse is even more important with pass transistor.
I have looked at tip31c as the limit transistor. Its 10A. Havent done the math yet, so ill have to do some calcus to see what the actual needs are.
This is a good idea, and is very similar to my design. But current limiting / crowbar and fuse is even more important with pass transistor.
I have looked at tip31c as the limit transistor. Its 10A. Havent done the math yet, so ill have to do some calcus to see what the actual needs are.
This is what i have come up with so far:
This design should be sufficient for 5A on 5V+ line.
The TIP2955 is 15A. And the packaging is a bit bigger then the TIP31. I haven’t really specced the thyristor yet, just picked one from Eagle lib.
Edit: Meh, i need to add a short circuit protection as well.
Hi.
It might be that the pic is lowres or i cant see it properly on my shitty htc, but i cant seem to understand the meaning of the transistor that passes over the vreg. I mean, i see its the pass transistor, but how does it actually work? I thought its base was supposed to connect after the vreg and not before… again i am no expert though… hehe. Ur schem seems interesting though, a bit more advanced than my basic idea… could u explain what the intention with the design is? Good work. It would be cool if we could come up with a fairly simple but good design that everyone can use. When i started projecting the psu i soon discovered that it was far more to designing a good psu suited for this delicate application than i first thought. I really enjoy learning about it. I have struggled for many years to learn about psu, logics and electronics in general and really understand it. I never went to any school about this, so everything i know i learnt from the internet, my dad who used to design industrial robots for a living and from the good people in here. Especially smash and nils.
Im working on the schem tonight, and ill post it later on.
Sorry, cant find a way to edit post on my phone,
But another thing i thought about is that i have some huge mosfets lying in box here. They were intended for driving big dc motors on a precut saw. The technician on the site left them behind in the factory they built it along with other components. They are rated about 100amp if i remember correctly.
It would be cool to use them instead of the bipolars. A wee bit overkill yes. But since i have them… i havent studied mosfets very much, so if anyone has an idea on if mosfets can be used please speak up…
I will anyway make the design for bipolars, but i could use them in my supply and make the necesary changes for my own psu if they can be used.
Actually I think it would make more sense to put a current limiting transistor before the regulator? If you are using multiple regulators off of one transformer, for instance, you can have just one master current limiting circuit for the whole works?
Please have a look at the 78xx datasheet at page 24 figure 13.
As you can see there, the application of the transistor is not current limiting. If the current surpasses 1A the transistor will take over. This will increase the current output to a max of 3 to 5A depending on the transistor used. As i said the design is not short circuit protected. So i should adapt it to resemble figure 15. at page 24 of datasheet.
My intention is to use 2 transformers. 1 for 5V and 1 for 9/12V line. The 5volt line is most sensitive. Hence the more intricate SCR design. I intend to include a simpler (less sensitive) zener diode based scr design on the 9V/12V line.
Ah ok…yeah I dunno if you need 3-5Amps for our purposes Many regulators can go 1-2A with proper heatsinking and that’s likely more than you would need in most of the applications. Can’t hurt though, I suppose.