HOW does this 24V regulator put out 48V? - Page 1
Forum section: electronics / repair

author: jaunty
author_rank: Regular Contributor
timestamp: September 16, 2020, 11:18:31 pm
content: " alt="" class="bbc_img" />doing a repair on a power supply (in this case a 48v phantom power rail going to a mixing board). IC3 is an LM7824 (capable of 24V output regulated). I'm not getting the expected 48V output but rather more like 17V. Obviously there's a problem somewhere (already replaced all the capacitors as a matter of general service) but in order to diagnose the problem i need to understand how this LM7824 is generating 48V. Anyone?

author: bdunham7
author_rank: Super Contributor
timestamp: September 17, 2020, 12:16:32 am
content: Quote from: jaunty on September 16, 2020, 11:18:31 pm

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: September 17, 2020, 12:20:50 am
content: What is the voltage at the pins of the regulator.Measure the DC voltages around the circuit and update.The regulator will give 24V from its output to common pin if its input to common pin is > ~25Vthe common pin is lifted from ground which means you will get something greater than 24 V out.the gain is something similar to this page.http://www.learnerswings.com/2014/07/adjustable-output-voltage-regulator.htmlinside the lm7824 there already is a resistor divider, we have to take this into account.

author: jaunty
author_rank: Regular Contributor
timestamp: September 17, 2020, 02:08:01 am
content: thanks - I will reply later with test data when i get access, though i'm a bit ambivalent about supplying it only because SOMETHING isn't working properly and it might confuse issues - i was just hoping someone could give me a pointer if I supplied the basic circuit topology so i could understand how such a thing would be possible. my best guess is that this circuit is 'riding shotgun' on top of the neighbouring circuit providing pos 18V (indicated as 'V+' in the rail across the bottom, where one might ordinarily see a ground)

author: ledtester
author_rank: Super Contributor
timestamp: September 17, 2020, 02:16:30 am
content: What is the voltage at the input pin (pin 1) of IC3, and what are the values of R11 and R12? That will say a lot about how it is supposed to work.

author: bdunham7
author_rank: Super Contributor
timestamp: September 17, 2020, 03:02:18 am
content: Quote from: jaunty on September 17, 2020, 02:08:01 amThe LM7824 is going to try to maintain 24 volts between its output terminal and its ground or sense terminal. Less than 24 volts it will turn on more, more than that it shuts off more. The LM7824 has no way of knowing what the rest of the circuit is doing. In this case, adding R11 and R12 causes current to flow in those resistors which means a voltage drop. In normal operation, R12 will have 24 volts across it and R11 will have some other voltage that I can't determine without knowing its value and the current--and that the LM7824 has no way of knowing about. So if you measure from the output to the bottom of R11, which is the same as your V+, you should get 24 volts + the voltage drop across R11. If your V+ line has a potential to another point (lets call it ground) --you say 18 volts--again this circuit doesn't know or care about that. If you measure from the output to ground in that case you will get 24V + the R11 voltage + 18V.If there is another power supply in series like this and your circuit is an 'add-on', and the output is 17V measure to ground, while V+ is 18V, the most likely cause is that there is a complete failure, perhaps no power from the transformer, and the 17 volts is just your 18V from the V+ bleeding through the resistors and the unpowered regulator. Or, the regulator could be open, with the same result.

author: floobydust
author_rank: Super Contributor
timestamp: September 17, 2020, 03:13:29 am
content: I think the IC is better a LM317HV or TL783. To increase a 78xx output voltage I have seen a zener used in the GND terminal.

author: jaunty
author_rank: Regular Contributor
timestamp: September 17, 2020, 12:43:52 pm
content: Quote from: ledtester on September 17, 2020, 02:16:30 amwell not with a failed bridge rectifier - which seems to be the issue with the circuit. i just thought someone might recognize the topology. getting 0.919V DC across C11 but 32VAC coming in from the toroid winding. input to the rectifier is 17.x volts and output 15.x volts ... so i guess it partly answers our question. it seems the 17.x (18V really by design) is supposed to be borrowed from the other leg and added to whatever is generated by this particular leg of the circuit by the rectifier (?) - still makes no sense to me - thought this would have been a big 'no no' adding voltages like this ...

author: jaunty
author_rank: Regular Contributor
timestamp: September 17, 2020, 12:48:03 pm
content: Quote from: bdunham7 on September 17, 2020, 03:02:18 amyes that seems to be what's going on mostly ... 32VAC out of the trafo and nil coming out of the diode bridge. but still scratching my head over the philosophy behind this - how the two legs are added together through the LM7824, if that's what's going on.the resistors are R11 = 470 ohm, R12 = 3K7 and R13 = 3K7

author: jaunty
author_rank: Regular Contributor
timestamp: September 17, 2020, 12:55:07 pm
content: posted just above. nil (0.9v across C11) and 32VAC out of the toroid (thank goodness). dead diode bridge i reckon - but thats an easy fix. knowing the potentials across R11 and R12 isn't going to tell us much it would seem. Guess the thing to do is put in a BY164 (closest thing i have to a B80C1500) and fire it up again and figure out how this thing is working. I may disable this circuit or mod it if it's taking a current draw from the +18V leg as i'd rather not compromise the feed to the op amps in the mixing board. if that's what's going on it seems it could be a much better design!

author: jaunty
author_rank: Regular Contributor
timestamp: September 17, 2020, 01:12:08 pm
content: this may help - the full power supply for confirmation (should have posted earlier but i was trying to keep things simple)" alt="" class="bbc_img" />it does not appear to be drawing power from the +18V circuit however (according to this)

author: tunk
author_rank: Super Contributor
timestamp: September 17, 2020, 01:29:14 pm
content: The voltage across R12 is 24V and the current is 6.5mA.The current through R11 is this plus the "Quiescent Current"(max 8mA, typically lower, maybe 4-5mA) giving ~5V.The total: 24 + 5 + 18 = 47V

author: bdunham7
author_rank: Super Contributor
timestamp: September 17, 2020, 05:02:32 pm
content: Quote from: jaunty on September 17, 2020, 01:12:08 pmDon't do that. The complete picture is always helpful.This is not a complicated issue. If the power supplies are truly isolated, then you can connect them in series and add up voltages just as if you were using 9 volt batteries. Nothing odd or wrong about it. I'm assuming the 48V supply is very low current.As for the bridge rectifier failing in that manner, I would take it out and test it thoroughly before coming to that conclusion. There might be some other strange issue causing a loss of isolation, or something. It would have to have failed open somwhere for it to behave this way and open is not the typical failure mode. Hopefully it is the failure in your case.

author: jaunty
author_rank: Regular Contributor
timestamp: September 22, 2020, 03:21:13 am
content: well after a ridiculous amount of running around and fooling myself - thinking the fault must have been a short somewhere - i discovered that the circuit had a blown fuse - so i popped a new one in and blew that immediately too (!). Having checked over nearly everything I finally resorted to my dim bulb tester - and yep sure enough that thing lit up BRIGHT when i put a fuse back in! So i decided to second guess myself and pull the secondary from their solder connections and GUESS WHAT? 1-2 megs is what i measured!!well at least i feel some satisfaction at least of having found the issue. But here's my last question then (i gave up trying to understand how an LM7824 generates 48V) - how the hell does an open secondary generate a fault condition??? the fuse is located between the secondary and the diode bridge! Shouldn't an open secondary behave like any open circuit?? not making sense to me - but it sure explains the earlier symptoms!!

author: jaunty
author_rank: Regular Contributor
timestamp: September 22, 2020, 03:43:08 am
content: also - is it dangerous to operate the transformer with the open secondary? i was reading something about this condition causing abnormally high core EM saturation and therefore messing everything else up (?) or no?

author: bdunham7
author_rank: Super Contributor
timestamp: September 22, 2020, 04:27:55 am
content: Not making a lot of sense here.On your schematic, is it F1, F2 or F3 that is blowing? Or is there another fuse?An open secondary would be open, no current would flow and your fuse would not blow. And no, your open/disconnected secondary shouldn't cause any serious issues--as in nobody will die.

author: jaunty
author_rank: Regular Contributor
timestamp: September 22, 2020, 03:10:17 pm
content: not making sense to me either! not a great schematic sorry - it's a totally different fuse - let's call it F4 - located between the secondary for the 48V rail and the diode bridge! but it's creating a fault (via dim bulb tester) in the main line when that fuse is in. I disconnected the secondary and measure well over 1 megohm. I don't see how that could be creating the fault but there seems to be lots of little gaps in my knowledge like that ... what do you surmise from that? I'd love to know

author: bdunham7
author_rank: Super Contributor
timestamp: September 22, 2020, 03:16:55 pm
content: You previously measured voltage at this transformer output. The fuse is blown, indicating a short. You've checked with a dim-bulb tester with the fuse in and it indicates high draw, indicating a short, assuming you have an appropriate bulb and all.I would guess that there is a short in the bridge rectifier and that you are making some error in measuring the resistance of the transformer. First, try the dim-bulb test without the fuse in place--if it still lights up that means something else is going on. If that is good--no short with fuse out--then pull the bridge rectifier out and test that.

author: jaunty
author_rank: Regular Contributor
timestamp: September 22, 2020, 03:48:01 pm
content: yes! already done. The dim bulb tester behaves BEAUTIFULLY with the fuse out. just a slight glow for half a second and then off. earlier - suspecting the bridge rectifier i removed it and tested it (fine) and then put in a new one with higher rating anyway - just to be safe. I've also replaced the reservoir cap and the LM7824 with new ... but yeah there sure is something not making sense here! thanks. I'll double check the winding resistance - who knows maybe it was a lacquer conductivity issue - i did notice it was jumping around a bit at the time ...amendment: dammit! you're right!! 1.3 ohms! gahhh ... i felt better when i decided the winding was open. hmmm... i guess the next step (after double checking the new bridge rectifier) would be to put the secondary leads back on the board and then cut the traces after the reservoir cap and see how things behave then? I was about to do that anyway. Sorry for the confusion.

author: jaunty
author_rank: Regular Contributor
timestamp: September 22, 2020, 04:03:38 pm
content: what is making this thing a bit complicated is that the +18V rail seems to be feeding the rail with the LM7824 on it to make up the 48V. Although it's not being shown in the schematics. But both the +18V and -18V rails seem to be performing nicely - which is making me think the fault isn't likely to be coming from that end of things ... i feel like i need fresh eyes on this.

author: bdunham7
author_rank: Super Contributor
timestamp: September 22, 2020, 04:38:53 pm
content: Quote from: jaunty on September 22, 2020, 03:48:01 pmWhoa! Slow down! More thinking, less cutting and soldering!Since your soldering iron is warm, take out the LM7824 completely and redo your dim bulb test. That should narrow things down a bit. If it still fails, disconnect the transformer secondaries again and test for continuity between those and F2 and F3.

author: jaunty
author_rank: Regular Contributor
timestamp: September 22, 2020, 05:03:41 pm
content: well i hadn't cut anything yet - but what i mentioned was previously tested.Anyway- i just did what you recommended - pulled the 7824 and reconnected the secondary for that rail and yep ... bulb lit up like the sun - which would have been my prediction i suppose.one really ODD thing going on here that i'm not sure has anything to do with the problem - but differs from every other PS i've worked on is that I'm getting open measurements IN CIRCUIT with the bridge rectifier - both the old one and new one i put in. when it remove them they test fine. But I've never seen that before - i've always had a fair amount of success testing them in circuit in the past. I can't think of any condition that might cause this to be the case.

author: bdunham7
author_rank: Super Contributor
timestamp: September 22, 2020, 05:16:16 pm
content: Quote from: jaunty on September 22, 2020, 05:03:41 pmPerhaps you have ohmmeter/test lead issues or the capacitor is still charged and interfering. From your description, either there is a cross-short in the transformer or the bridge rectifier is not installed correctly. Can you post a clear photo of this thing?

author: jaunty
author_rank: Regular Contributor
timestamp: September 22, 2020, 05:30:23 pm
content: I dunno - could be the awkward angle i'm working at ... but i doubt there are test lead issues. more likely to be my error but if i'm able to successfully test them out of circuit every time but never IN circuit ...well who knows.here are some pictures.overview of rectifier, transformer leads, etcok seems i cannot post more than one upload at a time.

author: jaunty
author_rank: Regular Contributor
timestamp: September 22, 2020, 05:32:01 pm
content: heat sink that holds 7824.

author: jaunty
author_rank: Regular Contributor
timestamp: September 22, 2020, 05:32:51 pm
content: bottom of diode bridges and transformer winding couplingsecondary is soldered in the way i found it at (grey) 15V terminal and (red) 15V terminal. The secondary makes a loop up to the fuse and back - one side connects to the bridge rectifier at rightmost terminal (left at top of board). the other side of the secondary connects to second terminal of BR. the the reservoir cap (yes its in the right way lol - as per markings on the board anyway) is connected to terminals 1 and 3 of the BR.

author: jaunty
author_rank: Regular Contributor
timestamp: September 22, 2020, 05:33:28 pm
content: view of fuse holders (end one is the blowing fuse) for the 48V rail. 4700µF/63V cap in foreground is the 48V rail reservoir cap.

author: jaunty
author_rank: Regular Contributor
timestamp: September 22, 2020, 05:34:09 pm
content: that's about it as far as images go.

author: bdunham7
author_rank: Super Contributor
timestamp: September 22, 2020, 06:06:23 pm
content: It's still not clear, perhaps a larger all-in-one shot to get the general layout? Could you point out the relevant components? I assume the green and purple wires are the relevant transformer leads, but is C11 the single large black axial? If so, I can't see the traces clearly, but is the (+) on the bridge connected to the (-) on that capacitor? Also, I see you pointed out the heat sink for the regulator, but I don't see the connection pads?EDIT: Just saw rest of photos. It's annoying when audiophoolery stuff uses unnecessarily huge boards.

author: bdunham7
author_rank: Super Contributor
timestamp: September 22, 2020, 06:24:38 pm
content: You have the wrong bridge rectifier. The markings on the bridge need to match the function of the traces--the AC inputs are marked with a ~ and the outputs are marked + and -. If I've compared correctly, the actual terminals of the bridge are as I've drawn them, which won't work. And, although this bridge won't work unless you bend and swap two of the leads, you also have it in backwards. At least that is what I'm seeing.

author: jaunty
author_rank: Regular Contributor
timestamp: September 22, 2020, 06:55:35 pm
content: 'audiophoolery stuff' ? not at all. it's not a piece of hifi equipment. just a humble mixing board. i was not involved in the manufacture of it nor the design however. the pictures are all captioned with relevant information (i.e. how no capacitors were reversed etc etc) and which component is what.the bridge rectifier is a BY164 and pretty much the same spec as the original B80C1500 - pinout is the same. if you reverse it (from what i understand at least) it shouldn't make a difference (or?). But i can put the original back in for you if you like. The symptoms will be the same. - ok i will try that then ... it would certainly be fantastic if this solved it but i don't really see how

author: jaunty
author_rank: Regular Contributor
timestamp: September 22, 2020, 07:06:00 pm
content: ok i see what you mean now.hmmm. ok i'll put the old one back in and see ...https://www.radio741.com/en/b80c1500-1000-bridge-rectifier-fagor/23851-b80c1500-1000-bridge-rectifier-fagor.html

author: jaunty
author_rank: Regular Contributor
timestamp: September 22, 2020, 08:09:17 pm
content: WOW - thanks for being the 'fresh eyes' i needed ... i guess i was concentrating so much on the other regulator specs I didn't notice the pinout was all wrong ... and then all of the steps i was troubleshooting before discovering this were moot since I hadn't realized the fuse was blown (assuming the power good light meant something). So i have a very healthy 61V going into the LM7824 and 42V coming out (not sure how that works still but hey - its better than before) - numbers still aren't perfect but I feel confident i can chase down any issues now. I'm not sure if others have been through this but it's amazing sometimes what a wild goose chase we can create for ourselves (speaking only for myself here)i owe you a bottle of something. THANK YOU!

author: bdunham7
author_rank: Super Contributor
timestamp: September 23, 2020, 02:22:54 pm
content: It has the feel of audiophoolery--strange board layout, using standard components improperly, unnecessary dependence on specific or matched components such as a particular brand or even individual unit of a common part--in this case the LM7824.You still have some issues.First, if you measure 61V to ground, that means that your LM7824 is seeing 43V input, but it is only rated to 40V. That is poor design, although you might get away with it for a while.Secondly, the reason you have 42V input makes sense at first glance because you have a 24V regulator floated up 18V, so 24+18=42. I guess the answer is always 42.However, the two resistors are supposed to tweak that--but resistors will only work for one particular LM7824, not all of them. The reason such so-called standard parts will vary like this is that they have their own internal resistor set to set the voltage and while the ratios have to be the same between models, the exact values used do not. The way to do this correctly, if this circuit can ever be called 'correct', is to replace R11 with a 5.6 to 6.3V zener diode.If the current requirements of the circuit are low--you didn't mention the value for F4, but 'phantom power' seems to imply low power--then you could also put a 5.6V zener between the positive side of C11 and the regulator as well as a current-limiting resistor to protect against output shorts.

author: jaunty
author_rank: Regular Contributor
timestamp: September 23, 2020, 06:51:47 pm
content: amazingly (maybe not amazing to you) i went through two more LM7824s but FINALLY got a stable and accurate output voltage!! so that's great. 46.5V but the spec requires ± 4.0V only. I think i found the main source of my issues - which was a bad trace that i just jumpered over.as for the design - well who knows ... maybe it's a 'dutch thing' (made in holland). The 'audiophool' designs that i've seen before all look pretty rational to me with beefy traces and massive amounts of attention paid to star grounding etc. Usually overkill is the operative word with that stuff. This one doesn't seem terribly like that to me - but hey what would i know?anyway - thanks again for the help! couldn't have done it without you (well possibly but i would have wasted way more time probably)PS - forgot to answer your question - yes that fourth fuse was a 1A. it's a very low current circuit.incidentally - service manuals for the models that came out after this all use an LM724 which is probably a much better choice - i agree that the 7824 is sketchy - at least as i understand it. either that or it's very clever. i can't imagine it was a cost saving measure as an LM723 or LM317 would have been just as little likely

author: Technomaniac
author_rank: Contributor
timestamp: September 26, 2020, 12:13:47 am
content: Been trying to follow the conversation and getting befuddled. But AS I SEE IT, the bottom power supply uses a 24v chip, but its ground pin is returned to ground through a resistor network that lifts it well above ground, by the amount of extra voltage above 24 that's required. The output from the transformer wasn't enough, so instead of returning the negative of the bridge rectifier to ground, it is returned to +18, which therefore provides an input to the 24v reg of whatever the bottom transformer provides, PLUS an extra 18 volts from the top supply.(That arrow that says + is pointed the wrong way. It's an INPUT from the top supply.) Presumably these two voltages add to something higher than 48 volts. The bottom regulator, because it's ground pin is not grounded, is lifted above ground by the resistors in its ground circuit, must have 24 volts on this pin. Then the 24v regulators OUTPUT pin will instead have 24 volt PLUS whatever's on its ground pin.First thing to do, put a lamp in place of the fuse, and measure the voltage on the regulators ground pin. It needs to be +24 volts. But remember, it has to come THROUGH the 24v reg. But if you're blowing fuses with the reg removed, it has to be the bridge or the input electro - I wonder if you're using fast blow fuses when you need a SLOW BLOW which will handle the initial charging of the first electro at switchon?

author: jaunty
author_rank: Regular Contributor
timestamp: September 27, 2020, 07:06:21 am
content: Thanks that was a reasonable explanation - it was kind of my understanding so far (that the 'ground' or 'common' ref for the middle pin of the LM7824 was well above zero so the regulator wasn't getting destroyed due to the 'floating' ground etc.no fuses are blowing anymore though, luckily. (crossing fingers, knocking on wood!)- it was a stupid oversight - i'd tested various parts of the circuit (like changing to the wrong bridge rectifier) at a time when i did not realize there was a blown fuse. So it led me to believe there was no issue with the rectifier - when i found the blown fuse i replaced it and immediately started blowing fuses. With the original bridge rectifier back in i am no longer blowing fuses. I should probably look at slow blow fuses though in the future since i beefed up the reservoir caps by a good amount ...