I have a short ( i think) and don't know where to start - Page 1
Forum section: electronics / projects

author: Hextejas
author_rank: Frequent Contributor
timestamp: December 15, 2018, 06:21:32 pm
content: I am building this power supply and so far I have exploded 8 diodes, mur860. I tried narrowing it down but I am stumped.When I attach diodes 1-4, it seems to be ok, but when i attach diodes 5-8 and apply the power, then WHAMO ,!A MUR860 has caught fire or exploded or somesuch. Scary.I have tried it with lesser voltage as mains. This is with a light in between. It furnishes 52VAc but apparently is not enough for the transformer.The transformer is furnishing 38VAc into the diodes.So,,,,,where to begin ?I ordered 20 more MUR860s but they won't be here until next week.Thanks for the guidance.

author: Andy Watson
author_rank: Super Contributor
timestamp: December 15, 2018, 06:35:35 pm
content: Can you confirm that the transformer has two isolated secondaries rather than a centre tap ?There's not much else to go wrong.

author: PA0PBZ
author_rank: Super Contributor
timestamp: December 15, 2018, 06:48:13 pm
content: Did you mount the diodes isolated? The flange is connected to the cathode so if you just mount them on a heatsink the result can be 'interesting'.

author: Hextejas
author_rank: Frequent Contributor
timestamp: December 15, 2018, 07:18:34 pm
content: well, I dont know exactly. The xformer has 4 wires coming out of the secondary.This is it, 500vahttps://4donline.ihs.com/images/VipMasterCap/Capacitor/AVEL/AVELS00030/AVELS00030-1.pdf

author: Hextejas
author_rank: Frequent Contributor
timestamp: December 15, 2018, 07:22:07 pm
content: I dont know what you mean by isolated. I have a pcb that includes the diodes.And a question.What would happen if a diode leg was not connected at all, or if the diode was completely dead or missing ? Obviously the board would not work, but would it act like it was shorted ? Smoke and fire and scary ?

author: PA0PBZ
author_rank: Super Contributor
timestamp: December 15, 2018, 07:34:43 pm
content: So you didn't mount the diodes on a heatsink? Did you check the datasheet to see how much current can flow without a heatsink?Also, make sure that the metal part from one diode is not touching another one, picture maybe?If a diode was not connected or missing you would not get fireworks, it one is shorted you can expect scary stuff.Edit: Your transformer seems to be fine.

author: fsr
author_rank: Regular Contributor
timestamp: December 15, 2018, 08:40:09 pm
content: When you connect the first 4 diodes, is the output voltage the expected one?I don't see how can the other diodes cause a problem with the 1st bridge, if the outputs aren't connected to anything.I don't get why the transformer has two secondaries, if they label the inner taps as "0V", and the corresponding outputs as "GND". So, is that connected afterwards on the circuit? Then why not to just use a center-tap transformer, and only one bridge?

author: nick_d
author_rank: Regular Contributor
timestamp: December 15, 2018, 09:21:16 pm
content: Disconnect the transformer and check it separately.To check your board, put DC voltage on U1/U2, by say a 9V battery, or something that doesn't have the juice to blow anything up, and check the output (V+) does what you expect. Swap U1/U2 and you should see no change. (It may be prudent to discharge those big caps between tests for reliable results, or wait for your discharge resistor to do it).Do the same on U3/U4 with U1/U2 disconnected. And then finally to check that U1/U2 is isolated from U3/U4 you can repeat the same checks with an extra battery connected from U2 to U3. This ensures the circuit is capable of operating with an arbitrary voltage between the isolated parts. Ideally check both parts work simultaneously with the 9V battery U2->U3, then move it onto U3->U2, U1->U3 etc. This ensures they can float independently of each other.Be suspicious of the load as well. Disconnect during the above tests and check it separately. (Especially if it connects the grounds together, which would stop your independent floating supplies floating relative to each other).What is the reason for the 1 ohm resistors and the small value cap connecting V+ to V-? Just curious.It looks like you intend the grounds to be connected to each other at the load, if that's the case there are probably easier ways to achieve the circuit using the centre tap as GND.cheers, Nick

author: Hextejas
author_rank: Frequent Contributor
timestamp: December 16, 2018, 02:08:31 pm
content: Thank you nick for your thoughts.This new photo is what the board looks like minus one mur860. I have the blank board below the populated board.Since I am waiting for replacement mur860s, could I put a jumper into where D6, diode 6 should go ? This is so I can try and troubleshoot it.I figured that with a missing diode I would still get a partial rectifier. 3/4 of the waveform maybe.What do you think.

author: sokoloff
author_rank: Super Contributor
timestamp: December 16, 2018, 02:52:10 pm
content: A full wave bridge rectifier uses two diodes for each half (positive going or negative going) cycle of the AC wave.Taking one diode away doesn’t pass 3/4 of the wave, but rather takes out an entire pair of diodes or half the full-wave rectifier.

author: Hextejas
author_rank: Frequent Contributor
timestamp: December 16, 2018, 03:04:15 pm
content: Nick, as i follow your suggestion of putting a small DC voltage on U1, where do i put the other side of the DC voltage ? The ---- minus side of the 9V battery ?As an aside, this is not my design. I am following anothers. Got it from here.http://www.circuitbasics.com/design-hi-fi-audio-amplifier-lm3886/#The-Final-Schematic

author: Hextejas
author_rank: Frequent Contributor
timestamp: December 16, 2018, 03:11:31 pm
content: Quote from: sokoloff on December 16, 2018, 02:52:10 pmI put a jumper in its place so I think that it will not act the same as if I had taken it away. It's gonna do something.Actually, when I get my new supply, I will breadboard it to see what happens.

author: PA0PBZ
author_rank: Super Contributor
timestamp: December 16, 2018, 03:48:43 pm
content: Ehm… if what I see is correct then I found your problem… Are the 4 pads with the red line all connected to each other?

author: Hextejas
author_rank: Frequent Contributor
timestamp: December 16, 2018, 04:08:13 pm
content: Quote from: PA0PBZ on December 16, 2018, 03:48:43 pmNo, they are not. But thanks for looking.

author: sokoloff
author_rank: Super Contributor
timestamp: December 16, 2018, 04:28:40 pm
content: Do you have any 9VAC to 24VAC wall transformer available?If so, I'd first remove R3 and R4.Then, put the output of that wall wart across U1 and U2 and measure the output and stability of that upper stage.Then, remove it from there and connect across U3 and U4 and measure the output and stability of the lower stage.Then, remove and jumper (short) U2 and U3 together and put the wall wart across U1 and U4.Perhaps first measure the resistance across U5 and U6 and U7 and U8 on an unstuffed board and on a board with everything except the diodes and R3/R4 stuffed.If all of that is OK, then see if the board can be powered from the intended transformer with R3 and R4 removed.

author: sokoloff
author_rank: Super Contributor
timestamp: December 16, 2018, 04:31:36 pm
content: IOW, Take a simplify, divide, and conquer approach. That tends to let less smoke out of things, though frustratingly still not zero...

author: mikerj
author_rank: Super Contributor
timestamp: December 16, 2018, 06:36:29 pm
content: Are you just replacing the one blown up diode each time? It's quite possible that one or more of the other diodes has gone short circuit internally.

author: nick_d
author_rank: Regular Contributor
timestamp: December 16, 2018, 09:08:52 pm
content: I think you need all diodes, however if you are just trying to validate the low voltage, low current operation you can put small signal diodes, like 1N4148 instead. I realize it is a hassle assembling the board up like this (try not to get solder in the through-holes) but much cheaper and less frustrating when things are not working right.As to where to put the DC voltage, when I said put a DC voltage U1/U2 I meant ACROSS U1 AND U2, I realize that can be confusing as it could have meant EITHER U1 OR U2. Anyway put it across U1 and U2 in a polarity that simulates the positive half of the cycle then reverse the polarity to simulate negative half, discharging the cap in between to check you can recharge it with either polarity input (or better, use a dummy load such as a lamp, but be careful not to introduce new problems or shorts via the load, and don't use the real load as it may be the cause of the original issue).cheers, Nick

author: fsr
author_rank: Regular Contributor
timestamp: December 17, 2018, 01:15:55 am
content: Quote from: Hextejas on December 16, 2018, 04:08:13 pmIt looks like there isn't a lot of clearance between the big track and the pads inside of it. Make sure that only the pads that must be connected to the big track are actually connected. Solder can cause short-circuits with such a small clearance (the solder mask should avoid it, but it doesn't harm to check). Even the terminals themselves could make a short to the track, if the wide section of the terminals bite into the solder mask.Two pads from two of the diodes of the bridge should be connected to the big track which should go to one terminal of the capacitors, while the other two diodes are connected to the other terminal of the capacitors. The catodes of the diodes go to the positive terminal of the capacitors, while the anodes go to the negative terminal.Possible source of confusion: C10 must be connected only between U9/U5 and U8/U12. When wires are crossed, they must be connected only if there is a red circle over the crossing. If there's NO red circle, the wires just pass over each other, and they should NOT be connected.With no load connected, there should be no interaction between the top and bottom diode bridges, besides C10, which won't do anything until the grounds are connected together.

author: james_s
author_rank: Super Contributor
timestamp: December 17, 2018, 02:25:00 am
content: For testing, wire a 60-100W incandescent lightbulb in series with the power input, that will stop you from losing all the magic smoke each time something goes wrong, and you'll have some time to take measurements under power and see what's happening.

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 17, 2018, 02:45:00 am
content: Quote from: james_s on December 17, 2018, 02:25:00 amdid he make any? what his multimeter (continuity tester) told him?

author: Hextejas
author_rank: Frequent Contributor
timestamp: December 17, 2018, 02:43:53 pm
content: This is strange. I am using a benchtop power supply ( ps ), a GPC3020, that can put out 0-35v DC. I have it set to 20v and this is what happens.It has 3 output connections. (--- GND +++)I connected the +++ to U1 and when i connected the --- to U2, the ps blinked and turned off the C.V. led and turned on the C.C. led.I think that it means it switched from constant voltage to constant current. I don't know if that was good or bad and the implications. Anyhow, what measured 20Vdc before now measures a lot less.I am thinking I need to do something to fix that.Secondly, when I try the same thing with U4 +++ and U3 ----, the ps stays at CV (20v8 dc), but none of the output shows any voltage.Let me see if I can find a 1N4148 or some such.

author: Hextejas
author_rank: Frequent Contributor
timestamp: December 17, 2018, 02:46:36 pm
content: Whoa, I found a tube of 8, MUR860sso here we go.

author: sokoloff
author_rank: Super Contributor
timestamp: December 17, 2018, 02:51:52 pm
content: Quote from: Hextejas on December 17, 2018, 02:43:53 pmA typical bench power supply will supply a set voltage or a set current,whichever is less. If you set it to 20V and a max of 100mA, it will provide 20V and 0mA when not connected to anything (and will be in "constant voltage mode"). If dead shorted, it will supply 100mA and way less than 1 volt. (It will be limited the voltage to whatever can push 100mA through the short. This is "constant current mode".)In your case, it's telling you that you have a very low resistance path (maybe a dead short) on the U1/U2 side.Quote from: Hextejas on December 17, 2018, 02:43:53 pmYes. Find the short.

author: Hextejas
author_rank: Frequent Contributor
timestamp: December 17, 2018, 03:29:14 pm
content: Well, I found a 9v AC wall wart so here goes.Hmmmm, how do I wire this ?I have the 2 wires coming from the wall wart and I need ac1, ac2, and 0v1 and 0v2 ? Where do I get 0v ?And thank you very much sokolof. I wondered how that worked.

author: Hextejas
author_rank: Frequent Contributor
timestamp: December 17, 2018, 03:49:25 pm
content: Progress ! The V-, left and right show -13V !Hmmm, can I check the MUR860 while still in the circuit ? I checked the resistance of a new one before soldering and it showed a resistance of 0.The leg with the new diodes and that yields -13v, the 4 diodes all read 0 resistance. The other leg, all the diodes show some resistance.I think that I will replace all 4 of them.

author: sokoloff
author_rank: Super Contributor
timestamp: December 17, 2018, 04:15:02 pm
content: Your circuit has two logical "halves".(If you wanted to, you would) Connect the 9VAC wall wart across either U1 and U2 or U3 and U4.However, given that you have a DC bench power supply with current limiting, just use that to find the short. It's not quite as useful for troubleshooting the bridge rectifier, but your problem is not in the AC rectification part of the circuit. (I asked if you had a wall wart because I bet many more people can put their hands on that than on a bench power supply. You have the bench supply. That's way more effective in short-finding.)

author: Hextejas
author_rank: Frequent Contributor
timestamp: December 17, 2018, 06:02:30 pm
content: I think its fixed. I hooked up the full mains voltage and no smoke or fire. Success as the output voltages read correctly.Sheesh, what a hassle but finally solved and thanks to all the excellent help here.Thank you again. I hope that all my projects will go smoother.

author: sokoloff
author_rank: Super Contributor
timestamp: December 17, 2018, 11:27:10 pm
content: What did the problem end up being?

author: nick_d
author_rank: Regular Contributor
timestamp: December 18, 2018, 04:07:03 am
content: To check the diodes use the diode setting on the meter. Ohms is meaningless for diodes, since current increases super-linearly with voltage, not linearly. The diode setting puts a specified current through the diode and tells you the voltage. In forward orientation it should show about 0.6V, larger for beefy power diodes (up to about 1V) or LEDs (up to about 3V). In reverse orientation it should show infinite. You should be able to do this test with them soldered into the board and no supply or load connected. If result is correct then it is a pretty good indication of diode health (though not absolutely definitive). Likely your previous issue was with damaged but not visually obviously damaged diodes.cheers, Nick

author: alanb
author_rank: Frequent Contributor
timestamp: December 18, 2018, 01:50:07 pm
content: There is nothing between the diodes and the caps to limit the current. The caps are very large and therefore at start up the current is going to be high. Could this be the cause of your troubles rather than a short?

author: xavier60
author_rank: Super Contributor
timestamp: December 18, 2018, 02:01:12 pm
content: Quote from: alanb on December 18, 2018, 01:50:07 pmAlso, fast recovery diodes usually don't tolerate current surges as well as standard diodes.

author: Hextejas
author_rank: Frequent Contributor
timestamp: December 18, 2018, 03:18:29 pm
content: Quote from: alanb on December 18, 2018, 01:50:07 pmI still have a problem. When I wire up both sides, I blow the 3.15a fuse. I can power either side but not both.I am wondering if I have too large a transformer for the fuse.The spec called for this :Avel Lindberg Y236651 250VA 18V+18V Toroidal TransformerAnd I have this:Avel Lindberg Y236803 500VA 35V+35V Toroidal TransformerCould I be overpowering the fuse ?I wired a meter into the circuit to see what kind of current was being pulled and it blew a 10amp fuse in the meter.So, how could I slow down the initial rush of current ?

author: alanb
author_rank: Frequent Contributor
timestamp: December 18, 2018, 04:17:49 pm
content: A resistor in series would limit the current this will waste energy as heat although this may be acceptable. You could also use a thermistor this would also waste energy but using the correct value would waste less than a normal resistor as the resistance drops as it warms up.

author: james_s
author_rank: Super Contributor
timestamp: December 18, 2018, 06:09:39 pm
content: When you say "wire up" do you mean when you put a load on them? Or when you just hook something up to them? If connecting wires blows the fuse then it sounds like you're missing the insulators between one or more power transistors and the case.

author: Hextejas
author_rank: Frequent Contributor
timestamp: December 19, 2018, 01:15:45 am
content: Quote from: james_s on December 18, 2018, 06:09:39 pmYes, wired up meant to put a load to both sides.Question. How do i go about sizing a thermistor and would i need one for each side (rail) ?

author: SiliconWizard
author_rank: Super Contributor
timestamp: December 19, 2018, 01:30:07 am
content: Could you try without C10? (Maybe you already did?)

author: nick_d
author_rank: Regular Contributor
timestamp: December 19, 2018, 07:08:55 am
content: I feel that something obvious must be getting missed here.I suggest to get a low value high wattage resistor and put it in series with the incoming AC. If all works totally fine with a moderate load on both outputs then yeah maybe I buy the theory that it's inrush current to the many large caps that causes the issue. In that case, vary the resistance to see what you need to stop it misbehaving. And test the hell out of it -- that the supplies CAN float independently of each other and that it can drive almost the designed load on both channels independently once this possible startup issue is worked around with the small resistor to limit the inrush. You might also try a slow-blow fuse if it's really the inrush issue.I bet that won't be the case. There's gotta be another issue.cheers, Nick

author: fsr
author_rank: Regular Contributor
timestamp: December 19, 2018, 11:12:54 am
content: If connecting the load blows fuses, then maybe your load has a problem. What is your load, anyways?Also, if you replaced a 18+18v transformer by a 35+35v, there can be problems. Over a resistive load, the current is the voltage divided by the resistance, and you doubled the voltage.You can test the psu by connecting both grounds together. I would first connect the grounds at the transformer, with nothing else connected, just the transformer by itself, and chech that you have your 35+35v. Of course, before connecting the grounds, make sure that they're two independent secondaries, like you think. Measuring voltage over the wires of separate secondaries should give you no voltage at all. With the transformer unpowered you can also test resistance. It should be infinite. The transformer's datasheet should tell you this, anyways.With the grounds connected at the transformer, you can connect the grounds at the output of your psu. Nothing should blow, or you still have a problem in your psu. Then you can test by using two power resistors as loads, from + to gnd and from gnd to -Use resistors of enough power rating, and their resistance should be enough for the current to be below the limits of your psu.I also think that using fast recovery diodes on a bridge rectifier is strange.

author: AngraMelo
author_rank: Regular Contributor
timestamp: December 19, 2018, 12:54:35 pm
content: Quote from: sokoloff on December 17, 2018, 02:51:52 pmSo it seems your board has a short, if you are having problems with 20V I doubt it is a clearance issue. It might just be a dead short.

author: AngraMelo
author_rank: Regular Contributor
timestamp: December 19, 2018, 01:01:20 pm
content: Wait a minute, the circuit says you should use a 18V transforme and you are using a 35V one?Your caps are rated for more than 35V?I have waaaaay larger capacitor banks here that never had that problem.They do make a fun noise when turning on but they dont blow any fuses (normally a slow blow fuse is needed)

author: Hextejas
author_rank: Frequent Contributor
timestamp: December 19, 2018, 05:30:52 pm
content: Hooray, I think that we got it !!!I added a small ohm (2ohm) resistor, large Watt, I dont know what it is, to the schematic as shown and it seems to work ok. It's a very beefy resistor and it did not get hot after powering up.The power supply does not blow fuses and is putting out the proper DC voltages.My question now is:1) Should I add a similar resistor to the other rail ? Does it need to be "balanced" with the other side ? Is there such a thing as balance between the 2 sides ? This electronics hobby is a strange beast.Picture attached.

author: metrologist
author_rank: Super Contributor
timestamp: December 19, 2018, 06:55:22 pm
content: I suspect that resistor is rated 2 watts, maybe 5W. Your transformer is rated 500W (input)?

author: Hextejas
author_rank: Frequent Contributor
timestamp: December 19, 2018, 07:08:56 pm
content: Quote from: metrologist on December 19, 2018, 06:55:22 pmYes on the transformer.And I spoke too soon. Alas.I put another 2 ohm in front of AC2, plugged everything in, bolted it into the chassis, turned it on, and smoke and a gunshot.Maybe adding the 2nd resistor caused it bu i am at a loss to explain it.Soooooo , I will take out the resistor, replace the exploded diode and try again,

author: sokoloff
author_rank: Super Contributor
timestamp: December 19, 2018, 07:16:21 pm
content: If you're not discharging all the caps between tests, it's possible that a previous test has left some charge in the caps and the subsequent power-on is not the same test as a power-on from a "cold" (all discharged) state.Adding the second resistor in that location did notcausethe failure.

author: metrologist
author_rank: Super Contributor
timestamp: December 19, 2018, 07:38:24 pm
content: You said when you put your bench supply to U1 - U2, it went to CC mode. You need to identify what caused that. In that state, if it is not drawing too much current, because you set the current limit to a decent value, you might be able to detect something getting hot. A thermal camera is great for that, or maybe a finger...

author: Hextejas
author_rank: Frequent Contributor
timestamp: December 19, 2018, 08:52:58 pm
content: Quote from: metrologist on December 19, 2018, 07:38:24 pmExcellent advice and i will give it a try.

author: nick_d
author_rank: Regular Contributor
timestamp: December 19, 2018, 09:40:23 pm
content: Yes you need both resistors (for the test at any rate, I am not sure I would recommend the resistors for production). Now let us think about this. A fully discharged capacitor charging from 35V through 2 ohms will take 17.5A initially, this dropping off as the capacitor charges. According tohttp://www.mouser.com/ds/2/149/RURP860-244175.pdfthe peak non-repetitive surge current of MUR860 is 100A. So I believe you have proved that inrush current isn't the issue. You may want to check RC time constant to be sure that this counts as a "surge" but realistically I think that it would.I think more likely you have an intermittent short or a PCB design rule issue or a reject PCB or all three. Can you make up another PCB the same and test it? And in case the halves are shorted together, maybe try running with the top half (+ve supply) from the first PCB and the bottom half (-ve supply) from the second PCB.Otherwise, the original advice applies. Validate everything with the 9V battery and possibly small signal diodes instead of the MUR860s (but keep the MUR860s if they are already soldered in and pass multimeter tests on diode setting).Then, if that's okay give it low voltage AC with much current limiting. What I do sometimes for this is grab a 12V AC plugpack and then use the 12VAC as if it was 240VAC. So basically you have an extra step down and then your normal transformer, result about 2V. And put some resistors in too. Then see if you actually have a reliable 2V supply over time.cheers, Nick

author: ebastler
author_rank: Super Contributor
timestamp: December 19, 2018, 10:05:05 pm
content: Quote from: Hextejas on December 19, 2018, 05:30:52 pmBut you did not actually install the resistor as shown in your schematic (in parallel with the wire), right? It needs to be in series, i.e. you must disconnect the original wire or trace. Apologies if this was already clear.

author: nick_d
author_rank: Regular Contributor
timestamp: December 19, 2018, 10:21:53 pm
content: Another thought: check your sums carefully. If you increased transformer voltage increase capacitor voltage in proportion. That's because 35VAC means 35V RMS so the peak is 1.414 times 35 in VDC, I often see 400V capacitors used in 240VAC supplies, 450V or 500V are also common. If you use say 35V rated capacitor with 35VAC input, the capacitor charges normally until the AC gets near the peak and then the capacitor begins to short as the AC peak exceeds 35VDC. It will short as long as more than the rated voltage is across it, which tends to limit the voltage stored in the capacitor to approximately its rated voltage. There may be some margin of safety but typically the margin is tiny.Another possibility is that your capacitors could be damaged from previous experiments causing them to either short (easy to detect) or short above some voltage that is less than the rated voltage (much harder to detect). Although it would be a major pain a good test would be to try running the circuit with all capacitors removed. The low voltage test won't pick up this issue. The low current test MIGHT but the problem is fairly benign at low currents so may be missed.cheers, Nick

author: Gregg
author_rank: Super Contributor
timestamp: December 20, 2018, 12:07:21 am
content: I think nick_d is on the right track; capacitors seem to have been left out of the discussion but certainly could be the problem. You could try connecting your 35V DC power supply at full voltage but a couple amps current limit to each half of your power supply output and see if it will charge the caps or not. This would be quicker than unsoldering all of them. If the caps are good, the power supply should go into current limit at a low voltage and slowly increase voltage until it reaches full voltage as the caps charge. You may find only one half of your supply is at fault (pun intended).If you have to take the caps off the board, you can test each one with the variable bench power supply in a similar manner, they should accept a charge and hold a slowly dropping charge with a high impedance multimeter when the power is removed.Follow the traces on the actual board to make sure none of the electrolytics are installed reverse polarity.

author: Hextejas
author_rank: Frequent Contributor
timestamp: December 20, 2018, 12:42:09 am
content: Quote from: ebastler on December 19, 2018, 10:05:05 pmIt was installed in series.

author: Hextejas
author_rank: Frequent Contributor
timestamp: December 20, 2018, 01:25:05 pm
content: Before I start at this again, I think I will post some of the specs.Capacitors:C1 ~ C4 = 8200uf 50vC5, C6, C11, C12 = 100nf 850vdcC10 = 1.5uf 450vdcC13, C14 = 100uf 50v-------------And some readings that I took before it burned up.Transformer out = 39vacV+ = 53V- = 53Why I chose such an oversized toroid, i cannot recall, but if i continue to struggle with this i think that i will exchange it.Ok, that's it and I replaced 2 of the diodes so let me get back to testing.Thanks again

author: GeoffreyF
author_rank: Regular Contributor
timestamp: December 20, 2018, 03:32:39 pm
content: Quote from: Hextejas on December 20, 2018, 12:42:09 amYou expect help with a schematic where it is in parallel? If you installed it in series, please provide an actual schematic which goes with your project! The schematic you provided, as I stated awhile back, is rather strange. I would suggest you get a book such as "the Art of Electronics", "Practical Electronics for inventors", learn about capacitance, inductance, resistance. Look at other designs for power supplies. I don't think you will solve the problems you are having without more learning of the fundamentals.

author: nick_d
author_rank: Regular Contributor
timestamp: December 20, 2018, 08:42:46 pm
content: If the transformer output is 39VAC then the peak is 39 * 1.414 = 55.146 VDC. In the bridge rectifier 2 diodes are conducting at a time and these are in series with the 55.146 VDC, each giving a drop of about 1 VDC under load. Thus the 39 VAC measurement at the secondary is consistent with the stated 53 VDC output measurement. You didn't notice that this is over-voltage for your 8200uF 50 VDC reservoir capacitors? As I mentioned they don't give any real margin, you are allowed to operate them at 50 VDC and no more. Otherwise, bad things happen. Problems can be immediate or take time to manifest. Capacitor over voltage may be damaged over time reducing their voltage limit even further.cheers, Nick

author: Hextejas
author_rank: Frequent Contributor
timestamp: December 21, 2018, 05:34:04 pm
content: What a dummy I am and what a lesson learned.There was more thanv1 thing wrong with this and I am going to take the advice to desolder everything, test the components and start with a new board.The picture says it all. 2 bad resistors and 1 bad diode. Plus apparently, it blew the wall wart as well.Sheesh.Oh Yeah, I built a very handy $15.00 kit that let me test the capacitors. It could also test the diodes if my meter did not have that feature.De-soldering is not fun 😦😡

author: james_s
author_rank: Super Contributor
timestamp: December 21, 2018, 05:50:19 pm
content: With proper tools desoldering is a piece of cake. Hakko 808 is some of the best money I ever spent.

author: AngraMelo
author_rank: Regular Contributor
timestamp: December 21, 2018, 07:18:55 pm
content: Buddy, you are trying to build a project with the wrong parts. If you have more than 50Vdc on a 50V cap you are asking for trouble.I second GeoffreyF's advice, go read some theory about what you are doing, and learn to at least be safe.The books he recommended are great!

author: ebastler
author_rank: Super Contributor
timestamp: December 21, 2018, 08:39:17 pm
content: Quote from: AngraMelo on December 21, 2018, 07:18:55 pmProbably just a language issue, but you do come across as a bit condescending. For future posts, I would suggest omitting the "buddy" part.

author: james_s
author_rank: Super Contributor
timestamp: December 21, 2018, 10:43:18 pm
content: I didn't think it sounded condescending, it's a fairly normal way of speaking in some regions.

author: fsr
author_rank: Regular Contributor
timestamp: December 21, 2018, 11:50:08 pm
content: I suggest that you follow the ratings on the original schematic, specially the transformer's parameters. You cannot expect something designed to operate with a 18v transformer to work with a 35v one.And still, there are some design choices with this PSU that don't make a lot of sense, like using a transformer with two separate secondaries, and discrete fast recovery diodes to make two different bridges, essentially creating two separate PSUs, but then connect them to get a split PSU, which could be made with just a center-tapped transformer, a diode bridge and some big capacitors.

author: nick_d
author_rank: Regular Contributor
timestamp: December 21, 2018, 11:57:11 pm
content: It is unusually complicated in the way you explained. On the other hand, if he already has the PCBs there it would be simplest just to build and use it as designed. Otherwise he has to make his own PCB. Also, having the 2 full wave rectifiers could give a better ripple spec than simpler designs with fewer diodes (or equivalently use smaller capacitors).cheers, Nick

author: nick_d
author_rank: Regular Contributor
timestamp: December 22, 2018, 12:10:58 am
content: As well as that, I think OP is doing a fabulous job here. Each and every one of us posting on this forum has had many such facepalm momentsThe most important thing for success in electronics is perseverance and this OP has amply demonstrated. In my experiencs very design must be redone at least three times and so must every repairNick

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 22, 2018, 06:04:49 am
content: Quote from: ebastler on December 21, 2018, 08:39:17 pmthe way i learnt from how US people talk and... from watching movies, buddy means close friend or good friend. if its a cultural issue, maybe i will stop calling German people buddy, Sir... if its age issue, yes sometime its show disrespect for a very young people to call buddy to an older guy, but this is internet we cant see how old are you, we assume all friend of the same age, ymmv Sir.Quote from: nick_d on December 22, 2018, 12:10:58 amyes 3 pages to find a short.. 3 pages of people speculating of whats going on and at last the OP showed some measurement, not hitting any of the speculation made, epic. due to lack of pictures, schematics and methodology description, rather than speculating, some people just made simple reply earlier...Quote from: Mechatrommer on December 17, 2018, 02:45:00 am