Author Topic: I have a short ( i think) and don't know where to start  (Read 6370 times)

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Offline HextejasTopic starter

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I have a short ( i think) and don't know where to start
« on: December 15, 2018, 06:21:32 pm »
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.
 

Online Andy Watson

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Re: I have a short ( i think) and don't know where to start
« Reply #1 on: December 15, 2018, 06:35:35 pm »
Can you confirm that the transformer has two isolated secondaries rather than a centre tap ?
There's not much else to go wrong.
 

Offline PA0PBZ

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Re: I have a short ( i think) and don't know where to start
« Reply #2 on: December 15, 2018, 06:48:13 pm »
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'.
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Offline HextejasTopic starter

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Re: I have a short ( i think) and don't know where to start
« Reply #3 on: December 15, 2018, 07:18:34 pm »
well, I dont know exactly. The xformer has 4 wires coming out of the secondary.
This is it, 500va https://4donline.ihs.com/images/VipMasterCap/Capacitor/AVEL/AVELS00030/AVELS00030-1.pdf
 

Offline HextejasTopic starter

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Re: I have a short ( i think) and don't know where to start
« Reply #4 on: December 15, 2018, 07:22:07 pm »
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 ?
 

Offline PA0PBZ

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Re: I have a short ( i think) and don't know where to start
« Reply #5 on: December 15, 2018, 07:34:43 pm »
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.
« Last Edit: December 15, 2018, 07:37:22 pm by PA0PBZ »
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Offline fsr

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Re: I have a short ( i think) and don't know where to start
« Reply #6 on: December 15, 2018, 08:40:09 pm »
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?
 

Offline nick_d

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Re: I have a short ( i think) and don't know where to start
« Reply #7 on: December 15, 2018, 09:21:16 pm »
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
 

Offline HextejasTopic starter

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Re: I have a short ( i think) and don't know where to start
« Reply #8 on: December 16, 2018, 02:08:31 pm »
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.
 

Offline sokoloff

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Re: I have a short ( i think) and don't know where to start
« Reply #9 on: December 16, 2018, 02:52:10 pm »
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.
 
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Offline HextejasTopic starter

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Re: I have a short ( i think) and don't know where to start
« Reply #10 on: December 16, 2018, 03:04:15 pm »
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
« Last Edit: December 16, 2018, 03:08:48 pm by Hextejas »
 

Offline HextejasTopic starter

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Re: I have a short ( i think) and don't know where to start
« Reply #11 on: December 16, 2018, 03:11:31 pm »
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.
I 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.
 

Offline PA0PBZ

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Re: I have a short ( i think) and don't know where to start
« Reply #12 on: December 16, 2018, 03:48:43 pm »
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?

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Offline HextejasTopic starter

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Re: I have a short ( i think) and don't know where to start
« Reply #13 on: December 16, 2018, 04:08:13 pm »
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?



No, they are not. But thanks for looking.
 

Offline sokoloff

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Re: I have a short ( i think) and don't know where to start
« Reply #14 on: December 16, 2018, 04:28:40 pm »
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.
 

Offline sokoloff

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Re: I have a short ( i think) and don't know where to start
« Reply #15 on: December 16, 2018, 04:31:36 pm »
IOW, Take a simplify, divide, and conquer approach. That tends to let less smoke out of things, though frustratingly still not zero...  :palm:
 

Offline mikerj

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Re: I have a short ( i think) and don't know where to start
« Reply #16 on: December 16, 2018, 06:36:29 pm »
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.
 

Offline nick_d

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Re: I have a short ( i think) and don't know where to start
« Reply #17 on: December 16, 2018, 09:08:52 pm »
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
 

Offline fsr

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Re: I have a short ( i think) and don't know where to start
« Reply #18 on: December 17, 2018, 01:15:55 am »
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?



No, they are not. But thanks for looking.
It 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.
 

Offline james_s

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Re: I have a short ( i think) and don't know where to start
« Reply #19 on: December 17, 2018, 02:25:00 am »
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.
 

Online Mechatrommer

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Re: I have a short ( i think) and don't know where to start
« Reply #20 on: December 17, 2018, 02:45:00 am »
and you'll have some time to take measurements..
did he make any? what his multimeter (continuity tester) told him?
Nature: Evolution and the Illusion of Randomness (Stephen L. Talbott): Its now indisputable that... organisms “expertise” contextualizes its genome, and its nonsense to say that these powers are under the control of the genome being contextualized - Barbara McClintock
 

Offline HextejasTopic starter

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Re: I have a short ( i think) and don't know where to start
« Reply #21 on: December 17, 2018, 02:43:53 pm »
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.
 

Offline HextejasTopic starter

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Re: I have a short ( i think) and don't know where to start
« Reply #22 on: December 17, 2018, 02:46:36 pm »
Whoa, I found a tube of 8, MUR860s  :-DD so here we go.
 

Offline sokoloff

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Re: I have a short ( i think) and don't know where to start
« Reply #23 on: December 17, 2018, 02:51:52 pm »
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.
A 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.
I am thinking I need to do something to fix that.
Yes. Find the short.  :-DD
 
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Offline HextejasTopic starter

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Re: I have a short ( i think) and don't know where to start
« Reply #24 on: December 17, 2018, 03:29:14 pm »
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.
« Last Edit: December 17, 2018, 03:35:44 pm by Hextejas »
 


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