EEVblog Electronics Community Forum

EEVblog => EEVblog Specific => Topic started by: EEVblog on November 06, 2017, 07:36:20 am

Title: EEVblog #1036 - PSU Fire PCB Repair
Post by: EEVblog on November 06, 2017, 07:36:20 am
PCB Repair of the RD Tech DPS5020 Power supply module after the flaming capacitor failure.

https://www.youtube.com/watch?v=VwdnGbI5ls8 (https://www.youtube.com/watch?v=VwdnGbI5ls8)

Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: Faith on November 06, 2017, 07:57:16 am
Thanks for the follow-up!~ Was originally going to get one of these; but now I'm not so sure.

Mistakes happen but what made me :palm: was when RD Tech admitted that this isn't the first time they've encountered precisely this method of failure but yet never bothered to correct the issue until now.

Oops.
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: bjcuizon on November 06, 2017, 08:02:13 am
Sorry but this is just regarding the video image-wise :D.
IMO The video quality is great and all shots look nice, with only the important bits in focus.  :-+ :-+
..And I think the lighting is a bit different. It looks brighter and things look clear.
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: EEVblog on November 06, 2017, 08:12:55 am
Sorry but this is just regarding the video image-wise :D.
IMO The video quality is great and all shots look nice, with only the important bits in focus.  :-+ :-+

I personally don't like that, and usually deliberately set the depth of field on my camera to avoid any bokeh effect.

Quote
..And I think the lighting is a bit different. It looks brighter and things look clear.

Lighting is the same, just a different camera and lens. The early shots on the video were actually shot with a macro lens.

The audio sounded a bit more echoey to me as I'm basically behind a shotgun mic instead of on top of the top mounted mic like on my Canon.
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: Muttley Snickers on November 06, 2017, 08:23:40 am
These videos were excellent with drama, suspense and a happy ending.   :-+ :)

If one of these capacitors can cause this much damage and potentially become a fire hazard by going short across a high current output then wouldn't it be a better idea to add these filter capacitors by way of dedicated smaller sacrificial tracks, the only down fall I see in doing the circuit this way might be a bit more noise and the risk that these tracks could blow out or go open without giving any feedback as the main output terminals would remain intact.   
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: trophosphere on November 06, 2017, 09:39:34 am
Some ceramic capacitor manufacturers have MLCC SMD offerings which are much more resistant to PCB flexing and have a failure mode with a preference for failing open rather than being a short. Examples being AVX (http://www.avx.com/products/ceramic-capacitors/surface-mount/automotive-mlcc-with-flexiterm/?) and Kemet (http://www.kemet.com/Lists/ApplicationBriefs/Attachments/58/Flexible%20Termination%20Technology%20(FT-CAP).pdf).
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: EEVblog on November 06, 2017, 09:40:53 am
Some ceramic capacitor manufacturers have MLCC SMD offerings which are much more resistant to PCB flexing and have a failure mode with a preference for failing open rather than being a short. Examples being AVX (http://www.avx.com/products/ceramic-capacitors/surface-mount/automotive-mlcc-with-flexiterm/?) and Kemet (http://www.kemet.com/Lists/ApplicationBriefs/Attachments/58/Flexible%20Termination%20Technology%20(FT-CAP).pdf).

Yeah, forgot about those. Would have made a nice addition to the video
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: NANDBlog on November 06, 2017, 10:06:00 am
OK, he stopped selling it, good.
But: Placing capacitors in high stress area?
Placing fuses between connectors? You know, something that you actually have to access to replace.
And placing fuses in parallel?? Seriously?
Placing components before the fuse?
How many death traps are there left in this circuit?

A working circuit and a proper circuit is two very different things.
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: Brumby on November 06, 2017, 10:09:16 am
Thank you for the follow up.

It is good to see the issue is a straightforward one, the repair simple (even if a little inelegant) and the result of success.

Even better that corrective action is being taken from the manufacturing end.


These modules are back on my "of interest" list.
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: firewalker on November 06, 2017, 10:21:45 am
I have seem high rating fuses (car type) paralleled in many good quality UPSs. Something like 3x30A fuses.

Alexander.
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: Psi on November 06, 2017, 11:25:17 am
One way to fix the design with minimal changes might be some routed slots around the cap so board flex doesn't result in much cap flex.
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: Brumby on November 06, 2017, 01:13:39 pm
I'd move the cap, myself.
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: NANDBlog on November 06, 2017, 01:48:08 pm
I have seem high rating fuses (car type) paralleled in many good quality UPSs. Something like 3x30A fuses.

Alexander.
If you are using a bigger, not SMD fuse, the story is different.
For a blade type fuse, the connection and the trace leading to it is massive, and it has small resistance, while the fusing element has a big resistance. For example it is almost 3.5 mOhm for Littlefuse ATOF.

If you are using a tiny SMD fuse, the connection the soldering and everything else has a big portion of the total resistance. You need to route high current through a 1206 component. The Littlefuse 501 has less than 2 mOhms resistance. It will dissipate 1W at rated current.
If you connect two SMD fuses in parallel, you get a fuse, which has "whatever, probably higher than just one" rating both for voltage and for current.

And this made me realize, I never saw and SMD fuse (chip type, not glass type) with 50+ DC voltage rating and high current.
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: MisterDiodes on November 06, 2017, 04:12:55 pm

If you are using a bigger, not SMD fuse, the story is different.
For a blade type fuse, the connection and the trace leading to it is massive, and it has small resistance, while the fusing element has a big resistance. For example it is almost 3.5 mOhm for Littlefuse ATOF.

If you are using a tiny SMD fuse, the connection the soldering and everything else has a big portion of the total resistance. You need to route high current through a 1206 component. The Littlefuse 501 has less than 2 mOhms resistance. It will dissipate 1W at rated current.
If you connect two SMD fuses in parallel, you get a fuse, which has "whatever, probably higher than just one" rating both for voltage and for current.

And this made me realize, I never saw and SMD fuse (chip type, not glass type) with 50+ DC voltage rating and high current.

Thank you.  Parallel SMD fuses are not good design practice, and generally they won't be rated for high power break / higher voltage.  A real fuse of the correct rating is called for here.

Also the advice to use two SMT MLCC caps in series is a bit bogus - that might delay the fire, does not eliminate the problem.  Even if you use different manufacturers for each cap it is just a race to see which one fails first in the series group.  Those caps that supposedly fail "open" can be just as bad we've found in our tests - and if they do fail "open" that can cause other problems on the board.  So we try to avoid all SMT MLCC caps on a high power rail in the first place, use good quality TH caps if possible, or else plan a proper reliable circuit protection method if you have to use SMT ceramics on a high available power rail.

I love how the manufacturer said it happened before, but they never bothered to fix it until it shows up on EEVblog! 

Big Thanks to Dave for the follow up video - that made the whole story even more interesting.  It just goes to show if you pay sh*t for a power supply, you'll probably get sh*t.  This one worked like sh*t and even smells like sh*t now.  There is a pattern there...

Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: IanMacdonald on November 06, 2017, 05:57:05 pm
The actual safety margin on fuses varies quite a bit between types.  If the two fuses in parallel have a safety margin of 1.5x (typical for mains fuses) then with two you could be looking at as much as 60A to blow them in a reasonable time, which is way too much.

The other option would be to use a polyswitch. They also have a safety margin before they open, but at least you could use a single 20A rated one without that being such a nuisance if it does trip occasionally. The best answer  for this sort of current though is an MCB, space permitting.

Also I cringe at leaving burned board in place. Better to cut the whole lot out and either rebuild it with glassfibre and hardwiring, or else mount the components elsewhere.
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: ChrisLX200 on November 06, 2017, 06:05:39 pm
Given the modest cost of these things I would not have attempted a repair anyway and just binned it. Plus - the manufacturer should send you a new modified one FOC (at least). How many of these things are out there owned by folk who don't watch EEVBlog on YouTube or read this forum? Has a recall been issued for this potentially hazardous device, can buyers get a refund if they can prove they bought one? The manufacturer has admitted fault so they don't really have a leg to stand on.
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: Freelander on November 06, 2017, 06:15:09 pm

Quote
that made the whole story even more interesting.  It just goes to show if you pay sh*t for a power supply, you'll probably get sh*t.  This one worked like sh*t and even smells like sh*t now.  There is a pattern there...

 :wtf: What a happy soul.. :popcorn: ... The only pattern seems to be the sh*t that you are writing there.  :palm:
 
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: gardner on November 06, 2017, 06:29:42 pm
Dave mentioned torsional force on the screws as a contributing factor.  I would say that it's more likely flexing of the board due to downward force of the screwdriver.  The board is quite rigid torsionally, but thin and flexy the other, normal, direction, and it is supported off to the sides, not directly underneath the screw terminals.  Pressing down with the screwdriver puts the MLCC under tension.
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: Cerebus on November 06, 2017, 07:09:21 pm
I'm feeling very smug, I called it right (low resistance through the charring)  in the thread that discussed the previous video.  8)
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: Freelander on November 06, 2017, 08:12:58 pm
I'm feeling very smug, I called it right (low resistance through the charring)  in the thread that discussed the previous video.  8)
nice one  ;D

Note also the unit running at a measly 100W and the tiny heatsink fan isn't even running. These are superbly efficient little units.  The cap issue is only on the 5020 units and has already been corrected. (it is not fitted to the smaller units). AFAIK there was just one previous instance of an issue in the factory that was put down to a faulty cap (live n learn). The clickbait flames video leader was a tad OTT and not really fair but I am sure the clicky baity part will and has increase(d) sales even more as people see the durability and shear value for money and performance of these little gems. A huge untapped market there and nothing even close competing. I bet there are a a few people wishing they had come close to designing and producing something even remotely as good and with such massive sales potential.
A little gold mine.
I have mounted the cap from my 5020c on a little plinth with a painted mushroom cloud over it............... ;)
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: Mr. Scram on November 06, 2017, 09:21:16 pm
Thanks for the follow-up!~ Was originally going to get one of these; but now I'm not so sure.

Mistakes happen but what made me :palm: was when RD Tech admitted that this isn't the first time they've encountered precisely this method of failure but yet never bothered to correct the issue until now.

Oops.
It took three Boeing 737s going down with the same problem before they picked up on the rudder issue that caused them.
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: jnissen on November 06, 2017, 10:48:22 pm
As for the case fan being noisy... I also purchased a box very similar to the one in the video. The fan operates off a 5V LDO on the input board. I reduced the voltage that is generated by the small LDO so the fan only operates at 4V. The result is the fan is slower and not as noisy. It still moves a lot of air so I'm not concerned it's a bad thing.

Replace the 3K ohm SMT resistor (labeled 302) with a 2.2K ohm 603 resistor. The voltage to the fan is calculated by:  Fan Voltage = 1.25V (1+R2/R1) 

The R2 is the 3K ohm while the R1 is a 1K ohm on my board.  With the factory values 1.25*(1+3) gives you 5V. So to get 4V just use a 2.2K ohm resistor!

New Fan = 1.25 (1+2.2K/1K) or 4V

You can try lower values but I found my fan needs about 4V to properly startup. Too low and the fan will cog once and not spin.
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: Someone on November 06, 2017, 11:14:09 pm
So we try to avoid all SMT MLCC caps on a high power rail in the first place, use good quality TH caps if possible, or else plan a proper reliable circuit protection method if you have to use SMT ceramics on a high available power rail.
Its not a problem exclusive to "high power" rails, a supply rated for just a few watts is capable of violently destroying MLCC caps as the heating is extremely localised.
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: Brumby on November 06, 2017, 11:59:31 pm
Given the modest cost of these things I would not have attempted a repair anyway and just binned it.

A lot of people looking at a low cost solution like these modules is not going to be so cavalier.  They will see a problem like this and want to repair the module, if at all reasonably possible.

By having one of these units "blow up", Dave has done the manufacturer a favour.  He has caught the attention of a lot of people who view such cheap modules with suspicion and has activated their "AHA!!" button.

By then successfully pursuing the repair, he has demonstrated the basic robustness of the design - other than the poor placement of a single MLCC cap (which only occurs on the 50V 20A version).  The response from the designer/manufacturer has only added to the positive vibe about them.

The repair itself is another plus.  It was very simple (for this fault) and could be done by anyone with a modicum of skill.


All these things add up to a more positive view of these modules.  Sure, something went wrong - but by having something go wrong, we have been able to see the Achilles' heel we so often suspect is lurking in low cost gear - and we can fix it.  Better still, we can prevent it.

Tell me you don't have a more positive attitude about these modules because of all this.....
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: Tek_TDS220 on November 07, 2017, 03:39:20 am
The printed circuit board shouldn't burn (and continue to burn).  Whoever made the board is cutting corners on flame retardants.  My guess is that the board would not pass the tests necessary to import them into the US and the EU.  I don't know about Australia.
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: ChrisLX200 on November 07, 2017, 09:28:52 am
Quote from: Brumby on Today at 10:59:31 AM (https://www.eevblog.com/forum/index.php?topic=98153.msg1343119#msg1343119)>Quote from: ChrisLX200 on Today at 05:05:39 AM (https://www.eevblog.com/forum/index.php?topic=98153.msg1342811#msg1342811)
Given the modest cost of these things I would not have attempted a repair anyway and just binned it.

A lot of people looking at a low cost solution like these modules is not going to be so cavalier.  They will see a problem like this and want to repair the module, if at all reasonably possible.

By having one of these units "blow up", Dave has done the manufacturer a favour.  He has caught the attention of a lot of people who view such cheap modules with suspicion and has activated their "AHA!!" button.

By then successfully pursuing the repair, he has demonstrated the basic robustness of the design - other than the poor placement of a single MLCC cap (which only occurs on the 50V 20A version).  The response from the designer/manufacturer has only added to the positive vibe about them.

The repair itself is another plus.  It was very simple (for this fault) and could be done by anyone with a modicum of skill.


All these things add up to a more positive view of these modules.  Sure, something went wrong - but by having something go wrong, we have been able to see the Achilles' heel we so often suspect is lurking in low cost gear - and we can fix it.  Better still, we can prevent it.

Tell me you don't have a more positive attitude about these modules because of all this.....




You missed my point from your quote - the person interested in a low cost solution is also entitled to a free replacement so does not need to repair anything. There's nothing cavalier about that, you don't really know what other components have been stressed by this event. But sure, with a replacement in hand, then they are free to do whatever they like with the burned unit. Would you really have a positive attitude about walking away from one of these units and leaving it powered up?

I actually own one of the cheaper units in this series but seeing this one self-destruct doesn't make me feel warm and fuzzy inside..
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: Ian.M on November 07, 2017, 09:52:47 am
The damage was localised to the area around the failed cap.  The fire did not spread to the rest of the PCB.  I have seen similar damaged PCBs from reputable manufacturers of consumer electronics sold in the EU, with similar localoisation of the damage, so its is extremely unlikely that the PCB laminate was made with a non-self-extinguishing resin.  Whether or not it meets EU and US standards for PCB flammability would require testing, but I think we can assume that if it does not, it isn't far short of meeting them.

Would I be happy to have one of these specific board version PSUs (assuming I had a suitable raw DC supply to run it from)?  Yes, but the first thing I'd do would be to remove the offending cap and fit a suitable low ESR electrolytic across the output terminals.  Its obviously not got the build quality of a comparable lab grade PSU from a leading test equipment manufacturer, but as its an order of magnitude cheaper, seems reasonably robust apart from an easily rectified design flaw, and has the 'grunt' you need if you are doing stuff like robotics on a budget, IMHO its still a good deal.  If the manufacturer did a recall offering exchange or partial credit if you remove the cap yourself, and also reworked existing stock to remove the ceramic cap and supplied them with a suitable leaded cap + fitting instructions, it would be an entirely satisfactory resolution to the issue.

Would I repair it?  Yes, if the manufacturer didn't want the failed board back or if it was out of warrenty.   Depending on the extent of the damage, once I'd confirmed it was savable,  I might choose to move the fusing off-board - depopulate and link out the SMD fuses then fit an inline holder and a 15A fuse.
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: Freelander on November 07, 2017, 12:58:30 pm
The damage was localised to the area around the failed cap.  The fire did not spread to the rest of the PCB.  I have seen similar damaged PCBs from reputable manufacturers of consumer electronics sold in the EU, with similar localoisation of the damage, so its is extremely unlikely that the PCB laminate was made with a non-self-extinguishing resin.  Whether or not it meets EU and US standards for PCB flammability would require testing, but I think we can assume that if it does not, it isn't far short of meeting them.

Would I be happy to have one of these specific board version PSUs (assuming I had a suitable raw DC supply to run it from)?  Yes, but the first thing I'd do would be to remove the offending cap and fit a suitable low ESR electrolytic across the output terminals.  Its obviously not got the build quality of a comparable lab grade PSU from a leading test equipment manufacturer, but as its an order of magnitude cheaper, seems reasonably robust apart from an easily rectified design flaw, and has the 'grunt' you need if you are doing stuff like robotics on a budget, IMHO its still a good deal.  If the manufacturer did a recall offering exchange or partial credit if you remove the cap yourself, and also reworked existing stock to remove the ceramic cap and supplied them with a suitable leaded cap + fitting instructions, it would be an entirely satisfactory resolution to the issue.

Would I repair it?  Yes, if the manufacturer didn't want the failed board back or if it was out of warrenty.   Depending on the extent of the damage, once I'd confirmed it was savable,  I might choose to move the fusing off-board - depopulate and link out the SMD fuses then fit an inline holder and a 15A fuse.

Agreed ! - the other point that people have happily and conveniently failed to mention is that the 'supply' - for the DPS5020 was NOT fused!!!. it was constant current limited. There is a major difference. Anyone building a unit such as this should indeed have a fuse in the supply that feeds to the DPS.  To not have such protection mechanism is simply poor practice and asking for trouble.
Had such an absolutely normal, safe and standard practice been in place during the tests then there would have been absolutely no drama at all. Often it is not the gun it is the gunner!
In normal use the fuse that SHOULD have been fitted in the supply to the unit would have done its job.
In any other 'experiment; with a similar result I would expect people to jump on the fact that the setup was totally wrong in the first place. The drama ONLY 'happened'  due to poor practice -it was not 'CAUSED' by the DPS unit. the fault was of the DPS, yes, but -NOT the 'DRAMA'. Much ado about nothing and blown out of proportion by lack of standard safeguards and practice. (all IMHO of course) ;)

edit - cannot splelllll ...
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: Ian.M on November 07, 2017, 02:15:25 pm
Although the input *should* be fused, it wouldn't have stopped any of the drama if the DPS5020 was suitably fused to permit its full specification to be utilised.  Once the cap failed and it went into current limiting, I would expect that the output voltage collapsed considerably, limiting the power.  This would have happened quickly enough that a 20A fuse on the input would have been unlikely to blow. Any lesser fuse woudn't permit full output current at full voltage.

Edit: thanks Brumby for reminding me the module is capable of 20A output, so needs a 20A input fuse when running at max output voltage with minimum headroom.
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: Brumby on November 07, 2017, 02:45:38 pm
Although the input *should* be fused, it wouldn't have stopped any of the drama if the DPS5020 was suitably fused to permit its full specification to be utilised.
This, I thought, was somewhat obvious and critiquing the lack of a fuse is, essentially, irrelevant.

The module was rated to deliver 50V @ 20A.  Put any fusing to allow operation in that range and there would still have been a magic smoke show.  1000W is several times the power necessary to cook that PCB.
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: thm_w on November 07, 2017, 08:11:57 pm
Thanks for the follow-up!~ Was originally going to get one of these; but now I'm not so sure.

Mistakes happen but what made me :palm: was when RD Tech admitted that this isn't the first time they've encountered precisely this method of failure but yet never bothered to correct the issue until now.

If you have hundreds or thousands of sales, and a few fail, its generally not worth looking into. Of course if you have free time go for it. I can tell you for my job unless I see 5+ failures of the same type, its not worth the time to investigate the fault.

They may have also not got the PSU's back, as shipping to china is expensive.
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: Kleinstein on November 07, 2017, 09:06:38 pm
If they don't get the board back, they probably get at least a picture back.  Looking at the burnt board and the new one should make it obvious what happened.

With a product sold to hobby customers, there is a chance that some of the reported failures are not due to a problem in the product. With just a few failures it may not be worth the investigation.
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: Brumby on November 07, 2017, 11:26:33 pm
With a product sold to hobby customers, there is a chance that some of the reported failures are not due to a problem in the product. With just a few failures it may not be worth the investigation.
This was the thought I often have with "reported failures".

In this case, Dave has some credentials - plus video - to back up his experience.  The manufacturer would certainly respond to this - which is exactly what happened.
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: rymannphilippe on November 08, 2017, 06:38:38 am
Hello all

I will order this DPS5020 and found it on Aliexpress.
Dave's Board Version is V2.3 (https://www.youtube.com/watch?v=Q2rvAoO-MIA (https://www.youtube.com/watch?v=Q2rvAoO-MIA) 2:10Minutes Position).

The one i found on Aliexpress is V2.2 on the backside without the CAP.
Is this a good or a bad idea? ;-)

Anybody know some more news, maybe about V2.4 (new design)?

Greetings from Swiss
Philippe
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: Fungus on November 08, 2017, 08:17:27 am
I will order this DPS5020

Dave's Board Version is V2.3.

The one i found on Aliexpress is V2.2

Anybody know some more news, maybe about V2.4 (new design)?

This video is only 2 days old and you want the next revision today? While the sellers are still on the previous revision?  :o
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: T3sl4co1l on November 08, 2017, 08:47:26 am
Why put fuses at the end, at all?  It's current limited, isn't it?  I hope it's current mode, anyway... that TL598 isn't a good sign though...

If it's for short protection (pass device fails), the fuse should be at the input, where a bigass capacitor or battery can supply clearing fault current.

Tim
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: rymannphilippe on November 08, 2017, 09:19:46 am
Dear Fungus

You are right  :-DD
Sorry I'm stuck in my business and we are only do some emergency manufacuring and 24h is nothing.... but you are right.

By the way:
I have some contact with RD STORE and the told me, THE CAP IS CHANGED.
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: NANDBlog on November 08, 2017, 10:29:24 am
Why put fuses at the end, at all?  It's current limited, isn't it?  I hope it's current mode, anyway... that TL598 isn't a good sign though...

If it's for short protection (pass device fails), the fuse should be at the input, where a bigass capacitor or battery can supply clearing fault current.

Tim
The pass transistor can fail short. Also, people can use this to charge batteries so in case a failure, power can be pumped in from the output terminal. Imagine a 24V li-ion battery pack, with 10mOhm resistance charged.
When I was designing battery testing system, we had a fuse for every single pass transistor.
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: Fungus on November 08, 2017, 02:02:09 pm
You are right  :-DD
Sorry I'm stuck in my business and we are only do some emergency manufacuring and 24h is nothing.... but you are right.

Less than 24h to manufacture a product revision and destroy all the old ones? That must be expensive.

I have some contact with RD STORE and the told me, THE CAP IS CHANGED.

Maybe you can work something out to get one of the new ones.

Or... get one of the old ones and substitute an old-fashioned cap. One with long, flexible metal legs.  :popcorn:
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: Freelander on November 08, 2017, 05:08:34 pm
You are right  :-DD
Sorry I'm stuck in my business and we are only do some emergency manufacuring and 24h is nothing.... but you are right.

Less than 24h to manufacture a product revision and destroy all the old ones? That must be expensive.

I have some contact with RD STORE and the told me, THE CAP IS CHANGED.

Maybe you can work something out to get one of the new ones.

Or... get one of the old ones and substitute an old-fashioned cap. One with long, flexible metal legs.  :popcorn:

The version for sale now HAS the cap with old fashioned 'bendy' legs ;). Simple replacement. All sorted. - Also,the cap is only (and was only) fitted on the DPS5020/C I believe.
Sorted. :)
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: paulca on November 25, 2017, 09:50:04 am
I just bought one of these.  For some reason I assumed the video was older and a new revision would be out.

However RDTech did say they had stopped selling the one Dave tested, but yet AliExpress's "official" RDTech store is selling them.

I am hoping to run it with a 48V 10A DC power supply (https://www.ebay.co.uk/itm/282556976292 (https://www.ebay.co.uk/itm/282556976292)).  I don't mind that I won't get the full 50V 20A.

So, when it arrives how would I check the board is safe?

For protection should I fuse the DC supply input to the DPS5020, or would I be better with a reset-able breaker to prevent overloading the 10A supply?  So if I do try and draw more then the 48V/10A the breaker will pop.

Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: Fungus on November 25, 2017, 10:14:50 am
I just bought one of these.  For some reason I assumed the video was older and a new revision would be out.

However RDTech did say they had stopped selling the one Dave tested, but yet AliExpress's "official" RDTech store is selling them.

AFAIK they're not doing a product recall. It will take a while for the supply chains to clear and the new version to trickle down.

I am hoping to run it with a 48V 10A DC power supply.

So, when it arrives how would I check the board is safe?

Dave's was due to a cracked capacitor that shorted out two pins. The only way to be sure yours isn't similarly cracked is to power it up. Be sure to record the whole thing for posterity.  :popcorn:

For protection should I fuse the DC supply input to the DPS5020

Where's the fun in that?

It might only be half-shorted. Even if there's no instant flameage you could still look at the unloaded current draw with a multimeter.
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: Brumby on November 25, 2017, 10:22:27 am
So, when it arrives how would I check the board is safe?

Remove the cap.

RD tech has said this will not affect the operation - and the capacitor only reduced the output ripple a bit.

You can always replace the MLCC with another type.
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: paulca on November 25, 2017, 02:52:27 pm
So, when it arrives how would I check the board is safe?

Remove the cap.

RD tech has said this will not affect the operation - and the capacitor only reduced the output ripple a bit.

You can always replace the MLCC with another type.

Could I not measure the cap isn't shorted and reads something like it should?  I mean if it's not cracked it should be fine right?  If it is cracked then it should read funny???
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: paulca on November 25, 2017, 02:54:47 pm
Dave's was due to a cracked capacitor that shorted out two pins. The only way to be sure yours isn't similarly cracked is to power it up. Be sure to record the whole thing for posterity.  :popcorn:

For protection should I fuse the DC supply input to the DPS5020

Where's the fun in that?

It might only be half-shorted. Even if there's no instant flameage you could still look at the unloaded current draw with a multimeter.

I think I might need to put a latex glove over the smoke alarm for this.  Embarressingly it's a building alarm so both my neighbours know full well when someone burns the toast (or toasts a PSU).

Anyway, I think I'll power it up on the bench supply first and limit the current so see how she goes before give it the full 10A supply.
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: Brumby on November 26, 2017, 12:06:18 am
So, when it arrives how would I check the board is safe?

Remove the cap.

RD tech has said this will not affect the operation - and the capacitor only reduced the output ripple a bit.

You can always replace the MLCC with another type.

Could I not measure the cap isn't shorted and reads something like it should?  I mean if it's not cracked it should be fine right?  If it is cracked then it should read funny???

The capacitor in question is at risk because it can easily be subjected to mechanical stress that will cause it to fail.  This stress is applied when the screw terminals for the output are tightened.

You may receive a board that is perfectly fine - until you tighten the terminals ... for the first? ... third? ...ninth? time ... ... ... or it might never fail.

Removing or replacing the cap seems cheap insurance.
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: paulca on December 05, 2017, 06:16:39 pm
Don't want to celebrate too soon, but it looks like they have hand reworked the board.

Pretty sure the offending cap is gone and poly of some kind has been soldered to the bottom of the board.

If you want more photos, just ask.  Although I am about to assemble it and find a rubber glove for the smoke alarm! :D

(http://i.imgur.com/D2Uz4Xs.jpg) (https://imgur.com/D2Uz4Xs)
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: glarsson on December 05, 2017, 07:03:09 pm
Pretty sure the offending cap is gone and poly of some kind has been soldered to the bottom of the board.
The fire happy capacitor was C34. The new bodge capacitor is not replacing C34.
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: Nusa on December 05, 2017, 08:49:11 pm
Pretty sure the offending cap is gone and poly of some kind has been soldered to the bottom of the board.
The fire happy capacitor was C34. The new bodge capacitor is not replacing C34.
You're right, it's technically C33, which was previously unpopulated. Now it's C34 that's unpopulated. C33 functionally replaces it, just on the other side of the 20A fuses.
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: paulca on December 05, 2017, 09:03:11 pm
It works.  The only downside so far is they are so tight fisted they give you EXACTLY enough wire and no more.  So things are pretty tight to solder and connect.

Other than that, it powers on and seems to function.  It's a bit short on max volts.  My bench supply is providing it with 30V, it allows me to set 29.0V but registers 27.15V.  Although it has no load.  The DMM confers to within a few tens of millivolts with what it reads.

Also, a lesson for myself this evening.  If you want to test a power supplies response to a short circuit by stuffing you pliers in the binding posts.

DO NOT DO IT ON 30V!    :-[

Scared the living piss out of myself as sparks shot across the room.  I thought I was safe setting the current limit to 100mA.  Forgot about the 30V (or 27.15V whatever).  Lovely smell of ozone though :)

Dialled back to 3V it responds fine to the pliers, volts drop to some near zero amount, CC lights up.
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: Nusa on December 05, 2017, 09:31:31 pm
Other than that, it powers on and seems to function.  It's a bit short on max volts.  My bench supply is providing it with 30V, it allows me to set 29.0V but registers 27.15V.  Although it has no load.  The DMM confers to within a few tens of millivolts with what it reads.
That seems to match the specifications for all the DPS models, which state the input voltage must be 10% higher than the output voltage.
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: paulca on December 08, 2017, 08:01:51 am
Well, device works fine.  I've loaded it up to 12A, 12.60V with a LiPo and it works as advertised.

Nowhere near as precise as my Tenma bench supply though.  When charging the LiPo with it, it switched from CC to CV at 12.49V instead of 12.60V.   Thankfully it was that way and not the other!  Don't think I will trust it for finishing the LiPo charge, but it can charge a 5000mAh at 2C up till it gets to CV mode then I'll use something more trust worthy.

Oh and... the mains switch on the back failed already.  My suspicion is they don't use high temp plastic to hold things together and the spring has shifted during soldering.  You can push the switch, tap it, thump it, but it will not "click" and go off.  So it's stuck on now and I have to pull the plug out of the wall to kill it.

I'm adding a master switch to my TODO list to beable to kill both my power supplies with an easy to reach switch.  Currently if magic smoke starts to be released I have to climb under the desk and pull the plug, then come back up and switch the Tenma off too.
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: HKJ on December 08, 2017, 08:31:24 am
Nowhere near as precise as my Tenma bench supply though.  When charging the LiPo with it, it switched from CC to CV at 12.49V instead of 12.60V.

Could that be user error, i.e. you are using thin wires and measuring at the battery, not on the ps board output terminals?
Title: Re: EEVblog #1036 - PSU Fire PCB Repair
Post by: paulca on December 08, 2017, 09:24:17 am
Nowhere near as precise as my Tenma bench supply though.  When charging the LiPo with it, it switched from CC to CV at 12.49V instead of 12.60V.

Could that be user error, i.e. you are using thin wires and measuring at the battery, not on the ps board output terminals?

Nope.  Using a dedicated banana to XT60 LiPo lead, it's about 14 AWG, 6 inches long. 

I was going by what the DPS reading said.  It was saying 12.60V set, 12.49V output, CV.  This was literally on the corner between CC and CV, so it might be more to do with a linear region between the two where the display says CV when it's really in that linear region between CC and CV.  I stopped the experiment there, but I would expect as the battery charged and the amps fell more it would come up to 12.60V (ish). 

EDIT:  Input voltage from the DC supply still reported 48.7V