Author Topic: EEVblog #512 - Rigol DP832 Bad Design Investigation  (Read 222530 times)

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Offline ashplant

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Re: EEVblog #512 - Rigol DP832 Bad Design Investigation
« Reply #25 on: August 30, 2013, 10:18:02 AM »
Dave, this was one of your best one's. I like the I like the way it combined teardown and theory.

I wonder how Rigol will react. The cynic in me says they'll come out with new firmware that speeds the fan and slows the digital. If they do it cleverly - and remember that responsive and fast are different - it might stop the resets without much UI pain. But it won't change the fact that you shouldn't have electrolytic caps nestling up to a hot spot.
 

alm

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Re: EEVblog #512 - Rigol DP832 Bad Design Investigation
« Reply #26 on: August 30, 2013, 10:22:21 AM »
one thing i noticed from watching the video again : the metal tab on that lm317 is very thin .... much thinner than normal ... fake lm317 ?

I noticed that too, but there's not a whole lot to screw up for a fake here in my opinion. It's not like 100 mA is flowing through its adjust pin. If 700 mA flows through a linear regulator with a 7 V difference between Vin and Vout, it's going to dissipate at least 3.5 W no matter what you do. It may increase Thetaj-c, though. I have seen the thin tabs from legitimate sources before, not sure if it was from ST.
 

Offline EEVblog

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Re: EEVblog #512 - Rigol DP832 Bad Design Investigation
« Reply #27 on: August 30, 2013, 10:22:41 AM »
It's a shame you didn't bother to just replace the small heatsink with a larger one.

I have not thought about the best fix for my unit yet.
I'd have to source a suitable heatsink in that case, I don't have suitable ones in the junk bin.
 

Offline johnnyfp

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Re: EEVblog #512 - Rigol DP832 Bad Design Investigation
« Reply #28 on: August 30, 2013, 10:29:07 AM »
Well it certainly looks a lot thinner than a not fake LM317.

Top LM317 by ST in malaysia
2nd LM317 by ST in morroco notice the tab difference
3rd 7812
4th 7805 by ST

So they are thicker. Or maybe mine are the fake ones!!!
 

Offline c4757p

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Re: EEVblog #512 - Rigol DP832 Bad Design Investigation
« Reply #29 on: August 30, 2013, 10:31:00 AM »
Yeah, I've always found the ST TO-220 tabs to be nice and thick. Now, Fairchild, on the other hand...
No longer active here - try the IRC channel if you just can't be without me :)
 

alm

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Re: EEVblog #512 - Rigol DP832 Bad Design Investigation
« Reply #30 on: August 30, 2013, 10:32:25 AM »
Or yours are older. What's the date code?

But even if it is a fake, what would a genuine LM317 do differently that makes this problem disappear? We saw it put out a fairly stable 5V DC, so it's not like a fake LM317 is going to magically have the LCD draw twice the current. Are you seriously blaming the LM317 for going into thermal shutdown at what might be a junction temperature in excess of 140°C?
 

Offline johnnyfp

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Re: EEVblog #512 - Rigol DP832 Bad Design Investigation
« Reply #31 on: August 30, 2013, 10:41:51 AM »
Yeah ok, mine are quite old, 97-99.

Maybe it's the old adage of "They don't make them like they use too"
 

Offline staggerlee

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Re: EEVblog #512 - Rigol DP832 Bad Design Investigation
« Reply #32 on: August 30, 2013, 10:47:49 AM »
Transformer tap voltage too high or wrong tap?
 

Offline johnnyfp

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Re: EEVblog #512 - Rigol DP832 Bad Design Investigation
« Reply #33 on: August 30, 2013, 10:49:13 AM »
Dave measured 12v in so suspect that is correct input voltage.
 

Offline staggerlee

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Re: EEVblog #512 - Rigol DP832 Bad Design Investigation
« Reply #34 on: August 30, 2013, 10:53:29 AM »
Is it normal to use 12v and regulate it down to 5?
 

Offline Rufus

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Re: EEVblog #512 - Rigol DP832 Bad Design Investigation
« Reply #35 on: August 30, 2013, 11:00:51 AM »
one thing i noticed from watching the video again : the metal tab on that lm317 is very thin .... much thinner than normal ... fake lm317 ?

Not fake, they call it a single gauge TO220 package, a drawing is in the ST data sheet.

Probably devised because of and definitely becoming popular with the rising price of copper.

It doesn't really change theta JC but there is less copper to spread heat across the tab so the effective case to heat sink thermal resistance is going to be a bit worse.
 

Offline johnnyfp

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Re: EEVblog #512 - Rigol DP832 Bad Design Investigation
« Reply #36 on: August 30, 2013, 11:20:28 AM »
Normal? Yes, if you want to put together something cheap and quick, but are powering a low wattage circuit, like a simple uController or low current circuit.

But at 700ma, no, pure power wastage madness.

I hope they don't want a Energy Star rating on this thing!
 

alm

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Re: EEVblog #512 - Rigol DP832 Bad Design Investigation
« Reply #37 on: August 30, 2013, 11:30:06 AM »
I think the 5 V supply is only driving the digital logic, so I really see no reason why they didn't go for a buck converter. Would have been much more efficient and would have easily fitted in that space.

In Dave's scope measurements it seemed like only a few mV or so increase in ripple caused the system to reset, but I think there's more going on (like a fast transient) that the scope didn't capture or gets clamped. Their noise margin can't be that poor. This is not a mobile device running on an extremely tight power and cost budget. Unless someone screwed up and designed in a 3.3 V regulator with a 4.99 V dropout voltage, but that would be plain stupid. And I doubt anything sensitive gets powered directly from 5V.
 

Offline EEVblog

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Re: EEVblog #512 - Rigol DP832 Bad Design Investigation
« Reply #38 on: August 30, 2013, 11:32:03 AM »
But even if it is a fake, what would a genuine LM317 do differently that makes this problem disappear?

Nothing. Fake or genuine, the die is still running at greater than the recommended max operating temperature.
 

Offline EEVblog

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Re: EEVblog #512 - Rigol DP832 Bad Design Investigation
« Reply #39 on: August 30, 2013, 11:33:18 AM »
Is it normal to use 12v and regulate it down to 5?

Nope. Normally you pick something like a 9V tap for that.
 

alm

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Re: EEVblog #512 - Rigol DP832 Bad Design Investigation
« Reply #40 on: August 30, 2013, 11:36:00 AM »
I assume the 12 V rail is also used for something like running the fan, so that would involve adding another winding to the already complex transformer.
 

Offline sync

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Re: EEVblog #512 - Rigol DP832 Bad Design Investigation
« Reply #41 on: August 30, 2013, 11:37:59 AM »
Is the USB host port powered by this LM317 too? Then load it with an additional 500mA.>:D
 

Offline Rufus

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Re: EEVblog #512 - Rigol DP832 Bad Design Investigation
« Reply #42 on: August 30, 2013, 11:40:05 AM »
At your mains voltage and 22V @ 3A output, just before the first tap switches

I was also going to mention worst case pass transistor dissipation will be at maximum current and a voltage just before one of the downwards tap changes.

On the reset problem - I'm not totally convinced that the regulator shutting down was resetting the processor without being able to see more than 100mV of ripple on the 5v rail. The ripple we did see was at 5Hz what is that from? Was there some high frequency oscillation the scope wasn't seeing? On the other hand air flow across the board stopping it and the crazy regulator temperature does point towards it.

On the regulator temperature yes it is crazy, the board ought to have a high temperature warning label if the regulator was intended to run that hot.

On the actual design issue I can see how it happens, the attraction of using a common heat sink, the load requirement being underestimated and creeping upwards. Maybe they decided minimum mains input voltage was too high and upped the transformer winding a bit. Maybe the fan was a bit loud and they turned down the minimum speed. What I don't understand is why it wasn't determined to be a problem and fixed before the went into production. I can't believe no one noticed there was a burn hazard on the boards they were developing then building and testing.

Rigol are competent enough to design a 5v regulator, if not their other products would be riddled with problems. Seems more like a management/quality/ethos failure.
 

Offline EEVblog

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Re: EEVblog #512 - Rigol DP832 Bad Design Investigation
« Reply #43 on: August 30, 2013, 11:44:29 AM »
Is the USB host port powered by this LM317 too? Then load it with an additional 500mA.>:D

Oops, yeah, it would be. Good point!
 

Offline sync

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Re: EEVblog #512 - Rigol DP832 Bad Design Investigation
« Reply #44 on: August 30, 2013, 11:48:33 AM »
Is the USB host port powered by this LM317 too? Then load it with an additional 500mA.>:D

Oops, yeah, it would be. Good point!

You can use it for remote reset. It's not a bug, it's a feature. ;D
 

Offline Fraser

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Re: EEVblog #512 - Rigol DP832 Bad Design Investigation
« Reply #45 on: August 30, 2013, 11:48:59 AM »
The comments on firmware changes makes me think of what Canon have done with my SX280 compact zoom camera. A great camera let down by a basic design flaw.

When running in video mode, if you used the zoom it would immediately indicate 'low battery' and this would remain until a full power off/on reset. How that fault got past QC I will never know. It wasn't as if the issue was hard to replicate...they all do it.

Canon came out with a new firmware to 'fix' the problem. My brand new camera arrived with the old firmware so I installed the new patched version. The result is less occurrences of the low battery warning when using the Zoom, but the problem does still occur now and then.

Why am I telling this story ? well the firmware update changes just one thing....it slowed the zoom motor and effectively lowered its current consumption. It appears that the battery in the SX280 is overloaded by video and Zoom functions operating at the same time and drops its p.d. as a result. Oooops !

The camera takes great pictures and the video issue is not a concern to me, but it is yet another example of very poor design and testing.

I suspect Rigol will not like the idea of a recall or any hardware changes, so the comments regarding crippling the processor speed are a definite possibility. The early ASUS 700 netbook was the subject of such a 'fix'. The first release overheated so the firmware was updated to hobble the processor speed and so reduce heat production inside the case. It worked but was then awfully sluggish.
« Last Edit: August 30, 2013, 11:52:35 AM by Aurora »
 

Offline Rufus

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Re: EEVblog #512 - Rigol DP832 Bad Design Investigation
« Reply #46 on: August 30, 2013, 11:49:16 AM »
Is it normal to use 12v and regulate it down to 5?

Nope. Normally you pick something like a 9V tap for that.

An LM317 needs 3v across it so that's 8v, add on the ripple and line input range and a bit less than 12v input average at the high end of the line range isn't far out.

Is the 12v used for anything else? A lower voltage tap and LDO regulator would almost be a fix. 
 

Offline twjtwj

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Re: EEVblog #512 - Rigol DP832 Bad Design Investigation
« Reply #47 on: August 30, 2013, 11:49:32 AM »
Maybe there is a software option upgrade to make it run cooler.  :-DD
 

alm

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Re: EEVblog #512 - Rigol DP832 Bad Design Investigation
« Reply #48 on: August 30, 2013, 11:53:58 AM »
Replacing all those transformers will be expensive, though.
 

Offline mickpah

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Re: EEVblog #512 - Rigol DP832 Bad Design Investigation
« Reply #49 on: August 30, 2013, 11:58:47 AM »
hmm,
noticed mine has a different transformer  500va not 400va and it has a 9v tap.
time to tear it apart again and measure I guess

http://www.eevblog.com/forum/testgear/new-rigol-dc-psu's/?action=dlattach;attach=55776[/url]
wrong link, trying again

http://www.eevblog.com/forum/testgear/new-rigol-dc-psu's/?action=dlattach;attach=55778
« Last Edit: August 30, 2013, 01:23:25 PM by mickpah »
 


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