Serious HDMI Problems. What's that smell? Burning Raspberry!


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by Tom1989 » Sat Jun 30, 2012 1:12 pm
OK. So I have just received my Neewer Black HDMI Input To VGA Adapter Converter For PC Laptop.

There was a post here:

viewtopic.php?p=111477#p111477

That reported it working and I have no reason to doubt that. However when I plugged mine in to my raspberry pi and VGA monitor the screen was black but lit i.e. the screen recognised a connection was present but wasn't receiving any data. A thin wisp of grey smoke appeared from the HDMI and the component labelled "D1" is looking like it has burn out. Suffice to say I very quickly removed it and all power!

With unbelievable luck I don't appear to have caused any other damage. The analogue output is working fine and everything else seems to be working fine too including graphics and the ethernet port.

So what the raspberry happened????

Please can someone help me!? I have no idea why this happened.

I have previously measured the output of my power supply using the raspberry pi's T1 and T2 connections and it is a seemingly stable 5.25Volts which, according to the wiki, should be ok just. The output is quoted on the power supply as being 5V 1A(max).

How should I proceed??

Thank you,

Tom
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by johnbeetem » Sat Jun 30, 2012 3:08 pm
My SWAG (Scientific Wild-A**sed Guess) from looking at the RasPi schematics is that the HDMI->VGA converter has an internal short between +5V (HDMI pin 18) and GND (HDMI pin 17), perhaps a solder bridge. This caused a large current (perhaps as much as 1A) from RasPi +5V (TP1) through BAT54 diode D1 which has a max current rating of 200 mA. So D1 quickly got hot and the magic smoke leaked out.

My guess is that D1 is now shorted or open, but it's easy to replace with moderate soldering skill. You can test it with a multimeter to see whether it's shorted or open. In the highly unlikely case that it's still working, it should conduct in one direction and block in the other.

I used the ON Semiconductor data sheet for the BAT54, available from onsemi.com. ON Semi used to be the part of Motorola that made discrete semiconductors.
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by zardoz99 » Sat Jun 30, 2012 3:59 pm
It seems that this is becoming a common problem. There are at least 3 posts that report the same.

Design/Manufacturing/Peripheral fault? Who knows....

Z.
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by mahjongg » Sat Jun 30, 2012 4:21 pm
zardoz99 wrote:It seems that this is becoming a common problem. There are at least 3 posts that report the same.

Design/Manufacturing/Peripheral fault? Who knows....

Z.



Not the same issue, those two had the diode burned out the moment the PI was turned on, this one is a situation where plugging in the HDMI cable caused the diode to burn out.

D2 is simply a diode connecting 5V to a pin (pin 18) on the HDMI connector, its there (instead of 5V connected directly to 5V) to make sure that if the TV itself puts a voltage on the pin it won't reach the RPI. Pin 17 of the same connector is connected to GND, so if for some reason pins 17 and 18 are connected, the diode will be connected (conducting) in between 5V and GND, shorting the 5V to GND,The short circuit current through the diode will burn it out. The short may be cause by any metal particle that may have found its way to short out pins 17 and 18 by accident, or by a soldering "touch up" that went wrong.

In this case the short was in the HDMI cable or the converter box not in the PI.

And "a common problem"? really? Common maybe for such cheap converter boxes.
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by jbeale » Sat Jun 30, 2012 4:30 pm
maybe not relevant to HDMI, but I was about to connect an analog TV input to my R-Pi using an RCA plug. I turned on the monitor first, connected the RCA plug to it and as I brought the other end towards the Pi, I felt a little "bite" as my finger brushed the plug. Concerned, I checked the AC voltage between the ground of the plug and the ground of my R-Pi. Hmm... 54 Vac.

I then decided NOT to connect that monitor. I don't know how much current would have flowed- maybe it was a very high-impedance leakage path- but still a worry! (fwiw, this was not a consumer display, it is a professional monitor, with a set of BNC connectors.)
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by Tom1989 » Sat Jun 30, 2012 4:59 pm
mahjong, thank you very much!

Hmm I will update the wiki with the info you have just told me. It sounds like this could happen to other people looking for the cheapest solution.

Ok so is it possible to replace this diode? I presume that my HDMI output is stuffed until this is fixed?
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by mahjongg » Sat Jun 30, 2012 5:07 pm
jbeale wrote:maybe not relevant to HDMI, but I was about to connect an analog TV input to my R-Pi using an RCA plug. I turned on the monitor first, connected the RCA plug to it and as I brought the other end towards the Pi, I felt a little "bite" as my finger brushed the plug. Concerned, I checked the AC voltage between the ground of the plug and the ground of my R-Pi. Hmm... 54 Vac.

I then decided NOT to connect that monitor. I don't know how much current would have flowed- maybe it was a very high-impedance leakage path- but still a worry! (fwiw, this was not a consumer display, it is a professional monitor, with a set of BNC connectors.)


Just use your monitor on a grounded mains plug, without a ground wire there will be half the mains voltage in between the whole monitor and earth, due to leakages in the power supply of the monitor, as it uses two small capacitors between the mains leads and the ground of the power supply (and thus the whole device). Very common, but not a real problem, as these (EMC protection) capacitors are very small. Again, just ground your monitor and the problem will disappear.
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by Tom1989 » Sat Jun 30, 2012 5:32 pm
My guess is that D1 is now shorted or open, but it's easy to replace with moderate soldering skill. You can test it with a multimeter to see whether it's shorted or open. In the highly unlikely case that it's still working, it should conduct in one direction and block in the other.

I used the ON Semiconductor data sheet for the BAT54, available from onsemi.com. ON Semi used to be the part of Motorola that made discrete semiconductors.


Sorry johnbeetem I didn't see your post. Thank you very much for the reply.

I had a look at BAT54 on onsemi.com and the only diode I could find there was a BAT54: 30 V Schottky Diode would that work as a replacement diode?

Update: Also had on RS Online and there are loads of different BAT54 diode there. How can I tell which one to use? The raspberry pi schematic just says BAT54.
Last edited by Tom1989 on Sat Jun 30, 2012 5:46 pm, edited 1 time in total.
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by mahjongg » Sat Jun 30, 2012 5:38 pm
Tom1989 wrote:mahjong, thank you very much!

Hmm I will update the wiki with the info you have just told me. It sounds like this could happen to other people looking for the cheapest solution.

Ok so is it possible to replace this diode? I presume that my HDMI output is stuffed until this is fixed?


Yes, the BAT54 is a simple SOT-23 schottky diode, as common as anything, maker is unimportant, dozens of manufacturers make this one. Use a cutter to cut off the one lone lead on the side of the package with one lead in the middle, then with a soldering iron you can heat both other legs, and remove the diode, then remove the remaining lead. Clean with solder wick, or pump, then you can solder another BAT54 diode, which should cost only a few dimes, but read on before proceeding.

There is probably still a short, or overload, in the power line of converter which must be removed or dealt with.
The raspi is designed to deliver the 50mA for a small EEPROM containing monitor capabilities in the TV, (which is according to HDMI specifications) and this power is (mis) used by this adapter, and also will probably be much larger than the 50mA allowed . A BAT54 is good for 200mA, but if the adapter uses much more than 50mA its a more fundamental design error in the adapter (which should work with the 50mA that is normally available to it). I guess the device is so cheap because its ignoring many specs, not being externally powered as it should be if it needs more than 50mA (which is almost certainly the case) is one of them I guess. I also guess that it might overtax the raspi's power supply, as it almost certainly doesn't draw 50mA.

What I would do in a situation like this is using the opportunity that a removed diode offers to measure the current the converter draws, (measuring on the solder pads of the diode) if its much more than 200mA (which I suspect) a BAT54 won't cut it, and you also will run into trouble with your PSU which must source the extra current, and with the input fuse of the Raspi, which is rated for normal use, that is 500mA for the R-PI and 2 x 100mA for the USB devices, nothing more!

Provided the PSU can source the extra current, and the polyfuse doesn't blow you could try to mount a heavier diode, for example a PMEG2010AET which can conduct 1A.
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by Tom1989 » Sat Jun 30, 2012 5:55 pm
Thank you for the information! That was very helpful indeed.

The maximum continuous forward current and peak voltages differ slightly for all the BAT54 diodes listed on RS Online. How can I find out which one to use? Or does it not matter as they only seem to differ by .2 of an Amp and Volt?

BTW I have no intention of re-using the converter I bought. That is going strait back! I will only be connecting it to expensive converters and actual HD Televisions and monitors.
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by mahjongg » Sat Jun 30, 2012 6:27 pm
In that case just use any generic BAT54, small differences in spec do not matter (and might lead you astray, as you have small lies, big lies and specs :mrgreen: ), the small drop voltage is just subtracted from the input voltage, and values like 0.2 Volt are OK. Non schottky diodes (Like the BAV99) have drop voltages in the range of 0.6 volt, which is more problematic. Just use a BAT54 and it will work.
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by johnbeetem » Sat Jun 30, 2012 7:36 pm
Tom1989 wrote:BTW I have no intention of re-using the converter I bought. That is going strait back! I will only be connecting it to expensive converters and actual HD Televisions and monitors.

Before sending it back, I'd test my hypothesis about HDMI pins 17 and 18 being shorted by the converter. It could also be the HDMI cable. It's easy to check this with an ohmmeter with all power off:

1. Disconnect RasPi power and all cables except for HDMI. Disconnect the VGA monitor.
2. Connect HDMI cable to converter box.
3. Check resistance between D1 pin K (it's the pin by itself nearest the text "D1") and GND (TP2). If there's a short between HDMI pins 17 and 18 this should be a very low resistance, well below 100 Ohms.
4. Unplug the converter box. If the short persists, it's in the HDMI cable. If the short goes away, it's in the converter box or a misaligned HDMI connection at the converter box.
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by mahjongg » Sun Jul 01, 2012 1:14 am
No need for the converter to short the pins, only for it to draw significantly more than 200 mA through the BAT54, you cannot really measure that with an ohm meter, as it would only put a fraction of a volt on its probes when measuring ohms, not enough to power the converter. You should measure how many milliamps the converter is using when is running, the only practical way to do that is by removing D1, so you can connect your multimeter there.
An indirect way would be to measure the power consumption with or without the converter plugged in.
Before measuring current consumption obviously you would check for any shorts first..
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by johnbeetem » Sun Jul 01, 2012 2:36 pm
mahjongg wrote:No need for the converter to short the pins, only for it to draw significantly more than 200 mA through the BAT54...

Yes, that's true. It's a pity that useful technical information like how much current a converter draws is not available at retail web sites or elsewhere. I didn't think of this since we have heard of the particular device working OK with other RasPis, so if the converter draws 200 mA or more you'd think it wouldn't work with any RasPi. Perhaps the lucky user has a particularly robust BAT54 which still contains its magic smoke, or perhaps power consumption depends heavily on VGA resolution.
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by mahjongg » Sun Jul 01, 2012 2:50 pm
Yes, all of the above is true.
Note that the more expensive (well built ones) use external PSU's, which is what I would expect if they contain frame buffers etc.
The extremely cheap "cable powered" ones probably only contain LVDS receivers, deserializers, and simple resistor ladder D/A converters for R G and B. And make no attempt to change anything in the signal resolutions and timings, just turn the HDMI LVDS signals into analog signals and sync signals.
They probably also usurp the power that would otherwise go to the monitors EEPROM, that is why you cannot get any monitor info, they need every mA.
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by gritz » Sun Jul 01, 2012 3:15 pm
It's possibly worth repeating that a Bat54 (or other Schottky) that is exposed to not quite enough overcurrent to reduce it to "smoking boots" mode may fail short circuit and then happily conduct in both directions. It may even fail more gracefully by exhibiting the correct forward voltage, but leaking like billy-ho when a reverse voltage is applied. Not pretty.

A bigger diode may let out magic smoke further upstream (like a pcb trace), especially if the input polyfuse had been bypassed, or if the board is now wearing decoupling capacitors that have been salvaged from industrial welding plant.
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by mahjongg » Sun Jul 01, 2012 3:20 pm
Okay, but a shorted or leaking diode will be of little consequence when the R-PI is used only with a cable powered converter. It might become a problem later on though if that Rasp is ever connected to a real HDMI (DVI-D) monitor.
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by gritz » Sun Jul 01, 2012 3:32 pm
Yeah, but bear in mind that "learning the hard way" with software is merely time consuming, but doing the same with hardware can can be downright messy. Let's be careful out there!
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by mad-hatter » Fri Jul 06, 2012 5:52 pm
Hello,

I've got one of these adaptors. Worked fine for 3 days, then powered up and no display.

Checked some voltages on the output of D1:-
3.83v connected with monitor on
3.65v adapter only
4.87v nothing connected
4.88v TPI to TP2

I don't know what the voltages should be or what they mean.
My knowledge of electonics is somewhat limited.

Does this suggest the adaptor is up the shoot?
Does anybody have any suggestions?

Regards
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by mahjongg » Fri Jul 06, 2012 6:11 pm
A BAT54 is a schottky diode, chosen because schottky diodes have a very low "forward voltage", that is when current is running through the diode there is very little voltage drop over the diode, for normal diodes it would be something like 0.6 Volt, for the BAT54 it normally is something like 0.25Volt.

The 1.23 Volt diode drop you see (4.88V - 3.65V=1.23V) means that the diode is damaged, and it is also clear the converter draws much more current than the TV, exactly how much I cannot say, but probably enough to burn out the BAT54.

The BAT54 needs to be replaced, probably with a much stronger Schottky diode, (or the diode must be bridged, but in that case its not safe anymore to connect it to a TV) also expect that your PSU has to deliver a lot more current than normal.
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by mad-hatter » Fri Jul 06, 2012 7:02 pm
Hello,

Thanks for the reply and information.

I've had a look at the data sheet :- http://www.fairchildsemi.com/ds/BA/BAT54.pdf
and can understand that its a low powered device.

Would you care to recommend a replacement with a higher current rating.

I think a standard package type would be OK, and easier for me to solder.

Perhaps I will replace the power supply, at the moment 5Volt 1A.

Regards
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by mahjongg » Fri Jul 06, 2012 9:55 pm
mad-hatter wrote:Hello,

Thanks for the reply and information.

I've had a look at the data sheet :- http://www.fairchildsemi.com/ds/BA/BAT54.pdf
and can understand that its a low powered device.

Would you care to recommend a replacement with a higher current rating.

I think a standard package type would be OK, and easier for me to solder.

Perhaps I will replace the power supply, at the moment 5Volt 1A.

Regards


As I said in an earlier post, the PMEG2010AET is a replacement that will do 1A instead of 0.2A.
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by davidw » Sat Jul 07, 2012 7:20 pm
If the Pi is damaged by a short circuit on the "HDMI" cable, it is not HDMI compliant. The specification requires that no combination of short circuits should damage either end of the connection.

If also requires the current from the pin in question to be limited to 500mA or less (a current limit is obviously needed to protect cable and connectors from a short).

The other problem I noted is that the maximum power off drain on the CEC line is very small, but that line is clamped to the 3.3v rail and I presume that there is a relatively low resistance path to earth through it.
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by pwinwood » Sat Jul 07, 2012 9:05 pm
For what it is worth I have measured the current consumption of the "Neewer Black HDMI Input To VGA Adapter Converter" to be 400mA. There are two 800mA switch mode power supply chips in the Converter. Therefore it is likely to damage the diode on the Raspberry Pi board. To fix this I have added a separate power lead to the Converter to feed it with 5v from the USB hub - the power lead has a USB connector on the end of it. No current is consumed now from the Raspberry Pi HDMI interface because the protection diode is reverse biased.
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by mahjongg » Sun Jul 08, 2012 12:44 am
Do note that when the BAT54 is burned out, it will no longer act as a diode, and should at least be removed, if not replaced.
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