Power Management: 2A fuse between battery and 12V blown - a short on the board?

[sabir]

Hi there,

on March 21st I installed the MacArthur Hat on my RasPi. Everything was working fine: When I switched on the navigation, the RasPi booted, when I switched of the navigation the RasPi performed a shutdown. The NMEA connection worked also. It was perfect, I was happy! The system was running continuously with only short intended reboots for about two weeks. It stopped working on 6th of April.

Yesterday I (finally) had the time to search for possible reasons, why it stopped (I expected the SD card to have died - as usual). I realised, that the 2A fuse, i placed between the battery and the board's 12V, has been blown. There seems to be a short circuit on the board, because a newly fitted 3A fuse also blew immediately. When I analysed the data of the RasPi in the InfluxDB, I realised, that the damage apparently happened at the moment, when I started the boat's engine.

I have the following components (stacked from bottom to top):
RasPi 4B
MacArthur HAT + power module with soldered headers
Moitessier HAT

The power module is directly connected to the battery (with the fuse to protect the cable and the RasPi). The two NMEA wires are connected. NMEA 12V goes to 12SW. USB-power is not connected.

I checked the cable from the battery to the black plug with a multimeter, there is no connection between the black and the red cable. After fitting a new fuse, I reconnected the black plug to the black connector on the MacArthur HAT and the fuse blew immediately. As mentioned in this reply from Adrian (Release the Magic Smoke), I tested 12V and GND on the black connector with my multimeter in continuous mode -> it beeped (also on the connector where the DC/DC convertor sits). I disconnected the DC/DC convertor and checked 12V and GND -> it seems to be ok (Multimeter says "0L").

Without the 12V and 12V SW connected, the RasPi and MacArthur and Moitessier still work perfectly with USB power connected. Even without the 12V connection the MacArthur HAT is the perfect solution for me on the boat! Thank you for all your work!

- What part could starting/cranking the engine have in the problem?
- What can I do to help finding the cause of the problem? (Please break it down "Barney style", I am more on the software side, than a hardware developer)
- When I power the RasPi with USB: Can I still leave the 12SW connected to startup/shutdown the Pi when switching on/off the navigation?

[Adrian]

It sounds like the DC/DC module died.

Starting/cranking can create voltage spikes (or drops) on the 12V. But the TVS diode D1 should take care of voltage spikes. What do you get if you put your multimeter into diode mode, and measure between 12V and GND on the black connector with 12V power removed? It should read about 0.6V when measured from GND to 12V, and open when measured from 12V to GND.

[sabir]

Thank you for the reply. I took the two measurements: 
The multimeter shows 0V in both cases.

[Adrian]

Thank you for the reply. I took the two measurements: 
The multimeter shows 0V in both cases.

You will have to remove the DC/DC module to make this measurement. We already established that the DC/DC module failed short.

Also, I recommend to shutdown the Pi and disconnect it from power before measuring anything except for voltages.

[ozone_one]

It sounds like the DC/DC module died.

Starting/cranking can create voltage spikes (or drops) on the 12V. But the TVS diode D1 should take care of voltage spikes. What do you get if you put your multimeter into diode mode, and measure between 12V and GND on the black connector with 12V power removed? It should read about 0.6V when measured from GND to 12V, and open when measured from 12V to GND.

I would always recommend installing a DC-DC converter/voltage stabilizer between the boat's 12V system and any sensitive electronics.  They are a buck/boost converter that will take in anything between 9v-36v and put out a fairly clean regulated 12 volts.  It will isolate your electronics from any spikes/dips of your 12v system.  Lots of stuff can happen - runaway voltage regulator, voltage spikes and dips from sudden loads like starting/refrigeration/compressor/bow thruster, etc.

They aren't expensive (here is one example that should be more than adequate for an OpenPlotter system), and can really make a difference

[sabir]

Oh wow, I had to look up, how a multimeter is testing a diode. I will never again try to use it with power connected to circuit...
Thank you also for the advice to use a DC/DC voltage converter/stabilizer to protect the RasPi.

I now removed the DC/DC module and repeated the measurement without power connected: I still get the same measurements, though...




Can you tell me, if I can still use the 12V SW to startup/shutdown the RasPi without the 12V connected?

[Adrian]

Hmm, what does your meter display in diode mode when the probes don't touch anything?

Measuring 0V in both directions would indicate the there's a short even with the DC/DC module removed, most likely because the TVS diode is dead. In that case, your fuse would blow again if you connected 12V.

I think the automatic shutdown should also work without a DC/DC module. However, you'd have to power-cycle the 5V supply to restart the Pi. But make sure to still connect GND when connecting 12V SW.

@ozone_one: I wasn't aware of voltage stabilizers. I don't know how well they hold up to abuse, but given the price it seems like cheap insurance.

[sabir]

Hmm, what does your meter display in diode mode when the probes don't touch anything?

The meter shows "0L" if it is not connected, so the meter should be ok. Is there anything else, I can do to check the board? Is the apparently dead diode "repairable" (meaning changeable)? For sure not by my, I would simply fry the whole board with my soldering iron... 

Measuring 0V in both directions would indicate the there's a short even with the DC/DC module removed, most likely because the TVS diode is dead. In that case, your fuse would blow again if you connected 12V.

Do you want me to check, if a new 2A fuse will be blown without the DC/DC module (I have some fuses left ?)?

Hasn't there been another post, that mentioned the TVS diode could be blown by a voltage peak of the alternator? Maybe a future revision of the board could take this into account or maybe the documentation could recommend using a voltage stabilizer, like @ozone_one proposed?

I think the automatic shutdown should also work without a DC/DC module. However, you'd have to power-cycle the 5V supply to restart the Pi. But make sure to still connect GND when connecting 12V SW.

You mean: Connect 12V GND of the NMEA cable to the black connector GND and NMEA 12V to 12V SW. Then the Pi will shutdown, if NMEA bus is powered down. To restart, I would have to disconnect/connect the USB power.

[ozone_one]

H
@ozone_one: I wasn't aware of voltage stabilizers. I don't know how well they hold up to abuse, but given the price it seems like cheap insurance.

Well, voltage stabilizer is really just a marketing name for buck/boost converter.  I don't have any significant data on lifespan...  My assumption is basically that the thing is designed and created to accept anything from 9v to 32v, so if you are only feeding it 11v-18v and not trying to pull max rated current from it 24/7/365.  it should last quite a while.  Most of them are fully potted in epoxy so they should be able to handle the vibration and damp environment of a boat no problem.

Anecdotally, I can say that I have at least a dozen of them in use on various projects over the past 7-8 years, and none have set the magic blue smoke free yet.  As with most buck and boost converters, they waste a little DC as heat, so there is a slight efficiency loss.  And that heat has to go somewhere. so I would definitely recommend getting one with an aluminum body/heat sink for anything over a couple of amps, and don't run them really close to the ragged edge of their spec - if you know you need 4.5 amps out of one, don't buy a 5 amp model, get the 7.5a or 10a.

Seeing 12v on the spec for a piece of electronics can be misleading.  When it refers to a 12v automotive or marine electrical system, chances are that it was created to work with the common voltage range for such a system - somewhere around 11.8v-14.6v more or less, so it is easy to assume that anything that says 12v will work in that voltage range.  But for many non-auto/marine devices, 12v means they accept a range from 11.9v-12.2v. A good example is a 12v LED strip. If you run it at 12.1v, it should last a very long time - years.  But if you hook it up to an auto or marine 12v electric system and it gets fed 11.8v-16v daily, with rapid excursions between those two limits whenever a large load connects, that strip is probably going to last a few months if you are lucky.

I just default to using a stabilizer on any auto/marine install when the equipment would otherwise be powered by a wall wart.

[Adrian]

The meter shows "0L" if it is not connected, so the meter should be ok. Is there anything else, I can do to check the board? Is the apparently dead diode "repairable"
(meaning changeable)? For sure not by my, I would simply fry the whole board with my soldering iron...

It can be done with the right tools. I would probably first remove the 1-Wire connector, then use hot air to desolder the diode. Where are you based?

Do you want me to check, if a new 2A fuse will be blown without the DC/DC module (I have some fuses left ?)?

Based on your meter readings, I'm pretty certain its the TVS diode.

I think the automatic shutdown should also work without a DC/DC module. However, you'd have to power-cycle the 5V supply to restart the Pi. But make sure to still connect GND when connecting 12V SW.

You mean: Connect 12V GND of the NMEA cable to the black connector GND and NMEA 12V to 12V SW. Then the Pi will shutdown, if NMEA bus is powered down. To restart, I would have to disconnect/connect the USB power.

Correct.