Category Archives: Design Analysis

The Earthwise Wood Chipper

I had a huge pile of sticks and brush that I needed to do something with, so I decided to purchase an electric wood chipper.  Even though this had mixed reviews, I decided on the Earthwise GS70015 Chipper/Shredder.  There were a few reviews about it not turning back on for a few people, but I figured they might be pushing it too hard, overheating the motor, etc.

Earthwise GS70015 Chipper/Shredder

After chipping two bins I opened up the top to clear a branch, closed it back up, and it wouldn’t turn back on. I checked the breaker, let it sit for a few minutes, and tried again but no luck. I then opened and tried securing the top, again with no luck. I noticed there was an interlock switch that makes sure the top has been secured to allow it to turn on, and given I had just opened it up before it stopped working this was highly suspect.
I decided it was worth a shot to take this interlock switch assembly apart to see if it was the issue, and turns out this is exactly the source. The switch itself has a cover over it to prevent liquid, dust, etc. to enter from the top, but under this assembly where the actual switch is there is NO protection of the actual switch.
When I opened this assembly I found that after only 2 bins of chipped material this area was already covered with dust. The switch is a simple sliding plunger that goes into the switch body. Whenever that switch is opened and closed, it allows dust to enter the switch body. If enough (and apparently not very much) material gets inside the switch body, then the contacts will be covered and it won’t be able to make electrical contact.

Interlock switch assembly.
Interlock switch with dust on plunger.

After verifying the issue with a DMM, I bypassed this interlock switch (reducing safety but increasing functionality) and was back in business. It worked for more than 2 more hours after this with no more on/off issues.

Switch bypassed by connecting both wires to the same terminal.

This really seems like a design oversight, unless their switch vendor/design changed recently and the change hadn’t been validated. While less safe, aside from this fatal design flaw, it operated very well and I cleared a large pile of branches rather quickly.

Dell Inspiron with Overstressed Parts

I was given a Dell Inspiron 3451 that wouldn’t turn on.  Some probing around showed that power is getting onto the motherboard, but none of the switchers that power the CPU would turn on.  After some time doing this I noticed that board was getting warm near the HDMI connector, so I then began to focus on closely checking the parts in that area.

What I found was one of the switching power supply controllers had a hole in the package.  This can happen when a part overheats, or has to deal with an electrical overstress event (which also turns into overheating and package cracking/venting).

TPS51225 with a tiny hole in the package.
Circuit with broken IC.

Some more searching found a second suspect part, in this case what appears to be a diode with a pit in the package.

Package with pitting.

From what I can tell, this traces a path back toward the HDMI port itself.

Location of suspect parts.

I haven’t tested all the diodes in that path, but I’m guessing there was an electrical overstress event (ESD or surge) through the HDMI port, which decided to take a path through these parts to GND.

Is this an issue with the design? It’s hard to say for sure, but ideally the energy wouldn’t make its way into major components.  Usually there are protection diodes (possibly all the other parts in the top left) that should provide a fast path to the board’s ground plane, but in this case it didn’t turn out well.

Given the damage done, it’s probably not worth attempting to repair, since I don’t know what other parts might also be damaged and this is a low-cost laptop to begin with.  Best option in this case is to salvage parts to repair other things.

The Dell Venue that Destroys Batteries

I have a Dell Venue tablet which has a removable battery.  It’s a good thing the battery is removable, because I’ve now how to replace it once a year over the last two years.

Dell battery label.

Each battery is made of two LiPO cells.  Over time the battery cells begin to puff up.  Since there’s no room for the expanding cells, the battery begins to push on the back of the LCD, causing discolored blotches on the screen.  Removing the battery makes this issue go away, but why are the batteries dying so fast?

Bulging battery.
More bulging battery.

This battery was disassembled, and the monitoring PCB was examined.  I didn’t find anything unusual there.  The on-board circuit more than likely only protects the battery from being discharged too much, and from overheating (given the thermistor attached).

Dell battery monitor circuit.

The voltage on each cell was probed and measured 4.3V.  A LiPo cell is fully charged at 4.2V, showing that the cells are being overcharged.  Overcharging a cell will stress it and can cause the cell to puff up like mine have.

Unfortunately the problem is with the tablet and not necessarily with the batteries or protection circuit, and that means battery 3 will eventually meet the same fate.  I don’t have a solution for this yet except for attempting to contact Dell.  Since it’s out of warranty they probably won’t care, but it’s unfortunate they’ve released a product that prematurely destroys batteries.

The Dead OBi200

After some thunderstorms came through, my OBi200 VoIP adapter stopped working.  The network end worked fine, but there was no longer any dial tone, and the device status page for the PHONE port no longer showed any information.

I only had the device for 11 months, so I promptly contacted support.  Since it was probably broken due to a surge/overstress event on the phone line, I decided to open it up and take a look.  I wanted to see if there was anything obviously broken that I could just replace and bring it back online, as well as I was curious what parts they had used in the design (and if there actually was any protection on the ports).

OBi200 Board

As expected, there really isn’t much inside.  There are three primary ICs:

  • Marvell MCU which provides the Ethernet interface, system control, config pages, etc.
  • RAM for the Marvell MCU
  • A Silicon Labs Si32260-FM1 ProSLIC telephone interface IC

The rest of the board is power supplies and a few components required by the primary ICs.  For what it’s worth there does appear to be an ESD protection IC on the USB port, but that’s good general practice for USB anyways.

I couldn’t find a full datasheet online for the Si32260-FM1.  The best I found was a couple-page datashort with a block diagram, pinout, and a summary of what the device does.  It essentially is everything necessary to provide a VoIP interface, including phone line voltage generation, DSP to encode and decode analog and FAX data, and a simple SPI interface for digital data transport (in this case to/from the MCU for transfer over Ethernet).

The SI3226x block diagram

Note that the block diagram shows two channels, but the OBi200 only provides one phone channel.  It appears that the second channel is connected and populated, so getting a second phone port is probably just a firmware change.

Unfortunately the datashort available didn’t have a reference circuit vendors usually provide, which more than likely is what Obihai used in this design.  From what I can tell (without a bunch of probing to determine actually connections) there appears to be a simple analog filter on the frontend made of 0805- and 0603-sized components.  The resistors appear to be either thick- or thin-film and the caps all MLCC.  A quick check with my DMM didn’t find anything that was obviously open, short, or different than a neighboring part with a matched circuit shape.

I did not see anything in the way of TVS diodes, spark gaps, or any other component that would provide significant protection from a high-voltage transient event, which is somewhat unfortunate.  Part of this is probably due to the small size, and the other due to there not being an actual ground lug anywhere on the product (the GND of the power port through a wall wart isn’t a true GND).

If I were to redesign this, knowing it’s probably going to connect to a set of phone lines that might be connected to a network of phone cable where lightning could possibly couple in, I would have probably added at least a couple TVS diodes and a GND lug.  Most customers probably wouldn’t connect the GND lug, but it’s better than nothing.

Fortunately, there are surge suppressors for phone lines available,  but of course it’s a separate product that needs to be purchased.  I decided to go with a Tripp-Lite DTEL2 suppressor, which connects between the OBi200 and the phone network in my home.

Tripp Lite DTEL2 Surge Suppressor

In the end, Obihai honored their 12-month warranty, and I sent the broken device back before doing any additional debugging.  I can only hope that adding an external suppressor will avoid another failure in the future.