rhills wrote: ↑
Fri, 26 Oct 2018, 01:17
OK, I checked these but am confused by my readings so maybe I'm not doing it right. With the unit plugged in to 240VAC, I used the 500VAC setting on my multimeter (the other AC scale was 200V) and measured...
Eek! I'm glad to know that AEVA still has a webmaster. Sorry, I neglected to say some really important things, that to me are really obvious, so I forget that beginners need to know them.
Chargers that connect to the mains contain dangerous voltages, that could lead to death. It's highly advisable that most if not all measurements be performed with the power OFF. For about a minute after the power is turned off, there will still be dangerous potentials from energy stored in capacitors.
Semiconductors are best tested with the power off, using the diode range of the multimeter. I assume that all multimeters have a diode range these days. The multimeter provides a tiny current from its battery to the device under test, and measures the voltage that results. For a forward biased ordinary diode, this should be about 0.4 - 0.5 for power types (the larger the power, the lower the voltage, really high power devices might read down to 0.35 V). In the other direction (reverse the muiltimeter leads), it should read infinite, usually displayed as "OL" for "OverLoad".
Diodes have an anode and a cathode. To forward bias a diode, you need the more positive voltage on the anode, and the more negative on the cathode. Usually, the cathode is marked with a band near one end; these are obvious on your two legged diodes in the photo (cathodes are all towards the bottom of the photo). Sometimes, two diodes are packaged together in one part; that's the case for D13-D15 at the bottom of the photo. Unfortunately, there is no consistency with anode and cathode connections with these; you have to find the part number and look up the datasheet. It may not be easy to see the part number in this case. But if you find about 0.4 V in one direction and OL in the other, you can be pretty confident that it is OK.
Usually, you need to test both halves of a dual diode independently. With three leads, two of the diode connections have to be commoned (they are connect to each other and share the same lead).
In your case, at least D13 appears to have the outer two pins connected together with thick printed circuit board track. It's a moderately safe bet to assume that the others are the same. With three of these, it's likely that the whole lot are paralleled, so you have six diodes in parallel. This is to share the current between them all. It looks like this is more than a three amp charger. So just treat all those bottom diodes as one diode, and use the middle and either outer pin to test. You should still see about 0.4 V one direction, and OL the other direction.
coulomb wrote: ↑
Thu, 25 Oct 2018, 20:23
...the MOSFET or transistor that drives the "transformer" (likely to really be a multi-winding inductor, and the split in the core indicates this),...
I'm sorry, I'm not sure which component(s) you're referring to here. Is the "transformer" the large yellow-tape-covered cylinder in the black plastic frame in the middle of the PCB or something else? I couldn't see anything that I could identify as having a "split in the core".
Yes, the transformer is the large thing in the middle with the yellow tape. For whatever reason, it's nearly always yellow for inductors and transformers. The MOSFET or other transistor will be on the heatsink to the left of that transformer.
I meant to add a little about coils confounding diode readings. Coils of copper wire have nearly zero resistance. In operation, they have voltages induced into them, but it's not practical or safe to do this while measuring. So sometimes something will appear to be short circuited, when it fact it merely has a coil connected to it, and that circuit disturbs the reading. Usually, it's not practical to take the component out of circuit to test it. So you just have to use your experience (or mine, or others) to decide whether that part is going to be so affected, or not. Of course, other parts can also do this. The other common confounding components are the large capacitors. Where you expect to see OL across some diode or semiconductor junction, you might see a low voltage that slowly climbs and reaches OL in about 10 seconds. This is due to the large value (100 μF or greater) capacitor appearing to be a short circuit until it is charged up, where it becomes effectively open circuit. At the very low currents from a multimeter, it can take seconds for this change to occur. Again, experience will tell you if this is expected. As a rough guide, if you see this, treat it as normal unless and until other evidence says otherwise.
Also, it may not be obvious in the picture and may not be relevant, but these diodes have quite a bit of white-brown grunge around their feet - ? cooked flux ?
It's just flux. This is surprisingly common (to me at least) in chargers. I can't stand it and often clean it off with a cotton bud soaked in metho, but it's safe to leave it alone.
OVF? For Diodes D4-D7 I measured 0 VAC
By OVF, I meant overload, which on most multimeters will show as OL, as already mentioned. The fact that you don't see any AC voltage across those diodes means that they are not getting any power. Measuring AC across diodes gives you a rough indication of the voltage that they are blocking. If they aren't blocking anything, then they're not doing anything useful, either because they are short circuited, or because some upstream failure causes them to receive no power.
coulomb wrote: ↑
Thu, 25 Oct 2018, 20:23
Since it's so dead, I'd also check the Negative Temperature Coefficient resistor...
Well this one at least I got right (I believe). I measured 13 Ohm which suggests to me it's OK based on your description.
coulomb wrote: ↑
Thu, 25 Oct 2018, 20:23
It looks like there is a 20-pin microcontroller in a socket...
Well, that was very difficult to get at and in trying to prise it part-way out, I not only flipped it out altogether, but also bent several legs, two of them to 90°
. Fortunately I managed to straighten the legs without snapping any and replace the IC in its socket without bending/damaging the legs further. Needless to say I didn't try reseating it again
Presumably, that didn't make it come back to life, or you would have reported that. Good to know that it seems to be OK.
FWIW, while there are no lights, something clicks when I power the charger up.
That's an indication that power is getting at least part way through the path from mains to battery. I see no evidence of a relay. My guess is that the fan receives power briefly. It might not get it long enough to turn the blades noticeably, unless you are looking very carefully at it. The other possibility is the film capacitors, like the grey one near the connector where the mains lead comes in. These can often be acoustically active, and it's not clear to me whether this indicates failure or not. If you're interested, you could skim "The mystery of the faint crackling sound solved
", where I describe Weber and I chasing this faint sound in another charger (an EV charger in this case). It could also be something arcing over. IF you can safely set it up without too much effort such that you can see under the board (it might even be above the board), you might
be able to spot a tiny spark in a totally darkened room that goes along with the sound. This is obviously not good, and could be a root cause. Clearing away any accumulated dust (again using a cotton bud soaked in metholated spirits, many minutes after power is removed) might cure the problem. But more likely, it will have cause some part to fail.
You might be able to use Weber's trick, mentioned in the linked post about the mystery sound above. That is to use a large drinking straw or similar to act as a crude stethoscope, so that you can be more selective about where you are listening for the sound. Any sort of non conducting tube will work, perhaps a short piece of conduit.
Good luck with it, and please proceed with more caution. I note that slipped multimeter probes is a common cause of problems; I recently reprogrammed a processor from a car that lost several components, including the processor, due to slipped multimeter probes. With large through-hole components like this, it's less of a problem, but still worth avoiding.
[ Edit: fixed missing quotes; added a few paragraphs about identifying the transformer, and coils and capacitors affecting measurements. ]