4Chan, the Four-Channel Isolated Charger

In a previous episode of the quadrotor show, I mentioned modifying a single four-channel Hobbyking battery charger (this one) into four isolated single-channel chargers. Well, that operation has been done… so at least now I can charge the quadrotor in one shot. The process involves cutting some traces, a small amount of surface-smount desoldering, and getting four isolated power supplies for the resultant charger channels.

So this is what the inside of the thing looks like. It’s mostly three copy and paste operations, with a few differences to accommodate mounting holes and the fan connection. The main power bus is on the left side – positive on top, negative on the bottom.

The power semiconductors are all on the underside, heatsunk to the aluminum case with silicone pads. The board standoffs keep the switches pressed against the chassis.

These common power traces are the ones to cut. I used a Dremel with a cutting wheel to separate the trace into four sections between the inductors. The fans receive power from the very end of the power trace, so it is now associated with one channel (it will still turn on according to temperature, but pulls from the power supply for that channel).

After this, it’s time to get some power supplies.

I bought these cheap switching power adapters (supposed to be used for LCD monitors) from Ebay for something like $10 each. They’re about as shady as you can possibly get a power supply, I suppose. In fact, they were so cheap that I cracked one open to see if it was really isolated or not. I see an optocoupler and isolation transformer, so I hope that means yes.

The rating is 12v and 5 amps, which is about 10 more watts than the HK Quattro should be charging at (50 watts) so it was a good fit.

Each channel needs to be connected to a power supply. I just used the long wire the PSUs came with to complete this operation. Positive on top, negative on the bottom.

This is where I decided to get Extra Fancy and made a supplementary case to hold the whole thing. It was designed in around 3 hours and then cut out of clear acrylic plastic on a laser cutter. This step is of course not necessary, but it organizes the wiring alot. I dug an IEC power jack out of a pile of old power supplies and made a 4-way split cable with the line cords that the PSUs came with. No, it’s not grounded.

It’s also really hard to take pictures of clear acrylic.

The charger mounts on top of the power supply case using its existing four rubber leg things. After making sure it powered on, I reassembled the case and put the whole thing together.

Unfortunately, the first test proved to be inconclusive. While the channels were DC isolated, clearly there was still some kind of coupling going on – whenever one channel started charging, the rest of the channels would freak out and some times reset. If I started another channel charging while one was already running, all of them would freak out and terminate.  The LCDs would also some times all blank if one battery was plugged in.

Clearly I missed something, so I took the charger apart again for a close flashlight inspection.

Here’s the culprit. I noticed a capacitor connected between each channel and the frame at each mounting hole. This is most likely a noise decoupling capacitor, but together they connect all the channels to chassis ground.

This is no problem if all the channels operate at pure DC offsets since capacitors are DC blocking. However, my guess is that the AC component of the switching power supply outputs is being transmitted across the capacitors on each channel, which is wreaking noisy havoc everywhere.

These caps had to go. There is one per mounting hole – just follow the lead emanating from the copper ring and remove the first capacitor you run into. And only that one, as far as I can tell. This can be done with a big soldering iron and a solder blob, or small iron and rapid back-and-forth heating of the two pads of the capacitor. Or you can dremel it off or something.

The behavior after this was much better. For some reason, the bottom channel LCD still has a contrast issue when any others are plugged in and running. However, it does not seem to affect the way it operates. I tried testing the isolation by trying random combinations of chargers and battery halves, and am proud to say that they all do work together.

The maximum charge rate for the Quattro on 5S lithium polymer batteries seems to be about 2.8 amps maximum. This works out to 60 watts or so, not accounting for inefficiencies (it does get quite hot for a 60 watt max charger – much much hotter than my 1010B+ ever gets, period). The shitty monitor power supplies also have thermal issues when charging at full capacity, so I think they’re either overrated or just that horrible. They only let the charger go full rate if there’s a fan pointed at them.

With these thermal issues in mind, I might append or redesign the base to house a small PC case fan. But in the mean time, it works!