Upgrading the EBC-A10H Electronic Load, Part One

I’d posted two reviews of SANMIM power supplies and was about to start on a third when I remembered that my main electronic load can only handle 60 W, while the power supply is rated for at least 70 W. So I decided to make a few improvements to the load while I was at it.


You may remember my recent review of EBC-A10H. It’s a decent load, if not for one “but”: it has a switching power supply. As it turned out, that supply can generate quite a bit of “noise.” This doesn’t matter at all when testing batteries, but it’s no good when using the load to measure ripple. That’s exactly why I modified my old EBC-A10.


There’s quite a long list of improvements I’d like to make:
1. Replace the power supply with a 50 Hz transformer.
2. Replace the power transistors.
3. Add an Ethernet-UART module.
4. Add galvanic isolation to the UART for direct control from a computer.
5. Add the option to switch the charger’s supply voltage between 12/24 V.
6. Add a connector for a battery holder.
7. Replace the power terminals.
8. Possibly redesign the front panel.
9. Add a fuse to the load’s power input circuit.

But I was eager to test the new power supply, so I decided to split the modifications across several reviews and start by replacing the switching power supply with a conventional transformer.

Of course, a transformer is heavy, less efficient, and so on. But the built-in charger is itself a regulator, so this solution isn’t all that much less efficient than a switching power supply. More importantly, there’ll be no interference from a high-frequency inverter!
For this modification, I bought a 2 × 15 V, 3 A transformer on OLX.


One of the wires came with a nice bonus: a fuse.


I bought two transformers. I’d already used one of them earlier; the only difference between them is the voltage and current: one is 12 V, 3.5 A, and the other is 15 V, 3 A.
Actually, there’s one more difference: the 15 V version has a fuse. I could have used one during the previous modification, since the A10H has its own fuse, so now I have two :)


Transformer dimensions. They’re not that critical here, since the enclosure is considerably larger—and, more importantly, taller.


With no load, the voltage is 16.5 V across each half-winding and 33 V overall. The no-load current is 60 mA, which is rather high for a transformer of this power. Unfortunately, that’s a common issue with transformers like this. Measurements were taken with a mains voltage of 222 V.


With no load, the transformer heats up to about 45 °C after an hour.


In my previous review, I wrote that using a transformer like this with a full-wave rectifier isn’t quite right, since each output winding supplies current to the load for only 50% of the time. So this time I decided to modify the configuration.


The modification is very simple: in this case, you need to change configuration C to D. It feels a bit sacrilegious, since this transformer would be perfect for an audio amplifier. But maybe I’ll order another one later, this time to use it for its intended purpose :).


1. Remove some of the tape. The winding ends are also secured with varnish.
2. Separate the windings at the point where they’re joined to the black wire.
3. Move the wire we don’t need out of the way, check the windings with a multimeter, and route the ends to the connection points with the yellow wires. While I was at it, I checked the wire. I can’t say for certain that it’s copper, but it feels very much like it. Also, after stripping it, the wire takes solder very well using only the flux in the solder.
4. Solder the wires. I routed the black wire back to its original spot so it wouldn’t get in the way—and in case I ever want to reverse the modification.


I carefully cover the joint and wrap it with tape. Ideally, a 2 × 9 V or 2 × 8 V transformer would be better suited to this load, but with winding currents of around 6–7 A. Then it would be easy to switch the supply voltage.


With an input voltage of 233 V, the no-load output is 17.4 V.


But the transformer needs at least a rectifier. I couldn’t be bothered to etch a simple circuit board, so I bought some small prototyping boards at our local “spaceport.” In the photo, they’re shown in order of increasing size: 20×80, 30×70, and 50×70 mm. I got three of the first size, two of the second, and one large one.


Along with the power rectifier, I decided to add a 5 V regulator to power the Ethernet module.
Almost all the components are used and came from my stash. The diode assemblies and regulator are from some ATX power supplies, the heatsink was just lying around at home, and the capacitor has been sitting around for ages. It’s an old Soviet-made K50-35, 4700 µF, 25 V, and is perfectly suitable for this job.


The result is a fairly neat little board. I even managed to fit a fuse ahead of the bridge rectifier and a power indicator LED, but there were a few snags. It turned out that you shouldn’t enlarge the holes near the edges of the boards too much: when I tried drilling them out to 2.6 mm and tapping them for M3, the PCB simply broke. I had to drill and tap the component holes instead.


The whole assembly, put together.


Now I have another “people’s” power supply, and this one has a “new” circuit, meaning it’s different from the power supplies I had before. Maybe I should test it and put together a short review?


I tried arranging the parts in different ways, but the only workable option was to position the transformer and board as shown in the photo, even though I’d originally planned to rotate the transformer by 90 degrees.


I installed and connected everything, using the original mounting standoffs too. I did have to add terminals with insulators: I wanted to connect the transformer neatly to the mains connector, and my heat-shrink tubing had gone missing somewhere, so I didn’t solder it.
I was drilling at night and trying to keep the noise down. Somehow I managed to break a drill bit, and in the process I also tore off the charger’s inductor, so it’s now sitting a little crooked. I might replace it altogether later.


The first stage of the modification is complete. With no load, the board outputs almost 23 V, which is more than enough for my needs, at least for now.


I’ve already tested the load in operation. The transformer does get hot, sure, but the load turns on its fan whenever the charger is running and the power is above 5 W anyway, so it’s nothing to worry about. The important thing is that the power supply interference problem is solved, and I’m very happy about that.

That’s all, thanks for reading :)