Finlandia Interface (Efore) electronic load CL 3050B

Any time I've needed to sink substantial current, be it from a battery or a power supply under test, I've had to improvise from car headlight bulbs or power resistors I have in shelf. I never got around to making a proper DC dummy load (though I'd already sourced a suitable el-cheapo volt/amp meter for the purpose), but then I found a vintage one going to garbage at the local Hacklab. There was a pile of them, in fact. I plugged one in, verified the voltage and current displays came to life, and brought it home. Perfect timing for testing an amateur rocket flight computer's buck converter I had in the works! Wow, now I don't need to have an incandescent bulb blinding me while I check out a new power supply!  :) It's a very nice and simple desktop unit manufactured by Finlandia Interface (which changed its name to Efore Oy in 1996). The front panel labels it as "Electronic CL model 3050B, and there's absolutely no information to be found on it online! No specs, no manual, no schematics, nothing! But it's simple enough—the front panel has red and black binding posts (also labeled "+" and "−", so probably polarity needs to be honored), two displays labeled "V" and "A", a "Mode" switch that toggles between "Auto" and "Manual", a single multi-turn potentiometer, a "Power limit" LED, and an I/V curve diagram showing its 400 W maximum safe operating area. The diagram's axes are labeled up to 70 V and to 60 A (yikes!), but no other mention is made of absolute maximum ratings, except in power.

Labeled in some mystery language

On the back of the unit, everything is labeled in Finnish! The markings state this is "Elektroninen keinokuorma malli CL 3050 B", i.e. that it's an electronic dummy load, model CL 3050 B (slightly contrary to the front panel, but no matter). Interestingly, there's a space for a serial number, but nothing stamped on it—could this be a pre-production or in-house model that never made it to market??? That might explain why there's no information on it anywhere. There's your typical IEC power input connector (input power specified 50 W)—and an output connector as well—and the main power switch. There's a diagram, annotated in Finnish, specifying that a maximum of five units may be daisy chained through the power connectors. There's also a prominent multi-pin connector (looks like DIN 41612, with 2×32 pins, center row unpopulated), not labeled in any way. (And there's also a warning—again in Finnish—that the heat sink, rather easily accessible by prying finger, is live. I don't know whether it means it's mains live, or only DC input live. No matter, just don't touch.)

But why should this kind of devices be designed to be daisy-chained in their power supplies? I can only posit that these units are designed to handle even higher currents by connecting several in parallel through their input binding posts. So you switch on the "master" unit, and up to four "slave" units come up as well. Each unit sinking a maximum of 400 W, that makes a total of 2 kW. I can see that DIN 41612 connector daisy-chained to the other units with a flat multi-conductor cable with five IDC connectors. I can also imagine the master unit set to "Manual" mode and the slaves to "Auto", allowing one to control how much current is sunk by the others. Alternatively, that connector might be some truly archaic computer control connector (GPIB this interface is not). Or, perhaps, both—if a "master" unit can set the current to be sunk by a "slave" unit, so could a computer, perhaps through a separate interface module.

Anyway, I only use the one unit in its "Manual" mode. With a lab power supply, I quickly determined that this unit slurps up constant current, which is set by the multi-turn pot. I repeat, constant current—it is not a high-power, programmable resistor, whose current consumption would respond to the input voltage. But perfect for load-testing that SMPS. The input binding posts are isolated from the chassis and mains ground / protective earth, so it should not cause any untoward weirdness when attaching other instrumentation such as oscilloscope probes.

Here's the unit drawing just shy of 2 A from the ugliest buck converter you're ever going to see. The IC is a TPS54202DDC, which is in a SOT-23 package—hard to prototype on solderless breadboard. So I whipped up a proto on an adapter PCB intended for SOP14 devices—wrong pitch, wrong everything, but close enough for this small number of legs. Good enough to check that it works, and approximately reasonable for thermal testing (i.e. is anything overheating) with my UTi260B thermal camera. And yes, at that current, the tiny IC did overheat, and rather quickly at that, but went cleanly into thermal shutdown. With this adjustable load, and the thermal camera, it was fast work to find that at around 1.1 A the IC hits 100°C or just above, but continues to operate normally. That's already more than the current I will need in my application, and once I build the same circuit on a proper PCB, with the proper thermal considerations (i.e. big copper pours on the recommended pins), I expect the chip to keep its cool much better still.

But no, honestly, yikes! This thing can draw some seriously substantial current! I mean, my amateur radio station's Diamond GZV-4000 power supply is rated for 40 amps continuous, but my measly 100 W radio doesn't consume anything near that even at peak!!! Well, this dummy load and that power supply are now well and truly tested! Note that at the typical 13.8 V of an amateur radio station, 40 A would exceed the maximum power rating of the load, which is why I tested at a lower voltage. Needlessly low, actually, but whatever. It's the raw amps that worried me more, not the absolute power being sunk. The latter is just a matter of heat sink, which I believe an instrument of this era didn't care to skimp on... But I'm not certain that even the banana plugs I used are rated for 40 A!

I only wish the main power switch were on the front panel. I guess that's easy enough to modify while I'm digging around inside the instrument—yes, I'll definitely be exploring in there, trying to find out more about it! Oh, and if you happen to know anything about this device, especially if you have actual schematics or such, do get in touch!  :)