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The bench electronic load is a type of equipment defined as "An electronic load designed to sink current and absorb power out of a power source. If a power supply is used to power a device, an electronic load is used to test the power supply by emulating the device under test (DUT)."
What I am confused about is why can't we just use a potentiometer or resistor as a load. Why do we need this very complex and expensive piece of equipment?
Have you looked at the prices of high-power resistors and rheostats (variable high-power resistors)? :) That's just a bit tongue in cheek, but those are not necessarily cheap - especially not if you want to mount them on a heatsink.
Electronic loads don't need to be either complex or expensive. A basic one, usable for up to 100W or so can be cobbled together from parts found inside a junker PC, quite literally. CPU heatsink + fan to dissipate the heat, mosfets from the ATX power supply and/or onboard supplies as series pass elements, and a couple cheap op-amps. Of course there's also the knowledge of how to put it all together. But if you had a bunch of junk desktop PCs, you could probably make one from nothing but parts scavenged from those PCs, quite literally.
Of course that would be a relatively nice adjustable load. If you only need a "dumb resistor" then of course you can use resistors - they just get expensive to implement, it's somewhat cheaper to use junk parts if you got them.
There are YouTube videos where you can see an electronic load being built start-to-finish. Some of those present quite reasonably performing designs built for very cheap.
If your time is free, then you can get quite a bit of lab gear made the DIY way, and it will be much "cheaper" in material cost than off-the-shelf devices. For most people professionally into electronics, though, their time is anything but free, and they may have different hobby projects in mind than what they need on the bench at work.
You can certainly use almost anything as a load, though using resistive loads helps avoid issues with non-linear loads.
There are high-power potentiometers/rheostats which can dissipate as much as 500W of power, in other words they can present up to 500W of power load.
If your needs are simple, there is no reason going with the more complex and more expensive digital loads.
A bench electronic load allows many more settings and a higher precision, covers a wider range and type of loads, and you can also set a load current or load voltage vs. only the load resistance with a resistor/potentiometer/rheostat.
If your specific requirements can be met with just a resistor, a bank of resistors or a potentiometer/rheostat, you would be perfectly fine going that route.
I sometimes use incandescent bulbs, hot plates or heater elements as loads, as they fit my test needs.
Back when I worked at MAXIM (Integrated Products) and we needed a few simple loads for testing (we had large, digital HP loads for all of our critical, detailed tests, but we needed a couple simple and cheap loads for more generic testing), the engineer gave me a schematic for a simple adjustable load using an op-amp and a MOSFET. The op-amp was of a very low quiescent power and a MOSFET in general needs almost no control current if used as a variable resistance. We used a 9V battery block as a supply for this circuit, which could last for months, if not longer.
Below is not the exact schematic, but what I remember of the basic circuit. Your dissipation power is limited by the maximum power dissipation of the MOSFET, plus the current sense resistor needs to dissipate significant power (it can also be changed according to your needs).
simulate this circuit – Schematic created using CircuitLab
Using a potentiometer at a very low portion of it's resistance is one problem.
At such a low resistance, say 5% of the full potentiometer resistance, you would only be able to dissipate 5% of the full rated power of the potentiometer.
If you are testing a power supply which involves DC-DC conversion then the voltage regulation in response to transients in the load current might be important to test. This is a case where an active electronic load allows improved testing compared to a fixed or manually adjusted rheostat.
Reference Designs: TIDM-DC-DC-BUCK C2000™ Digital Power BoosterPack™ is an example board which contains:
In the above linked design guide their are examples of plotting the output voltage in response to a load transient from the active load under different control-loop conditions for the software controlled DC-DC buck regulator. E.g.:
