DC Electronic Loads

An electronic load to test DC power supply devices up to 100W.

What do you use when you have to test a new power supply that you just built, or one that you bought and want to know if the declared specs are true?

One thing you’ll need is a passive load that you attach to the power supply output to drain a certain amount of current, both to verify that the power supply is capable of providing that amount of current, and to verify the amount of ripple that was not filtered away by the power supply itself.

rheostatA classic method for doing so is to use a rheostat, which is essentially a potentiometer capable of dissipating the amount of power produced by the power supply. The resistance of the rheostat can be changed and therefore different amount of currents can be used to test the power supply. However, rheostats are big, heavy and cumbersome.

An alternative to rheostats is to have a so-called Electronic Load. These are electronic circuits that are capable to emulate the functionality of a rheostat.

I’m proposing here two simple versions of an Electronic Load, functioning in DC, and capable of dissipating up to 100W, all of this in a very condensed space, and very light in weight.

The first version is a very simplistic one.

100W_el_load_v1

It uses a cascade of three transistors, in a configuration called Darlington. This configuration is effectively equivalent to a single transistor with a gain (hfe) that is the product of the gain of all the transistors in the configuration. This allows for a very little control current flowing into the potentiometer used to regulate the base current, and for a high current available between the collector and the emitter of the transistor Q3, in the above schematics.

This circuit does not need its own power supply, since it gets what it needs directly from the power supply under test. The resistor R1 is calculated based on the highest voltage that the circuit will be able to handle, in this case 50V. It is important to note, however, that using this load at lower voltages will prevent the possibility to use the full excursion of the potentiometer, thus limiting the sensitivity of the circuit for the control of the current.

The next circuit eliminates the sensitivity problem by providing it own power supply to control the base current of the Darlington, thus eliminating the dependency from the external power supply voltage.

100W_el_load_v2

In fact, in this case, the base current can be adjusted with the potentiometer RV1, which is polarized through the resistor R1 and the trimpot TR1, which can be adjusted to maximize the useful range of motion of the potentiometer, regardless of the voltage applied at the input terminals. This way the electronic load can have the same sensitivity for any input voltage. A digital Volt/Am-meter, powered by the same internal battery, provides a visualization of the voltage of the system under test and the current being drained from it.

I will soon publish a video on my YouTube channel that shows the Electronic Load I built for myself. Please watch for that video to come out. And, in the mean time, can you think which one of the above schematics I used? Did I use the simple one, because less expensive? O did I sacrifice a few extra bucks to gain more sensitivity on the regulation of the load?

I strongly suggest to subscribe for free to my YouTube channel, and also to click on the bell icon that appears after the subscription is done. This will allow you to automatically receive an e-mail whenever I publish a new video. This way you won’t have to go periodically to my channel to check for new posts.

Happy Experiments!

Lab Setup

Having a hobby like electronics generates a number of problems, like finding the space where to run the experiments and build your own creations, store all the measurements equipment and tools, and store all the needed electronic components. This has to be done with respect to the necessities of the rest of the family while, at the same time, provide a safe haven where to leave all the equipment and components in use for an experiment in an area where nobody will touch anything, especially if there are young kids around.

My personal choice was to setup a lab area in the basement of my house. Being that a finished basement, I didn’t have much space to fit the lab in, so I had to go with what I had available: a little unfinished corner room.

I built in there a workbench that occupies an entire wall of the space, and put a huge pegboard on its back to hold the bulk of my tools. A shelf built on top of the workbench would hold some of the measurements equipment, while the area underneath it would hold the remaining of the instruments. Other shelves would be mounted anywhere there is a place for them while the wall opposite to the workbench would hold the drawers cabinet with all the small components. For lack of space, I had to place that cabinet right behind the door that gives access to the lab, thus preventing the door opening completely. On that same wall, I built an enclosure for my self built 3D printer, and a bigger shelf system takes the remaining space of the tiny room.

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A few small cabinets mounted on casters crowd the floor of the room, while a small white board hangs on the remaining wall. I use the board to do quick draws of circuits and quick calculations, or to hang paper drawings with small magnets.

This is the space that I’ve been using since a while now, and the location from which I plan to record a number of vlogs to show what I do and how I do it, in the hope of generating some interest and create that spark that will allow other people to enter in this rewarding kind of hobby. There is nothing more satisfying then seeing your creations take shape and function, through the amazement of your family and your friends.

‘Till the next time…

(Watch the video on YouTube: https://www.youtube.com/watch?v=JUxwRB_MJ4E)