An electronic load is a measuring device to load power supplies. I wanted to build one to test my selfmade power supplies, but the schematics I found on the web were all way too complicated. Also, all the loads were powered themselves and I wanted a purely passive standalone device. Since I couldn’t find anything that fit, I designed my own one.

As resistive load I use two high power bipolar transistors which are driven analog. I also used a high power rectifier so that i can put load on AC.

In principle this is a finished load. Two potentiometers on the casing are base resistors for the transistors.

As rectifier I found one for 25A and which could be screwed directly onto a heatsink. As transistors I used two BD249C, which are bipolar npn transistors for up to 25A and 125W each. So I can put a power up to 250W on them. The rectifier itself is only rated for 100W but he doesn’t need much since his voltage drop is only about 2V (two diodes in series).

Everything should fit into a casing and I had a bigger BOPLA casing laying around, but fr the heatsinks to be effective, they had to be mounted on the outside. I put them on top for more stability.

But you not only want to put a resistive load on a power supply, but also a capacitive or inductive load, That’s why I added two capacitors (47µF and 100µF) as well as two inductors (63µF and 130µF, both made by myself) which can be added through switches on the front. The inductors both have a 50Ohm resistor in series to limit the current. They can be added individually or in series (with 50Ohm series resistance, no matter the configuration). The capacitors can be added alone or in parallel with a parallel resistance of 50Ohm each.

After adding the main switch for the resistive load, i installed a current meter. But since I didn’t want to add a power source for measuring devices I decided it is time for a ammeter. I found one laying around, but the scale only goes up to 5A… Enough for now.

But the design was not flawless. The rectifier only should rectify the voltage for resistive load, I however also wired up capacitors and inductors to the then DC voltage… That is disabling their reactive power rendering it to just a filter. Ideally the transistors should be bidirectional too, but that is near-impossible 😉

Again, schematic and finished product for you to look at: