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Electronic Load


This device is a DIY electronic load. I use it mostly to test power supplies together with my simple square wave generator.

Functional Description

I designed for the following features:



Electronic Load schematic


Static Measurements

In a static measurement, you demand from your device under test (DUT) to deliver a constant current. The use the electronic load in this scenario, you need in addition an adjustable voltage source, for example from a lab power supply.

Before you start, you need to choose an appropriate emitter resistor of the electronic load. Example:

The driving voltage always needs to be below the DUT output voltage. Otherwise the base-collector diode of the transistor will conduct. The base resistor means to limit the base current to protect the 2N3055. Be aware that the DUT output voltage may sharply decrease at high load currents due to built in overcurrent protection. In that region, you might want to chose a lower emitter resistor, to make sure the driving voltage always stays below the DUT output voltage.

You would start with a lab supply voltage of 0 V and monitor the voltage over the emitter resistor with a multimeter. This allows you to monitor and measure the load current.

At the same time, you monitor the DUT output voltage. By slowly increasing the lab supply voltage ‑ and hence increasing the load current ‑ you monitor the DUT voltage and can then calculate the DUT load regulation.

For a switching supply DUT, you might also want to monitor the DUT output ripple voltage for various load currents using an oscilloscope. Both the ripple amplitude and the ripple frequency might depend on the load current. I had various DUTs, where the ripple was especially high for intermediate loads, much below their maximum load current capability.

The same time you could also measure the power consumption on the primary (110 V or 230 V mains) side of the DUT. By measuring the load current and multiplying with the output voltage, you get the delivered power. Dividing the delivered power by the primary power, you get the efficiency of the DUT as a function of the load current.

Dynamic Measurements

The static measurements outlined above yield the load regulation of the DUT for slowly varying current. You can also measure the dynamic load regulation by driving the electronic load from a square wave generator. This can establish how the DUT output voltage behaves when the load current changes rapidly, especially whether there are any transients like undershot or overshot, which could be harmful to an electronic device powered by the DUT.


I mounted the parts on a piece of scrap wood. I took care to make sure both the transistor and the resistors have big enough heat sinks and that the heat sinks are firmly mounted and are isolated from the devices.

Mounting of the connectors is provisional. I fitted some silver coated ground wires to connect multi-meters and oscilloscope probe ground wires.

I did not implement a rotary switch as indicated on the schematic. Rather I simply use a stranded wire ending in a "crocodile" clamp to select the appropriate resistor. This is simple and also allows to use the resistors for other purposes, like using them as a resistive load for audio amplifiers.

View on device

Legal Notices


Copyright (C) 2020, by Dr. Thomas Redelberger, All rights reserved.

I do not intend to offer/sell this!

I publish here the electrical schematics and the mechanical design, such that you could build your own device. I put all the files of this project under the GPL2 open-source license. See the file COPYING.TXT. This is hence open hardware.


Actual construction of the circuit requires a significant knowledge of electronic circuit design and construction beyond what is presented here. The author, Dr. Thomas Redelberger, does not warrant operability, reliability, suitability or safety of any of the circuits. Anyone who constructs and/or uses these circuits accepts all responsibility for their operation and safety.

Last change: 2020-04-12
© 2002-2020 Dr. Thomas Redelberger