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Electronic Load (trelload)

Summary

This device is a simple do-it-yourself electronic load. For example it can be used to test power supplies.

Requirements

The following lab devices are necessary in addition to trelload:

For static tests you need a lab power supply and two multi-meters
For dynamic tests you need in addition an oscilloscope and a pulse- or function-generator. I am using my simple square wave generator
To measure the power efficiency of a power supply, you need a power-meter for the mains 230 V or 110 V side.

Functional Description

The trelload electronic load device has the following features:

The device sinks a constant current. This current is controlled by an external voltage. The voltage can be supplied by a lab power supply for example
Using a function generator as the external voltage source, the load current can be changed dynamically. In particular, using square waves, sudden load changes can be simulated
The device can consume power of up to 50 W, albeit only over a limited time period.

Design

The current sink is implemented as a classical voltage controlled current source using a bipolar transistor with emitter resistor. The transistor's base is the control voltage input
The current source is fed directly by the device under test (DUT) via the transistor's collector
Transistor and emitter resistors are mounted on heat sinks to dissipate power. The transistor is electrically isolated from the heat sink to keep stray capacitance low

Schematic

Electronic Load Schematics

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 to connect an adjustable control voltage to the electronic load, for example from a lab power supply. The current to be delivered is in the order of a few 10s of mA.

Before you start, you need to choose an appropriate emitter resistor of the electronic load (R2 - R5 in the schematic). The emitter resistor shall match the maximal output current of the DUT. Example:

The DUT be a 12 V switching power supply which provides a current of 2 A maximum
At maximum current, the emitter resistor should cover approximately half of the voltage, the 2N3055 transistor the other half. Hence R_e = 6 V / 2 A = 3 Ω
You choose the next bigger resistor, i.e. 4 Ω. This will yield a voltage of 8 V over the resistor and 4 V over the 2N3055 transistor
The 2N3055 I have available, has a beta of around 80. Hence the transistor base current at an emitter current of 2 A is around 25 mA. Your lab power supply will hence need to deliver about 8 V + 0.7 V + 56 Ω × 0.025 A = 10.1 V to achieve a maximum load current of 2 A
- 8 V shall be over the emitter resistor
- 0.7 V is the typical transistor base-emitter voltage
- 1.4 V are "lost" over the base protection resistor R1

Caveat: 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 R1 means to limit the base current to protect the transistor.

Be aware that the DUT output voltage may sharply decrease at high load currents due to built in over-current protection circuitry. 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. In the specific example above, you could choose 2 Ω. These could be achieved by connecting two 4 Ω resistors in parallel.

How to measure Load Regulation

You could measure the dependency of the DUT output voltage from the load current using simple resistors. But you needed different resistors for that. These resistors also need to stand the power to dissipate. Instead, using the proposed electronic load you can easily adjust the load current and measure the resulting DUT output voltage.

How to Connect the Various Devices

The following schematic shows how to connect the DUT, the trelload electronic load and the other test equipment:

Connection Principle Schematics

Voltage source V1 is the DUT, for example a switching power supply
Voltage source V2 is a lab power supply. With its output voltage you adjust the current that the electronic load sinks
Volt meter E1 is a multi-meter that is measuring the DUT output voltage
Volt meter E2 is a multi-meter that is used to indirectly measure the current the electronic load sinks: I_Load = V_E2/R2. This simple calculation neglects the base current into the transistor that also flows through the emitter.

Measurement Process

You would start with a lab supply voltage V2 of 0 V and increase it step by step, while you record the two voltage readings E1 and E2 from the two multi-meters.

As stated above, you can calculate the load current from voltage V_E2. Hence you can record the load current together with the DUT output voltage V_E1 in a table. From the results you can calculate the load regulation.

Measuring the DUT Efficiency

In important attribute of a power supply is its efficiency, i.e. how much power is drawn from the 230 V or 110 V mains (primary) side for a specific load on the output (secondary) side. The test set-up above can easily expanded to measure efficiency.

How to Connect the Various Devices

The following schematic shows the power meter that is added on the mains side of the DUT:

Schematics with Power Meter Added

Measurement Process

You measure like above, but in addition you record for each load current the power requirement on the primary (110 V or 230 V mains) side of the DUT. The power delivered on the output side of the DUT you calculate by multiplying the output voltage with the load current. The efficiency can then be calculated by dividing the delivered power by the primary power. Most often you will find that the efficiency depends on the load current.

How to Measure Output Ripple

When you measure the output voltage using an oscilloscope, you can determine the output voltage ripple. The ripple can also depend from the load current. The Y-input of the oscilloscope has be be switched to AC-coupling.

Usually, the following cases are relevant:

The output voltage of classical linear power supply is often modulated by a small 100 Hz ripple voltage, which sometimes increases with increasing load current
The output voltage of switching power supplies is by design modulated by a ripple voltage at the switching frequency. This ripple is often dependent from the load current. Sometimes the switching frequency itself also depends from the load current
Switching power supplies sometimes show switching "transients". These are short, fast pulses at the switching frequency, but with strong higher harmonics which could hamper connected electronic devices
Some switching power supplies exhibit a specific gap-type operation at some load situation. This results in ripple with lower frequency than the switching frequency.

How to Connect the Various Devices

Schematics with Oscilloscope Added

Measurement Process

You measure and records the output voltage using the oscilloscope, while you increase the load current step-by-step.

I had various DUTs, where ‑ surprisingly ‑ the ripple was especially high for intermediate loads, much below their maximum load current capability.

Dynamic Measurements

The static measurements described above yield the DUT's behaviour for a constant or slowly varying current. Using the electronic load, you can also measure the dynamic behaviour of the DUT for fast changes of the load current.

How to Connect the Various Devices

Instead to a lab power supply, you connect the control input of the electronic load to a pulse or function generator. The generator shall have an output impedance low enough

to deliver the necessary base current into the transistor
to change/discharge the parasitic capacitance of the transistor fast enough

Most generators have 50 Ω output impedance. That will do.

Schematics with Pulse Generator Added

Measurement Process

To measure a fast load current change from zero load current to specific load current, you set the pulse generator to a rectangular wave form output and set the amplitude to jump to a voltage that corresponds to the wanted specific current. If the pulse generator as an adjustable offset voltage, you can also observe DUT behaviour for switching between two defined load currents.

The dynamic test will reveal

how well the DUT will keep the output voltage constant, e.g. whether there is any voltage over-shot or under-shot and how large the peak pulse voltage of such transients are
how fast the DUT reacts to the load change.

Mechanics

I mounted the trelload parts on a piece of scrap wood. Both the transistor and the resistors are mounted on heat sinks. However the set-up is not suitable for continous operations.

I used insulated copper wires that go to posts which fix the wrires using screws. The DUT is connected using a socket for a "hollow" barrel plug, because many power supplies use these. For the control input I use a BNC-Connector, because my pulse generator can also generate fixed voltages. This is convenient, because I do not need a separate lab power supply for the static tests and I usually do the dynamic test anyway.

The multi-meters and oscilloscope I connect via "crocodile" clamps and a standard probe.

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. By using a short patch cable, I can also wire two 4 Ω resistors in parallel to get 2 Ω.

View on device

Legal Notices

Copyright

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.

Disclaimer

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.