Adjustable Resistive Load for 5 VDC 1000 mA Applications

2024-08-15 | By PTSolns

License: Attribution Resistors Test Equipment

INTRODUCTION

This project outlines an adjustable resistive load (ARL). The ARL consists of 20 resistors and a potentiometer to yield a desired current draw of up to 1000 mA (see NOTE 1 below). Although it is developed with 5 VDC applications in mind, it can be used for 3.3 V and above 5 VDC voltage applications provided the various power, current, and voltage ratings of each part or section are not violated. In particular, this tool was developed for the application of stress-testing power management circuitry commonly found on microcontroller development boards. Typically, the maximum allowable current draw is much below 1000 mA, and therefore this tool can be used appropriately.

DESIGN PHILOSOPHY

The design philosophy of the ARL is that it should be able to operate in a range of ratings and be made with predominantly commonly found parts (resistors, connectors, wires, etc.). The purpose of this is to make this project accessible to the most users. The ARL should be safe to operate (doesn't get too hot, doesn't give the user a shock) as well as convenient and easy to adjust the current draw.

With these considerations, the first step in designing the ARL is to determine what current range to operate in, and how many parallel resistive branches there should be. It was decided that 1000 mA is sufficient for testing of power management circuitry found onboard most microcontroller development boards. Next, it was decided that 50 mA should flow through any resistive branch. Intervals of 50 mA seem to be a good range for many tests. Therefore, we need 20 resistive branches, each containing a 5 VDC / 50 mA = 100 Ohm resistor. We also wanted to make one of the branches (the 20th) to be able to fine-tune the current draw. So, we added a 10 kOhm potentiometer in series with the 100 Ohm resistor. The 20th branch can provide a current draw between 0 mA to 50 mA.

Each branch needs to have an ON/OFF switch. The Proto-Half prototyping board conveniently fits 20 2-position slide DIP switches (see Parts list below). The ratings of these switches are 100 mA at 5 V, and so these are appropriate to use in the ARL tool.

The Proto-Half gives the user two options to provide power to the board via a 2-pin 2.54mm/0.1in screw terminal connector or a 2.1 mm/5.5 mm female jack. Both of the appropriate components are installed. However, they are not listed in the parts list below as they are quite common, and many different types will be applicable here.

To prevent the resistor from heating up too much at 50 mA current draw, we decided to use bulky 1 W resistors. At 5 VDC and 50 mA current, the power in these resistors is only 1/4 W, much within the specs and low enough that 1 W resistors will not burn out. All other parts on the ARL have ratings that allow 1000 mA current draw at 5 VDC. The voltage ratings can even be a bit higher as long as the current ratings are not exceeded. Careful consideration of the ARL for above 5 VDC applications is therefore recommended by the user.

The ARL schematic is shown below.

Adjustable Resistive Load Schmatic

Some points to consider:

Only two pins of the potentiometer are used, with the third only soldered to the PCB for mechanical stability. Therefore, the pot is used not as a voltage divider, but only as a variable resistor.
A main ON/OFF switch is added. However, when assembling the tool, it was moved to the negative branch, which in this case makes no difference.

The final product image is shown below.

Adjustable Resistive Load Cover

Note that the bulky 1 W resistors needed to be staggered so that 20 of them would fit along the full length of the Proto-Half. Also, note that the main ON/OFF switch is added to the negative rail simply due to space restrictions on the positive rail. Finally, a 2-pin jumper cap was installed on the positive rail ... well for no real reason other than to have another ON/OFF mechanism.

3D PRINTED BASE

As can be seen in the cover image, the ARL is mounted to a white 3D-printed base. The idea of the base is to reduce the chance that the electrical contacts on the back or bottom of the ARL do not get shorted out accidentally. The base is shown below.

Proto Half Base

The four posts have a diameter opening such that many M3 brass inserts can be installed inside. This is what is done for this project, and M3 screws are used to mount the ARL and the base together.

The 3D models for the Proto-Half Base, as well as the Proto-Full Base can be found on our sub-domain under the 3D Models section.

IMPROVEMENTS

The following is a non-exhaustive list of improvements one might make to the ARL presented in this project.

For some applications, it might be useful to have reverse polarity protection. This can be done by simply adding a diode on the input terminals.
An ON/OFF LED for the main board, as well as for each branch may be useful to visually indicate to the user that the current is being drawn and the device is ON.
Use a smaller potentiometer than 10 kOhm so that the fine control is better.
Adjustments to the branch resistors such that the resistance of the connectors and common current-carrying paths is accounted for and a full 1000 mA is realized. This might include more potentiometers for fine adjustments at each branch level or more complex circuitry.

NOTES

NOTE 1: During the testing of this tool, it was seen that a full 1000 mA of current draw at 5 VDC was not possible due to various resistances not being accounted for during the design phase. These resistances stem mostly from the connectors and the common current-carrying paths. Therefore, in practice, this ARL can provide around 850mA at 5 VDC, which is still sufficient for most testing.