Switched Load Resistor Bank With Smaller Step Size

Load Resistor Banks are required for testing power products, for characterization of solar panels, in test labs and in industries. Rheostats provide continuous variation in load resistance. However, as the value of resistance is reduced, the power rating also reduces. In addition, rheostats have series inductance.

Some of the desirable features of load resistor bank are:

1) Series inductance should be as small as possible

2) Smaller step size

3) As the load resistance is reduced, the power rating should go up.

Here, a design of load resistor bank is given. The special feature of this design is, smaller step size with less number of switches and resistors.

Following is the Bill of Material:

1) General Purpose PCB 12" x 2.5" - 1 pc

2) Rectangular Aluminium pipe (12" x 2.5" x 1.5") - 1 pc

3) Resistors 3300 Ohm 2W - 27 pcs

4) Toggle Switches - 15 pcs

5) M3 x 8 mm screws, washers and nuts - 12 sets

6) Wires

The circuit consists of 27 carbon film resistors of 2W power rating. The first resistor R1 is directly connected across the terminals T1 and T2 as shown in Fig 2. Circuit needs 15 toggle switches. Thirteen switches SW1 to SW13 are used to switch two resistors each, in the circuit. Two toggle switches J1 and J2 are used along with SW1 and SW2. SW1 connects R2 and R3. Here, R2 is directly connected to ground. R3 is connected to ground through J1 (when J1 is in ON position). Similarly, SW2 connects R4 and R5. Here also, R5 is directly connected to the ground. R4 connects to ground when J2 is in ON position. When J1 and J2 are moved to OFF position, resistors R3 and R4 come in series. Interconnections for SW1, SW2, J1 and J2 are shown in Fig. 3.

Following are the design specifications:

1) Max Resistance Req = 3300 ohm (All switches SW1 to SW13 are OFF)

2) Power rating at Max Resistance = 2 W

3) Minimum Resistance Req = 3300/27 = 122.2 ohm (SW1 to SW13 are ON, Jumpers J1 and J2 are ON)

4) Power rating at Min Resistance = 54 W

5) Number of steps = No of switches * 3 = 13 * 3 = 39

The table shows the values of equivalent resistance Req for different switch and jumper settings.

Notes for the table:

^ R3 and R4 are in series

* J1 OFF and J2 ON gives same result

** R4 not in the circuit.

In the Aluminium pipe, make a slot in the middle of the wider side. The slot should be about 1.5" wide, leaving 0.5" margin at the top and the bottom as shown in Fig. 4. Drill 12 mounting holes of 3 mm dia.

Take the general purpose PCB and drill 15 holes of 5 mm dia. These holes are located just below the top margin such that, when the toggle switches are mounted, will not touch the Aluminium pipe. Also drill 12 mounting holes on the PCB to match those on the Aluminium pipe. Fix all the toggle switches in the 5 mm holes.

Take on long bare copper wire and solder it to the top terminals of all toggle switches SW1 to SW13. Do not connect this wire to J1 and J2. Similarly take another bare copper wire and solder it to the PCB at some distance below the toggle switches. Take two resistors and join them at one of the ends. Then solder this to the middle terminal of the toggle switch SW3. Similarly solder 2 resistors each to all the toggle switches up to SW13. The other end of the resistors is soldered to the copper wire (Ground) as shown in Fig. 5.

Connections to SW1, SW2, J1 and J2 as per the circuit diagram of Fig 3 is shown in Fig 6. Solder two wires at the centre of the array and bring it out for external connections T1 and T2 as shown in the above figures.

Slide the assembled PCB into the Aluminium pipe. Ensure that none of the resistors touch the pipe. Fix the PCB to the pipe using 12 screws. The load resistor bank is ready for use.

Keep all the toggle switches OFF. Now turn ON SW1. Along with SW1, J1 can be used to reduce the resistance value. Next, turn ON SW2. Now J1 and J2, both will be effective. J1 and J2 in OFF condition gives maximum resistance value in this range setting. Turning ON J1 will reduce the resistance. Now turning ON J2, will further reduce the resistance. To go to the next lower values of Req, SW3 has to be turned ON. In this setting, again, we can go through three steps eg. J1, J2 OFF, next J1 ON and lastly J2 also ON.

Advantages:

1) Uses less number of switches and resistors and provides more number of steps.

2) All resistors are identical in value and power rating. This reduces the cost. Especially when high power resistors are to be used. High power resistors are quite expensive.

3) All resistors are uniformly loaded, hence better utilization of resistor power rating.

4) We can go on adding more switches and resistors to get the desired resistance range.

5) This circuit can be designed for any range of resistance values and any power rating.

This design is useful for all electrical/ electronics laboratories in teaching institutions, in test centres and in industries.

Vijay Deshpande

Bangalore, India

email: vijay.deshpande860@gmail.com