Decade boxes are test instruments which use a series of resistors, capacitors, or inductors to simulate very specific electrical values. They can be quickly and easily substituted into a circuit and replace any standard value component. Their ability to be configured to nearly any resistance, capacitance, or inductance makes decade boxes a convenient way to find the optimum value for circuit operation. Highly useful for laboratory, education, or design work; decade boxes are also ideal for verifying the accuracy of test equipment prior to use as well as troubleshooting in the field or on the factory floor.

The Technology of Decade Boxes

Decade boxes are passive devices that consist of switches and, depending upon the type of box, a series of resistors, capacitors, or inductors of different values arranged to form “decades”. Decades are set up in factors of ten and stepped such that any value 0 through 9 can be selected. For example, a resistance decade box may have a 5 ohm resistor, a 2 ohm resistor, and two 1 ohm resistors in the first decade. The second decade may have a 50 ohm resistor, a 20 ohm resistor, and two 10 ohm resistors. The third decade may have a 500 ohm resistor, a 200 ohm resistor, and two 100 ohm resistors. With this combination, this three decade box can be switched to any value from 1 ohm to 999 ohms in 1 ohm steps. Some decade boxes may have 6 or more decades to allow for very wide ranges and highly precise values. Decade boxes are characterized by their range, maximum resolution, and accuracy.

Using a decade box for testing consists of setting the box to the desired value and hooking up the input device such as a transmitter, controller, multimeter, etc. The display value of the input device should match the set value of the decade box. It’s that simple.

Types of Decade Boxes

Resistance Decade Boxes

Resistance is the opposition to the passage of an electric current through an electrical conductor. Measured in ohms, and defined by Ohm’s Law as equal to voltage divided by current, resistance is one of the most basic of electrical measurements. As it impedes the flow of electricity, resistance is responsible for some power loss across a circuit. That power is lost as heat which can melt wires, creating a fire hazard, if the resistance is beyond the rating for the wires.

There are a number of practical applications that rely on the properties of resistance. Resistance is what keeps our bread toasted because of its ability to create heat out of electricity. Resistance is also used in potentiometers to perform a number of tasks including controlling the volume of audio equipment. Since the temperature of metals and semiconductors affects their resistance, resistance-based temperature sensors such as RTDs are used widely. As so many applications are based on electrical resistance, it’s important that devices that read or rely upon resistance are periodically tested.

Resistance decade boxes can quickly and accurately simulate resistance for quickly testing the accuracy of thermostats, temperature controllers, multimeters, and others. Resistance decade boxes are also commonly used commonly used for product design as they can be easily inserted into a circuit and function as a resistor of any value (within its range) to assist in identifying the optimal resistor size for the circuit.

Capacitance Decade Boxes

Capacitance is the ability of a body to store an electrical charge. Measured in farads, capacitance is important in a number of applications. Capacitance in electric circuits is deliberately introduced by a device called a capacitor. Capacitors generally consist of two conductive plates separated by a dielectric. Capacitors store the electrical charge in the form of an electrostatic field between the plates.

As they are capable of storing electrical current, capacitors are often used in electronic devices to maintain power in case of an interruption to prevent the loss of information from a volatile memory. That same power storage can be used to assist in starting certain types of electric motors or to power amplifiers in audio systems. They are also widely used in electronic circuits for blocking direct current while allowing alternating current to pass. Other applications for capacitors include power factor correction, signal coupling, tuned circuits and many types of sensors.

Capacitance decade boxes can quickly and accurately simulate capacitance for quickly testing the accuracy of multimeters and other instruments that measure capacitance. Capacitance decade boxes are also commonly used for product design as they can be easily inserted into a circuit and function as a capacitor of any value (within its range) to assist in identifying the optimal size for the circuit.

Inductance Decade Boxes

Inductance is an electrical property by which voltage is induced in a circuit, or a nearby circuit, by a changing magnetic field. Measured in henrys, inductance can be classified as self-inductance when the voltage is induced in the circuit itself, or as mutual inductance when the voltage is induced in a nearby circuit. Inductors are the devices that are characterized by their inductance. Inductors are made of a conductor, often a wire, wound into a coil. When current passes through the inductor, energy is stored temporarily in a magnetic field in the coil. Any change to the current passing through the inductor induces voltage in the conductor according to Faraday’s law of electromagnetic induction, which opposes the change in current that created it.

Inductors are commonly found in analog circuits and signal processing. When used in conjunction with capacitors, inductors form tuned circuits which filter out or emphasize specific signal frequencies—critical for radio reception as well as filtering signal interference. Two or more inductors in mutual inductance form a transformer, a fundamental component of every electric utility power grid.

Inductance decade boxes can quickly and accurately simulate inductance for quickly testing the accuracy of multimeters and other instruments that directly measure inductance. Inductance decade boxes are also commonly used for product design as they can be easily inserted into a circuit and function as a inductor of any value (within its range) to assist in identifying the optimal inductor size for the circuit.

Things to Consider When Selecting a Decade Box:

  • Is this for simulating resistance, conductance or inductance?
  • What range is required?
  • How much resolution is needed?
  • What level accuracy is preferred?

If you have any questions regarding decade boxes please don't hesitate to speak with one of our engineers by e-mailing us at sales@instrumart.com or calling 1-800-884-4967.