13.8 Volt Load

This unit was built to test the output of high current 13.8vdc power supplies commonly used to power transceivers and amplifiers. The math is first day electronics, E=IR. We know the voltage to be used is 13.8vdc and we can select the desired current. For example the resistance needed to produce a current of 10 amperes at 13.8vdc is:

13.8vdc = 10a x R then R = 1.38ohms

An appropriate material to use to construct resistors at these values is number 18 nichrome wire. I determined three current values and the needed resistances for those values to be able to sequentially test the output of power supplies and observe the voltage regulation. This is such a unit.

Several concerns were addressed in the design. The first was obtaining stable characteristics. Next, dealing with the heat generated. Another was to reasonably obtain components. An old unused “Cantenna” served the purpose for mounting components and housing the cooling oil. In this case light weight machine oil (bulk, McMaster Carr). The nichrome wire was readily available from one of the electronics online suppliers. Concerned with heat I found three ceramic feed through insulators to terminate the nichrome resistors and a ¼” brass bolt and washers for a common connection point. The nichrome resistors have loops rolled on the ends and are physically attached to the appropriate bolts while being bathed in the oil bath to provide heat dissipation and electrical insulation.

Three current choices are available from the common terminal to a resistor. However, many choices can be had by selecting other terminations. Examples are such as wiring to the 10 and 30 ampere terminals and getting a termination for 7.5 amperes or wiring to the common and 10 ampere connections with the 10 and 30 ampere terminals bridged witch gives a termination for 40 amperes.

Most likely this unit would be used to evaluate a power supply’s ability to regulate the voltage under various loads. My test results have been very surprising.

EST 4-27-25