Power resistors, rheostats (variable resistors), and other electrical loads are useful for testing power supplies and regulators. Though functionally equivalent to a mechanical rheostat, electronic rheostats are based on active components. The advantages of these rheostats include many decades of load resistance in a single circuit, very fine resolution of resistance values, and better thermal management than the mechanical version.
One such load is the circuit of Figure 1:
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Figure 1: This electronic rheostat (variable resistor) provides several decades of constant resistance, and is very useful for bench testing power supplies and
Analysis of the circuit gives expressions for voltage at the op amp’s inverting and non-inverting terminals (Equation 1):
Because these voltages are equal, we see in Equation 2:,
The entire circuit acts like a resistance between the IN+ and IN- terminals. Equivalent resistances looking into the IN+ and IN– terminals are constant and independent of the test voltage, VIN.
Resistor RSENSE consists of several resistors in series, and provides a choice of ranges covering many decades of resistance. If you need 10Ω, for example, your terminals are IN+ and the “B” subterminal of IN– (leaving A, C, and D unconnected). For high-power loads, pay close attention to power-dissipation ratings for the sense resistors and n-channel FET, and add a heatsink for the n-channel FET if necessary. Components R3, R4, and C1 add stability to the feedback loop.
The op-amp power supply can be a battery or any DC power supply. A ground-isolated supply or battery has the advantage of floating the IN+ and IN– terminals (floating the resistor), thereby providing a load for positive and negative test voltages. The op amp shown draws a maximum supply current of only 20mA, which allows a standard 9V battery to provide the electronic rheostat with a year or two of normal use.
About the Author
Ken Yang is an application engineer at Maxim Integrated Products, www.maxim-ic.com