Thursday, August 25, 2011

Use Remote Sense to Regulate Voltage at Your Load

Have you ever set your power supply output voltage to a particular value and found the voltage at your load was lower than you expected? If this was acceptable for your test, then you probably just left it alone. But if you wanted the voltage at your load to be equal to the voltage you set, then you should have used remote sensing.

Remote sensing is a feature on many power supplies that allows the power supply to regulate the voltage right at your load (“remotely”). This is accomplished by using a set of remote sense leads that are in addition to your load leads. The power supply uses the voltage on the remote sense lead terminals to sense the voltage right at the load terminals and regulate the voltage right at the load by adjusting the output terminal voltage.

Consider the example in Figure 1 showing a power supply set for 5 V, the desired voltage at the load. If the load is located six feet away from the output terminals, and you are using 14 AWG wire (about 2.5 mΩ/ft), each load lead will have about 0.015 Ω of resistance. If 10 A is flowing through the load leads, each load lead will drop about 0.15 V (10 A x 0.015 Ω) for a total drop of 0.3 V. When the power supply regulates its output voltage right at the output terminals, the result at the load is 4.7 V instead of the desired 5 V.

Figure 2 shows the same setup using remote sensing. The remote sense terminals are connected to the load at the points where you want the 5 V setting to be regulated. In this case, the power supply regulates 5 V at the load by adjusting its output voltage to 5.3 V to make up for the drops in the load leads. It does this by using the voltage across the sense leads as part of the feedback loop inside the power supply to adjust the voltage on the output terminals. The purpose of the power supply is to keep the sense lead voltage constant at the setting; the power supply changes the output terminal voltage based on the sense terminal voltage. The input impedance of the sense terminals is high enough to prevent any significant current flow into the sense terminals – this makes any voltage drop on the sense leads themselves negligible.

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