The purpose of a voltage divider is to convert a high voltage into a related low voltage. The basic voltage divider circuit consists of two resistors connected in series. With the input voltage (high) connected across the resistor terminals, the output (low) voltage is obtained from the connection point between the two resistors.
Voltage dividers are used in a range of measurement applications. They can also be used in regulation circuits where the low voltage line provides a feedback loop.
A basic circuit is described above. In some applications, the construction can be more complex with a series of resistors. The ratio of the resistor values determines the output voltage. To maximise performance a primary high voltage resistor is often combined with a secondary low voltage resistor.
The selection of the resistor values often involves compromises. To minimise losses in the circuit the resistance values should be high as possible. Losses in the form of heat are wasteful and, if not dissipated, can compound circuit thermal issues (discussed below).
However, high resistance values deliver a relatively low output current which can compromise system performance. Errors can occur due to circuit noise and leakage currents. The current that flows through the voltage divider must, therefore, be high enough to make noise and leakage currents insignificant.
It cannot be assumed that the same current flows through both resistors. It is important to remember the input current to the load or device will flow through one resistor more than the other. The solution is to set the current through the divider resistors many times (typically 1000x) higher than the input current to the device.
Errors can also occur if one resistor changes its value in relation to the other. Hence, for precision application resistor tolerances, VCR and TCR all need consideration.
TCR and VCR values are directly related to the materials selected. Long resistors elements give low voltage stress and thus reduced VCR effect, but long resistor elements also tend to increase TCR.
To minimise the differences between the two resistors careful selection of materials is required. Resistor pairs for voltage divider circuits are often manufactured using the same resistor ink material.
In order to achieve the divider ratio with one resistor ink print the primary, high voltage, resistor is normally a long meander, whilst the secondary, low voltage, resistor is normally an inverse gain block, or rectangular, resistor.
Although resistor divider circuits are relatively simple in theory there are many issues to consider if measurement errors are to be avoided. If potential issues are identified high voltage resistors can be designed and manufactured to fit the unique demands of a particular application.