High voltage resistors are used in a wide range of applications including Power supplies, ESD protection, Electron Microscope, Air Ionising equipment, RADAR equipment and ATE. Example circuits include high voltage bleeder, voltage balancing, voltage regulation and voltage dividers. In this post, we discuss the key issues affecting the choice of a resistor in each circuit.
Voltage Bleeder Circuits
A Bleeder resistor is connected across a capacitor to drain its stored charge. It may be permanently connected or switched. In switched types, there is a trade-off between the time for the capacitor to discharge to a safe level and the quiescent power loss.
The challenge is to choose a bleeder resistor with a value low enough so it will discharge the capacitor in the shortest possible time but high enough so it does not compromise the normal operation of the system. Safety should always be a prime consideration when choosing the resistor value.
High Voltage Balancing Resistor
In high voltage DC bus applications, it is common to build a capacitive reservoir to eliminate supply line ripple effects. Generally, two capacitors in series are used each rated at half the bus voltage. Capacitors are often chosen with an equal value to share the voltage equally across them.
If capacitors could be selected with tight tolerance values there would be no need for balancing resistors but in practice, this is not possible. This results in a voltage imbalance that can lead to an overvoltage condition and damage the capacitor if not corrected.
The capacitor leakage current may be modelled as a resistor connected in parallel with the capacitor. This calculation is generally inaccurate but facilitates the choice of a balancing resistor value at less than 10% of the calculated resistance. The value of both balancing resistors may then be matched appropriately.
Although the basic model described above may suffice for some applications there are many potential high voltage resistor design issues that require a more complex design approach beyond the scope of this post.
Voltage Regulation Circuits
A voltage regulator protects sensitive components from overvoltage conditions. The device delivers a fixed output voltage that remains constant regardless of any changes in the input voltage or load conditions.
Voltage regulators are used to regulate AC or DC voltages but DC voltage regulators are the most common. The two main types of voltage regulators are linear and switching.
Linear regulators drop a voltage across some resistive element (active or passive) to maintain the output voltage at the required level. They often employ a negative feedback loop to compare the actual output voltage to a reference voltage.
Switching regulators turn on and off rapidly to control the output voltage. They require several control elements and charge storage elements (Capacitors). Switching regulators may be step up, step down or inverter voltage type.
Linear voltage regulators may be series or shunt. The shunt type is the simplest but also the most inefficient. The series voltage regulator is, therefore, more common. In switching regulators a variety of resistors are used as voltage divider, biassing and feedback elements. They are found in comparator, integrator and amplifier circuitry.
In linear shunt regulators, a high voltage thick film resistor is used to dissipate current to ground. In Linear series regulators a resistor is used to bias the base of a series voltage element (usually a FET).
Voltage regulators may be constructed using only discrete components or using an integrated circuit. An integrated circuit based approach is the most common as it has several potential advantages. The discrete high voltage resistor approach is used in more specialist applications.
The discrete component approach tends to deliver improved performance at a potentially lower cost. It may also deliver improved power handling. Discrete high voltage resistors (particularly when combined with a heatsink) may dump excess energy as heat.
Voltage Divider Circuits
One of the main application of high voltage resistor dividers is in power supplies with a feedback loop for regulation purposes. Appropriate choice of resistor values and their ratio determines the feedback voltage. For optimal performance a primary high voltage resistor is often combined with a secondary low voltage resistor. To prevent errors it is important to ensure one resistor does not change its value in relation to the other.
When calculating resistance values it is often assumed that the same current flows through both resistors. However, it is important to remember the input current to the voltage regulator or amplifier. This will flow through one resistor more than the other and cause an error.
The solution is to set the current through the divider resistors many times (typically 1000x) higher than the input current to the device. This approach tends to result in higher power losses so a trade-off is required.
Choosing the correct high voltage resistor for a particular application can be complex. The restricted standard range of resistor values, power ratings and package styles can further complicate the situation. Sometimes the only solution can be to use a custom high voltage resistor designed and manufactured by a specialist supplier.