01207 230 728 sales@tsec.ltd.uk

A high voltage resistor can be used in a wide range of applications. These include power supplies, power distribution systems, ESD protection, Electron microscopes, Air Ionising equipment, RADAR equipment, X-Ray generators and ATE.

Example circuits include high voltage bleeder, voltage balancing, voltage regulation and voltage dividers. In this post we discuss the key issues to consider when choosing a resistor.

Voltage Bleeder Circuits

A bleeder resistor is a safety device designed to prevent electric shock to maintenance and service personnel. Its purpose is to dissipate residual electric charge at switch off. Bleeder resistors are common in power supplies, braking systems and many other high-power applications.

A Bleeder resistor connected across a capacitor drains its stored charge. It can 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 high voltage resistor with a value low enough to 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.

Resistor tolerance is generally not an issue and 5% resistor tolerance is often adequate. The trade-off is the time to reach a safe voltage versus power loss. The resistor value calculation is as follows:

R(max) = DT / C x ln (Vs / Vi)
DT – Discharge time
C – Capacitor value
Vs – Safe threshold voltage
Vi – Initial voltage
For a given value of resistance R the initial (max) power is:
P = Vi2 / R

The chosen value of resistance R should minimise quiescent power loss whilst ensuring its value allows the voltage to decay to a safe level (Vs) within an acceptable period of time. The bleeder resistor should have a voltage rating that exceeds the maximum voltage it will be subjected to in the circuit.

Thick film resistor technology is a common choice in bleeder resistor applications, as it dissipates high power within a small area. Thermal performance can be improved by heat sinking and employing specialist substrate materials.

High Voltage Balancing Resistor

In high voltage DC bus applications, it is common to build a capacitive reservoir to eliminate supply line ripple effects. A simple circuit includes two (equal value) capacitors in series, each rated at half the bus voltage.

In practice, variations in capacitor leakage currents make it difficult to equally divide the voltage across the capacitors. A voltage imbalance can result in an overvoltage condition on one capacitor. As capacitors are not generally available with tight tolerances, a common tactic is to use (matched) balancing thick film high voltage resistors connected in parallel with each capacitor.

The current flow through the balancing resistor is set at a multiple of the capacitor leakage current. The aim is to swamp the leakage current and negate its impact. There are many issues to consider, and calculating a balancing resistor value is beyond the scope of this post.

Voltage Regulation Circuits

Voltage regulators protect sensitive components from overvoltage conditions. They deliver a fixed output voltage that remains constant regardless of changes in the input voltage or load conditions.

Voltage regulators can 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 a 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.

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.

Switching regulators turn on and off rapidly to control the output voltage. They require several control elements and charge storage elements (Capacitors).

Switching regulators can be step up, step down or inverter voltage type. High voltage resistors are used as voltage dividers, biasing and feedback elements.
Voltage regulators can use only discrete components or an integrated circuit. The integrated circuit is the most common approach with discretes used in specialist applications.

The discrete component approach tends to deliver improved performance at a lower cost. It also delivers improved power handling as discrete high voltage resistors (particularly when combined with a heatsink) dump excess energy as heat.

Voltage Divider Circuits

The purpose of a voltage divider is to convert a high voltage into a related low voltage. The most common voltage divider application is measurement systems. They are also found in regulation circuits where the low voltage line provides a feedback loop.

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. To maximise performance, a primary high voltage resistor is combined with a secondary low voltage resistor.

As input current to the load or device will flow through one resistor more than the other, it can cause an imbalance. The solution is to set current through the divider resistors many times (typically 1000x) higher than the input current to the device.

The selection of the resistor values often involves compromises. To maximise circuit performance, resistance values should be high. Losses in the form of heat are wasteful and, if not dissipated, can compound circuit thermal issues.

However, high resistance values restrict output current, which compromises system performance. Errors can occur due to circuit noise and leakage currents. Hence current flow through the voltage divider must be high enough to make noise and leakage currents insignificant.

For precision application resistor tolerances, VCR and TCR all need consideration. The voltage divider resistors should provide long-term stability to maintain consistent voltage division over time. It is, therefore, important to consider factors such as drift and ageing.

As errors can occur if one resistor changes its value relative to the other, careful selection of materials is required. Resistor pairs for voltage divider circuits are often manufactured using the same resistor ink material.

To achieve the divider ratio with one resistor ink print, the primary, thick film high voltage, resistor is normally a long meander, whilst the secondary, low voltage, resistor is an inverse gain block, or rectangular, resistor.

Choosing the correct thick film high voltage resistor for an application can be a challenge. The restricted standard range of resistor values, power ratings and package styles can further complicate the situation. It is important to avoid exceeding the high voltage resistor specification limits.

When a standard device fails to match the demands of the application, one solution is to use a custom high voltage resistor. It is wise to choose a specialist resistor manufacturer with a detailed understanding of thick film resistor technology and its performance under a range of electrical, mechanical and environmental stress conditions.