A thick film high voltage resistor can be used in a wide range of applications. These include 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 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 equipment switch off. Bleeder resistors are common in power supplies, braking systems and many other power applications.
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 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 taken 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 value of resistance R should be chosen to minimise quiescent power loss whilst ensuring its value allows the voltage to decay to a safe level (Vs) within an acceptable period of time.
Thick film resistor technology is an ideal choice for bleeder resistor applications as it can dissipate high power in a relatively small area. Heat sinking and an appropriate choice of substrate material can improve thermal performance.
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 basic 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, another solution is required. A common tactic is to use (matched) balancing thick film high voltage resistors connected in parallel with each capacitor.
The general principle is to set the current flow through the balancing resistor 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
A voltage regulator protects sensitive components from overvoltage conditions. The device delivers 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.
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. A variety of resistors are used as voltage dividers, biasing and feedback elements.
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 linear shunt regulators, a high voltage resistor dissipates current to ground. In Linear series regulators a resistor biases the base of a series voltage element (usually a FET).
Voltage regulators can use only discrete components or an integrated circuit. The integrated circuit based approach is the most common with the discrete approach 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. A discrete thick film high voltage resistor (particularly when combined with a heatsink) can 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 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.
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.
One of the main applications of thick film 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.
A basic circuit is described above. In some high voltage resistor 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.
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.
To minimise the differences between the two resistors careful selection of materials is required. Thick film 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.
The selection of the resistor values often involves compromises. To minimise losses in the circuit the 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 deliver a relatively low output current which can compromise system performance. Errors can occur due to circuit noise and leakage currents. Current flow through the voltage divider must, therefore, be high enough to make noise and leakage currents insignificant.
Errors can also occur if one resistor changes its value relative to the other. Hence, for precision application resistor tolerances, VCR and TCR all need consideration.
Choosing the correct thick film high voltage resistor for a application can be a challenge. The restricted standard range of resistor values, power ratings and package styles can further complicate the situation.
When a standard device will not 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 in thick film high voltage resistor applications.