Thick Film Bleeder Resistor Selection
A bleeder resistor is a safety device designed to prevent electric shock to system maintenance and service personnel. Correct bleeder resistor selection is vital to ensure electric charge stored in a system is dissipated when equipment is turned off. Bleeder resistors are common in power supplies, braking systems and many other power applications.
The electric charge is often stored in capacitors. Bleeder resistors tend to be either permanently connected in parallel with the capacitor or switched in when required. The switched arrangement dissipates less quiescent power but this advantage must be balanced against a consideration of the long term reliability of the switching mechanism and costs.
Resistor tolerance is generally not an issue in bleeder resistor applications and 5% resistor tolerance is often adequate. The key trade off is the time taken to reach a safe voltage versus power loss. The resistor value may be selected using the formula
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 may be calculated as
P = Vi2 / R
The value of resistance R should therefore 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 technology is an ideal choice for bleeder resistor applications as it can dissipate high power in a relatively small area. Suitable heat sinking and an appropriate choice of substrate material can improve thermal performance and ensure heat generated by the resistor may be dissipated without affecting any sensitive components in the immediate vicinity.
Bleeder resistors are safety critical components and device failure is therefore not an option. It is therefore wise to choose a resistor manufacturer with a detailed understanding of thick film resistor technology and its performance under a range of electrical, mechanical and environmental stress conditions.