The FTA Reversing DC Potential Drop System has been designed by Keith Donald to provide the precise current and amplifier stability necessary for the reversing DC potential drop method of monitoring crack size. Custom-manufactured at FTA’s facility, the system is intended to be used in conjunction with the FTA Adwin-Gold Fatigue Crack Growth Rate data acquisition and control testing software, which synchronizes current reversing and voltage measurement.


The system consists of two units:


The FTA programmable linear DC power supply has a current rating of 20 amps and a voltage rating of 5 volts. Solid-state polarity-reversing relays have been internally installed and are controlled by two digital logic signals from the Adwin-Gold. Polarity reversing has two distinct advantages over pulsed (on/off) current control. First, for a given constant current, the magnitude of the voltage drop is twice the difference obtained with pulsed current control. Second, the durability of the DC power supply is enhanced since the current output remains constant and the switching transients are minimal.


The FTA differential amplifier offers two channels for active and reference probes. The low-level signals are first conditioned using a pair of Ectron model 428-O precision differential amplifiers. The output of the Ectron amplifiers is further conditioned (10x gain) and filtered as necessary to provide a maximum gain of 25,000. Unique features include manual or auto zero of the input signal and a 4 Hz filter bandwidth. The amplifier offers isolated true differential input, excellent temperature stability and immunity to EMI/RFI noise.