What Is a C5707 Transistor?

A C5707 transistor is a low-saturation voltage transistor for high current switching applications. It has a large current capacitance and enables high-speed switching. The C5707 transistor is commonly used in liquid crystal display (LCD) computer monitors. The transistor has been known to fail in LCD monitor applications when put under a particularly high load by a faulty tuning capacitor or through dry joints on high-voltage transformer pins.

Transistors are constructed of solid semi-conductor materials with a minimum of three connection terminals. One transistor characteristic is the ability to amplify signals by providing an output power, known as a controlled power, in excess of the incoming, or controlling, power signal. One of the most common uses of a transistor is as a controlled electronic switch to turn current on and off. The C5707 transistor is mainly utilized as a controlled switch because of its high-speed switching capabilities.

There are many different types of transistors. Each one possesses individual combinations of output capacitance, collector cut-off current and transistor base-to-emitter saturation voltage, among other variables. When a consumer is looking to buy a transistor, the wide range of available combinations makes it imperative that either a matching transistor is sourced or in-depth research carried out to ensure that a suitable alternative is available prior to purchase.

The relatively common occurrence of C5707 transistor pair failures in LCD computer monitors can be caused by high equivalent series resistance (ESR) in the tuning capacitor, which will short the C5707 transistors. Failure or shorting of the transistors can also be caused by high voltage transformer pins having "dry" joints through insufficient solder being present on the pins. When investigating this problem on LCD monitors, the hfe value of the C5707 transistors is important because differing values across the four transistors will result in the problem persisting.

The term "hfe" is not an acronym, per se, although it is based on the terms "hybrid parameters;" "forward," as in the direction of the current; and "common emitter." Instead, the term was applied to the measurement by transistor manufacturers, and it is applied to indicate the alternating current (AC) gain across a transistor. There are two terms for current gain across a transistor where the capitalization indicates the actual measurement — "hfe" for AC gain and "hFE" for fixed direct current (DC) gain.