What is "foldback"

n. 1 A current-limiting feature in power supplies and amplifiers. 2 The use of rear-facing heavy-duty loudspeakers on stage during live music performances. 3 (context botany English) (rfdef: English) 4 (context genetics English) (rfdef: English)

Foldback may refer to:

• Foldback (power supply design), a current-limiting device in power amplifiers
• Foldback (sound engineering), a speaker used to direct sound to performers

Foldback is the use of rear-facing loudspeakers known as monitor speakers or stage monitors on stage during live music performances. The sound is amplified with power amplifiers or a public address system and the speakers are aimed at the on-stage performers rather than the audience. This sound signal may be produced on the same mixing console as the main mix for the audience (called the " front of house" mix), or there may be a separate sound engineer and mixing console on or beside the stage creating a separate mix for the monitor system.

Foldback is a current limiting feature (a type of overload protection) of power supplies and power amplifiers. When the load attempts to draw overcurrent from the supply, foldback reduces both the output voltage and current to well below the normal operating limits. Under a short circuit, where the output voltage has reduced to zero, the current is typically limited to a small fraction of the maximum current.

The prime purpose of foldback current limiting in linear power supplies is to keep the output transistor within its safe power dissipation limit. With a linear voltage regulator, the output voltage V and output current I are maintained by simply dissipating away the surplus of input voltage V:

$$P_{\rm dissipation\, in\, regulator} = (V_{\rm in} - V_{\rm out})\times I_{\rm out}$$
Under overload conditions the output voltage falls and so the difference V - V becomes larger, tending to increasing dissipation. For a simple current limit, safely handling the worst-case scenario (a short circuit) would therefore require a much larger heatsink and output transistor than would be required under normal operating conditions. Foldback partially solves this, helping to keep the normal-rated output transistor within its safe operating area under fault and overload conditions. Foldback also significantly reduces the power dissipation in the load in fault conditions, which can reduce the risks of fire and heat damage.

Many power supplies employ constant current limiting protection; foldback goes one step further by reducing the output current limit linearly as output voltage decreases. However it adds complexity to the power supply and can trigger "lockout" conditions with non- ohmic devices that draw a constant current independent of supply voltage (such as op-amps). Foldback in switched-mode power supplies is discouraged because of these disadvantages, and since the benefit of reducing the power dissipation does not apply. Despite this, foldback is still often implemented in them.