# Gary Ayton's camping and photography wiki

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science:voltage_droop

# voltage droop

• Voltage Drop is DIFFERENT to droop - voltage drop is the drop in voltage along a circuit due to resistance in the circuit
• in contrast, voltage droop is the drop in voltage at the power output in response to a current load

## Introduction

• campers with 12V battery systems generally keep a keen eye on their battery voltage as this gives a very rough idea of its state of charge
• they will note that as soon as there is a current draw, the voltage meter shows a significant drop in voltage - this is called voltage droop
• voltage droop is the intentional loss in output voltage from a device as it drives a load.
• without a designed voltage droop, when the current is activated, there will be a brief drop in voltage bit when it ceases there will be a brief “unexpected” rise in voltage which could be damaging to circuitry.
• adding droop in a voltage regulation circuit increases the headroom for load transients (the system is designed for the initial higher starting voltage and for the lower in use voltage) and allows a decrease in the number of output capacitors, or gain better regulation with the same number of capacitors.
• at high currents, as per Ohm's law, even a small resistance results in substantial voltage drop between the regulator and the load.

## What voltage to use to assess the state of charge of a battery?

• simple - use the voltage WITHOUT a current load (ie. with nothing connected to the battery, or everything turned off)

## Are there problems with voltage droop?

• the main issue is that it can cause instability and reduce the maximum current and power usage
• current = voltage / resistance
• assuming resistance stays constant, a falling voltage will result in less current
• the power available will thus also fall even faster as power in Watts = current in amps x voltage and BOTH of these fall with voltage droop as the current increases
• another potential issue is that the voltage droop may trigger the battery system's low voltage cut off and shut itself down to protect the battery

## AC-DC power adapters and voltage droop

• in general, AC-DC power adapters use diodes and capacitors to create an almost linear DC signal and are thus designed to deliver a specific current at a specific voltage (usually the “nominal voltage”), and depending upon design, many of these may have an initial voltage output 20-100% higher than this nominal voltage when there is no load.
• this is why you can't just use a “9V” power adapter with all 9V devices as it may not be optimised for that device's current draw