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Driver List Database code 16 December 2019 Footnotes 10 August 2016

• Step-up drivers • Low voltage step-down drivers

• Mains drivers • User interface & PWM drivers

Drivers not yet added 3 March 2012 Links 28 February 2012

Why use a driver? 20 February 2010

Driver types overview 15 September 2010

Types of White LED Drivers

Control Methods

Voltage Control

A voltage controlled driver presents a couple of problems.

1. A small variation in voltage can produce a large variation in current. A constant voltage driver may avoid this problem.
    
2. Thermal runaway. When an LED heats up its forward voltage reduces, which means more current, which means more heat, etc. For more on thermal runaway see Why Use a Driver?

Current Control

Using a constant current driver is much preferred, as it completely avoids the problem of thermal runaway.

The simplest kind of current control is using a resistor. Don't be put off by the simplicity and low cost; a resistor really may be the best solution for driving your LED. As well as being simple and low cost, they're easily obtainable and also often the smallest solution. If the input voltage is reasonably steady and not many volts (or watts) need to be dropped then a resistor will likely do the job very well. See ledcalc or similar sites on the Links page for an online resistor calculator.

Sometimes a resistor isn't needed as the power supply has a sufficiently high internal resistance that an external resistor isn't needed. Small batteries are often in this situation, with a good example being my keyring torch.

Step-up/Step-down Type

There are several kinds of driver types that are useful in different situations. Some increase the voltage to the required level, some decrease the voltage, either by using a voltage divider or by simply burning off excess volts as heat.

Step-up

Boost circuits increase the voltage to a level usable by the LED. The input current is higher than the output current.

Step-down

Buck circuits drop the voltage. (The mains voltage drivers listed here also incorporate a transformer to drop the input voltage by a set ratio to make it easier for the buck circuit.) The input current is often lower than the output current, even though the input power is greater than the output power.

Linear regulators act like a variable resistor. They are placed in series with the LED and automatically adjust their resistance to maintain a fixed current level.

Pulse width modulation (PWM). If the peak output voltage of a PWM driver is greater than the maximum specification of the LED being driven with it the driver should be used with a capacitor placed in parallel with the LED to smooth out the peaks.

Efficiency

No driver is perfectly efficient, so the input power (measured in watts) will always be greater than the output power. The difference is dissipated as heat (and sometimes a tiny bit as sound). It's not fair to say this is "wasted" power, though, since the regulator circuit being driven with that power is serving the important purpose of maintaining a regulated (or semi-regulated) output for our LED. It's a necessary cost of the regulation, although it's still desirable to keep that cost as low as possible.

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