There are two main types of external LED drivers, constant-current and constant-voltage, as well as a third type of driver called an AC LED driver which will also be discussed. Each type of driver is designed to operate LEDs with a different set of electrical requirements. When replacing a driver, the old driver’s input/output requirements must be matched as closely as possible. Key differences are detailed below.


Constant-Current Drivers

Constant-current drivers power LEDs that require a fixed output current and a range of output voltages. There will be only one output current specified, labeled in amps or milliamps, along with a range of voltages that will vary depending on the load (wattage) of the LED. In the example below to the left, the current output is 700mA, and the output voltage range is 4-13V DC (volts of direct current).


Constant-Voltage Drivers

Constant-voltage drivers power LEDs that require a fixed output voltage with a maximum output current. In these LEDs, the current is already regulated, either by simple resistors or an internal constant-current driver, within the LED module. These LEDs require one stable voltage, usually 12V DC or 24V DC. In the example below to the right, the output voltage is 24V DC, and the output current is a maximum of 1.04A.


AC LED Drivers

AC LED drivers are actually no-minimum load transformers, meaning they could technically operate low-voltage halogen or incandescent bulbs as well. LEDs, however, cannot operate with conventional transformers because conventional transformers were not made to detect the low wattage of LEDs. In other words, LEDs have such a small electrical load that regular transformers do not register that they’re wired to a bulb at all. AC LED drivers are typically used with bulbs that already contain an internal driver that converts the current from AC to DC, so an AC LED driver’s job is to register the low wattage of LEDs and step down the voltage to meet the bulb’s voltage requirements, usually 12 or 24 volts. AC LED drivers are typically used to power 12-24V AC input LED MR16 bulbs, but they can be used for any 12- 24V AC input LED bulb. The LED bulb datasheet must be carefully examined; if the LED bulb requires DC voltage input, it cannot be used with an AC LED driver.


Other Factors to Consider

Max Wattage

According to the NEC (National Electrical Code), LED drivers should be paired with LEDs that use 20% less than their maximum rated wattage (with the exception of AC LED drivers). Drivers should not be paired with an LED that is at or exceeds the driver’s maximum wattage to avoid overstressing the driver components. For example, if you have a driver that can operate a maximum of 96 watts, it should only operate LEDs that use 77 watts at most (96 x 0.8 = 76.8). Dimming

Both constant-current and constant-voltage LEDs and drivers can be made with a dimming capability, though both LEDs and drivers must specify that they are dimmable on the product datasheet for that assertion to be made. If the specs don’t mention dimming at all, it is safe to assume that the product is not dimmable. Dimmable external drivers often require an external dimmer, or other dimming control devices specified on the product datasheet (namely TRIAC, Trailing Edge, or 1-10v dimmers) to work. Since technologies are improving rapidly, it’s best to test specific LED/dimmable driver combinations for acceptable dimming performance before making large purchases if brand-specific dimmer compatibility charts are not available.

Power Factor

Power factor describes how efficiently an LED driver uses electricity. It is calculated by dividing the power being used by the driver (wattage) divided by the product of the input voltage times the current going in (volts x amps). The range for power factor is a decimal between 0 and 1. The closer to 1 the power factor is, the more efficient the driver is. A good power factor is 0.9 or above.

UL Class 1 vs UL Class 2

UL Class 2 drivers comply with standard UL1310, meaning output is considered safe to contact and no major safety protection is required at the LED / luminaire level. There is no risk of fire or electric shock. These drivers operate using less than 60 volts in dry applications, 30 volts in wet applications, less than 5 amps, and less than 100 watts. However, these limitations pose restrictions on the number of LEDs a Class 2 driver can operate. UL Class 1 drivers have output ranges outside UL Class 2 designations. An LED Driver with a UL Class 1 rating has a high-voltage output and safety protection is required within the fixture. A Class 1 driver can accommodate more LEDs, making it more efficient than a Class 2 driver.


Ingress Protection (IP) Rating

IP ratings tell users the environmental protection that a driver’s outer casing provides. The first number specifies protection against solid objects, and the second number specifies protection against water elements. For example, according to the chart below, a driver with an IP67 rating is protected against dust and temporary immersion in water.



Physical Size and Shape

Consider the physical dimensions of the driver. Make sure it will fit in the area you will place it.


Q & A

Isn’t a constant-current driver similar to a resistor?

Yes, in that they both regulate current. However, if you use a resistor, the current will slowly continue to rise over time. Constant-current drivers are preferable because they are more consistent, efficient, and flexible, especially for high-power LEDs.

Can a driver operate more than one LED fixture?

In some cases, an external driver can operate more than one light fixture. A driver’s datasheet should always specify how many fixtures it can operate.


What are the differences between TRIAC, Trailing Edge, and 1-10v Dimmers?

TRIAC is the most commonly used and cheapest dimmer. However, they generate undesirable amounts of Electro-Magnetic Interference (EMI).


Trailing Edge (Reverse Phase or ELV) dimmers generate much lower amounts of EMI, but they are more expensive than TRIAC dimmers. Also, in the U.S., most Trailing Edge dimmers require that a neutral wire be run to the dimmer.


0-10V dimmers use low-voltage control wires to dim. This usually means that an extra pair of wires must be connected to every driver the dimming module operates.



How to Choose a LED driver ?