How do I determine if the fixture is current or voltage driven?  How do I select the correct Power Supply?

posted in: Constant Current Constant Voltage Controls Drivers Power Modules Products Transformers


Was this answer helpful?

Let us know!

1A)      If voltage driven, the fixture will require 120 VAC, 12VDC, 24VAC, 24 VDC or some other voltage?
If voltage driven, the LED fixture has components built in that regulates the current to the LED. 
Lower power LEDs (20ma) are usually ballasted with resistors, which burn off the excess voltage as heat and determine the drive current to a string of 3 or 6 LEDs.
For Higher Power LEDs, an on board regulator or driver is in the fixture that calls out for a certain voltage to operate properly.
Low Voltage fixtures will require a Remote transformer (AC) or Power Supply (DC).

1B)      If Current driven, it will typically specify the drive current required; typically 350ma (0.35A) or 700ma (0.7A).
Current driven products can be made smaller since the LED is on its own and driven from a remote power supply.  They are usually less expensive and don?t normally suffer from voltage drop concerns so they can be fed with 18AWG wire from the driver.

2A)      If Voltage Driven ? What voltage is required and is AC or AC/DC acceptable or DC only?
Semper Fi?s standard transformers step down to low voltage (AC).
If DC voltage is required, can Full Wave (FW) Rectified DC (Unfiltered ? 120 Hz ripple ? and Unregulated) be used or is Constant DC voltage required?
Semper Fi Produces All of these solutions.

2B)      If Current Driven ? What is the drive current required?  (This is always DC)  
Semper Fi?s standard drivers are 350ma or 700ma, but also build 1000ma (1 Amp) and/or other amperages by request.

3A)      If Voltage Driven ? Is the fixture dimmable?
If yes, is it designed to be dimmed by reducing voltage to the fixture or is a separate feed required to the integral regulator/driver?s control circuit?
Semper Fi transformers and FW Rectified DC units can be dimmed using LV magnetic Dimming.  Power Modules are dimmed using 0-10VDC control.
Keep in mind that regulated voltage = constant voltage, therefore if a regulated/constant voltage power supply is going to be dimmable, it usually requires a separate control feed.
Semper Fi manufactures a dimmer interface that converts traditional dimming (incandescent of LV) to a dimmable 0-10VDC signal that can be used to control devices that require 0-10VDC control.
There are many products that claim to be dimmable, but their definition of dimming may be different than yours.  Technically, any reduction in light output could be coined ?dimming?.  Get VERY specific information on what they define to be dimming and what is required.  Just remember that LEDs are CURRENT DRIVEN DEVICES and you are feeding voltage to the fixture, therefore something else has to happen to create/simulate dimming.  Some power supplies can/should not be dimmed.  Others that say they can be dimmed, but may or may not meet customer expectations.

3B)      If Current Driven ? The LEDs at fixtures powered by Current/Amperage are dimmed when the amperage is reduced. 
Semper Fi?s RPK-D drivers are designed to be dimmed using traditional dimming (incandescent of LV)
Semper Fi?s RPK and RPK-LV drivers are designed to be dimmed using traditional dimming (incandescent of LV) when using a Dimmer Interface to provide 0-10VDC control feed to those drivers (separate port on the LV side of the Drivers)

4A)      If Voltage Driven ? How to size transformers or power supplies?
The fixture should have a Wattage rating at some specified voltage.  Beware of maxing out the power supply, wiring and other components based on these ratings because it is possible that the actual wattage could be Higher.  Possibly 25% or more in some cases.  This impacts all of your calculations and component requirements/sizing, so get a real world wattage or de-rate minimum 25% to reduce the odds of facing this issue on the job after installation.
If you are confident in the wattage per fixture, then selecting and sizing the power supply is almost identical to sizing a transformer (if it is using AC voltage).
If using DC voltage, there are some other considerations ? which I can address in a separate email.

4B)      If Current Driven ? How to size drivers?
The typical current driven fixture is either 350ma or 700ma, but could be something else ? this is critical to know since the wrong drive current could destroy the LED or produce less than desired light output.
When someone says "1Watt" LED, it is usually a Nominal number and rarely is 1W.  The Forward Voltage (Fv) of the LED determines actual Wattage when driven with a constant current. This is why Semper Fi sizes its drivers using Fv to avoid overloading.
The total Fv on the circuit is the sum of all the Fv on that wire run plus any voltage drop.  If there are 10 LEDs on a single wire run to a driver and each LED has a Fv of 4.0 VDC, then 10 x 4.0 = 40.0 VDC.  Wire AWG and distance will determine voltage drop, but using 100 feet of 18 AWG wire to power 350ma LEDs drops less than 0.5 volts.
Drivers are sized to Maximum Fv/load.  The RPK-120-60D driver can accept a dimmed 120 VAC input and have a load between 15VDC-60VDC.  Electronic devices have minimum load considerations when sizing; RPK drivers typically 25% minimum load required.  Therefore -60 driver is 60 VDC x 25% = 15 VDC minimum load, which is 4-6 LEDs; depending on the LED Fv.

What is Forward Voltage (Fv)?
Diodes allow electricity to flow in One direction, which is why LEDs are driven using Direct Current (DC).
The amperage thru an LED will result in a Forward Voltage, which changes with drive current and Junction Temperature (Tj).

When current driven fixtures are wired as Low voltage fixtures, indoor applications are limited to 60VDC (30VDC for Wet locations) per wire run.

Semper Fi can provide solutions for any voltages, amperages and frequencies around the world.