Do I choose a constant current or DC voltage led transformer for my led?

Summarize

• Led transformers regulate the current to the led circuits, the choice depends on the luminaire
• Constant Current (CC) and Constant Voltage (CV) transformers differ in current and voltage control
• Increase in temperature affects the power consumption and lifetime of the LED
• Standard current transformers are best suited for individual LEDs or series connections
• Constant voltage transformers are common for LED strips with built-in current limiter

With so many options available, choosing the right LED transformer can sometimes seem overwhelming. In this article we will go through the question of constant current or DC voltage. In English, constant current is Constant Current (CC) and constant voltage is Constant Voltage (CV). We answer questions about why and when they are needed, and go through their features.

What is a led transformer?

A led driver is a device that regulates the amount of current flowing to the led circuits.
All driverare either constant voltage (CV) or constant current (CC) or both. The question arises, which of the drivers do you need? The simple answer is that it depends mainly on your LED luminaire. We’ve written about Led transformers on the blog before, so there’s more information on the subject.

How does LED work?

LEDs are the most widely used light source today. LEDs are semiconductor diodes formed by bringing together two slightly different materials to create a PN junction (positive and negative).

P-type holes have an extra positive charge, and N-type electrons have an extra negative charge. By combining these into a PN junction, it produces light by applying a direct current to it.

The colour of the light depends on the materials used in the semiconductor. You can read more about LED as a technology in our article What is LED?

Like any other semiconductor, LEDs allow current to flow forward, but prevent current from flowing backwards. A Led produces light when an electric current passes through it from the anode (+) to the cathode (-).

Current characteristics of LEDs

Like traditional PN junction diodes, LEDs are also dependent on the amount of current flowing through the forward voltage. Small changes in voltage cause large changes in current.

Typically, the forward voltage (volts) of LEDs is 2-3.5 V at a forward current of 10-200 mA. In other words, the amount of current increases dramatically with even a small change in voltage.

Temperature coefficient of LEDs

One might think that when the voltage of an LED is fixed, the current is fixed. In other words, the constant voltage or constant current would be same.

In fact, the voltage to current ratio of LEDs is not fixed, but changes with temperature. So although the voltage is fixed, the current is not fixed, but changes with temperature. This is because the LED is a diode, and has a negative temperature coefficient.

The figure below shows 3 curves representing LEDs at different temperatures. At the same voltage of 3.3 volts, the currents are 8 mA, 20 mA and 37 mA.

Temperature coefficient, usually -2 mV/degree (-1.5 – 2.5 mV/°C), meaning that as the temperature rises, the current consumption of the LED increases proportionally more steeply.

In other words, in the design of an LED luminaire, the cooling element affects the current consumption of the LED, and to select the right transformer, the current consumption of the LED must be measured at its operating and running temperature.

We have discussed the issue in more detail in our article Why can’t leather stand the heat?

What is a constant current LED transformer?

The constant current LED transformer changes the voltage across the entire luminaire group to maintain a constant current. LEDs designed to work with a constant current transformer therefore need a certain amount of current to work properly.

Led light is brighter at higher current, but if the current is not the amount of current designed for the led, the led will burn brighter and hotter, which will cause the led to burn out quickly. In addition, burning too hot led to a loss of light properties such as colour rendering. This is why constant current transformers are so important for LEDs.

From the LED temperature coefficient graph earlier, we could see that it is better to use a fixed current LED driverto prevent temperature rise.

When do you need a constant current LED driver?

Standard current transformers are primarily used in LED luminaires, where the current output of the LED transformer directly supplies power to the LED circuit. This method provides high efficiency and low complexity.

Constant current LED transformers are the best choice if, for example, you are building the luminaire yourself.
These drivers can be used with a single bulb or in a LED chain in series. The choice of a Led driverdepends crucially on the purpose for which you intend to use it. The manufacturer of the LED circuit or luminaire always indicates the amount of current required to operate the LED.

For lighting control, constant current transformers are the best choice because they allow continuous control of brightness and light quality when the amount of current is known.

What is the constant voltage led transformer?

However, the most common LED transformers are constant voltage transformers, 12 VDC or 24 VDC.
These transformers are designed for DC voltage. The amount of current, in turn, varies. A constant voltage LED luminaire requires a certain amount of voltage to function properly.

These LED lamps have a built-in current limiter or regulator to keep the voltage constant. The current produced by the driver voltage may vary, as the number of LED lights may vary. For example, two metres of conductive tape requires twice as much power as one metre.

Can I use constant voltage led driver for any led directly?

We already know that as the operating temperature rises, the current consumption of the LED increases. If the voltage (V) is constant, the current (A) will naturally increase. If we use a constant voltage transformer for a constant current controlled LED, the temperature rise is very fast. When the power is switched on, the LED temperature can easily rise to over +80 or +90 degrees. So the led doesn’t last long, and goes out.

According to the graph shown earlier, as the current rises, the temperature rises, e.g. An LED designed for 20 mA current will draw 35 mA and the temperature will reach 85 degrees and keep it there, without increasing the brightness of the LED.

How then can a constant voltage transformer be used for a constant current transformer?

An additional current limiter can be added to the circuit. This way, the constant voltage becomes the constant current for the LED. This prevents the LED from over-currenting and overheating, which increases the lifetime of the LED.

When do I need a constant voltage led transformer?

You can use a constant voltage (V) led transformer with multiple LEDs connected in parallel, such as led strips. What these LED luminaires and LED strips have in common is that the manufacturer has installed current limiters in the luminaire.

The picture shows LED strip with a current limiting resistor installed. The resistor limits the current to a series of 3 LEDs with a 470 Ohm resistor. It is a white led circuit that uses a 3.2 V supply voltage. With a 24V led strip in this case, each led circuit receives 30 mA (24V-3×3.2V)/470 = 30 mA current.

This type of construction is suitable for DC to DC led transformers. Led strips always have a limit of 3-6 LEDs per series, which allows the strip to be cut between series.

Let’s take the example of an LED strip with 5 sets of 3 LEDs.

Each set of 3 LEDs requires 90 mA (30 mA per LED). So the LEDs work perfectly with 450 mA current.

If we run the strip at 450 mA, but cut off one piece, we are left with four series, and the current in each series increases to 112.5 mA (37.5 mA per LED).

Increased current will start to damage the led circuits, so it is better to use constant voltage led driver with led strips. Led strips are always manufactured for a specific DC voltage. 12 or 24 V. So use them with a DC voltage transformer.

Constant current or DC voltage

A short summary of the standard voltage CV and standard current CC transformers:

The pros and cons of DC voltage

Pros

• Fault tolerance: this means that when one light fails, the efficiency of the other lights is not affected.
• Easy installation: connect one or more led strips / luminaires.
• Price: in larger groups, one DC transformer is cheaper than many DC transformers.
• Models: are usually smaller than constant current drivers for high power, 12V, 24V are the most typical models.

Cons

• Efficiency: constant voltage transformers have a lower efficiency than constant current transformers.
• Additional technology: lights must be fitted with a current limiter.

The pros and cons of constant current:

Pros

• Limiter: standard current limiters avoid exceeding the maximum current limit specified for the LED. It saves the LED from burning out prematurely and extends its lifetime.
• Price: they have the lowest price and high efficiency.
• Standard: makes it easier for designers to control light output.
• For large: suitable for high-power LEDs.

Cons

• The Christmas tree effect: since the lights are connected in series with a constant current transformer, the failure of one light will lead to the failure of all lights.
• Inconsistency: uneven current and brightness for parallel LEDs.

What type of led driver do you need?

To choose the best LED transformer for your needs, you need to consider the following criteria:

• application of the led driver.
• LED voltage and current.
• Power

Summary

LEDs are inherently constant current controlled. But for many solutions, DC voltage is the best solution. So there is a place for both and they should not be mixed.

If you are interested in the development of (transformer) technology, here is a link to the American Institute of Physics website. After all, technology is constantly evolving.

Led lighting expert

LedStore has been an expert in led lighting since 2010. We have our own product design, so our products are technologically state-of-the-art.

We focus on light colour temperature controlled and high colour rendering index lights. We do around 500 lighting designs in a year.

We offer a service of custom-made led strips, i.e. made-to-order, easy to install led strips in profiles for everyone. Also installed.

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Janne Halttu Owner

LedStore.fi entrepreneur who has built 4 detached houses and renovated several properties.20 years of construction experience has given me a lot of knowledge about structures, and electrical work for several houses. I have worked with LED lighting for over 10 years.I am an expert in Led lighting, I write articles and I am a lighting designer.I often help clients with electrical and structural issues related to lighting and house construction.

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