ULN2803: all about the Darlington transistor pair


If you are doing transistor work, there is likely a combination of these semiconductor devices that interests you. It is the pair of transistors known as Darlington. This setup is quite interesting for many DIY electronics projects, and can be found cheaply in the IC ULN2803.

You will be able to find the ULN2803 manufactured by different companies, such as the mythical Texas Instruments, or the European STMicroelectronics, etc. And in this guide article, I will try to solve all the doubts about this product, tell you where you can buy it, and how to work with it ...

What is the ULN2803?

El ULN2803 is a chip, an integrated circuit with a traditional DIP packaging, like so many others. That is, with two stacks of pins on its sides. Well, up to here it may seem like so many others, but inside it does not have conventional logic doors, multiplexers, filtros, current sensor modules, shift register, nor a microcontroller...

Inside the ULN2803 you will find drivers, with a series of transistors, some devices that I have already talked about on other occasions with different types such as: MOSFET, BC547, 2N3055, 2N222, etc.

What is the Darlington transistor or pair?

El Darlington transistor It is not a transistor as such, but a pair of them connected in a very specific way. Two connected bipolar transistors will form the Darlington pair, which allows the current amplified by the first transistor to enter the base of the second transistor and be amplified again.

This type of amplification was used with two separate transistors, but an engineer from the Bell Labs named Sidney Darlington He patented the combination in 1952. The idea was to put two or three transistors on the same monolithic chip. An idea similar to that of creating a chip or integrated circuit, although this achievement is not recognized to him, as you well know ...

The Darlington pair behaves like a single conventional transistor, that is, after combining the two transistors it still has a single base, a collector and an emitter. Only that the current gain will be the combined one, and therefore greater than using only one transistor. Specifically, it is considered that the gain in the Darlington is approximately the result of the product between both gains of the transistors used separately.

HR and advantages to use this Darlington pair is clear, get a large current gain. This makes it possible to control currents of a higher magnitude with a small base current. But it also has its downsides, such as a greater phase shift at high frequencies than when using a single transistor, which makes using them in negative feedback circuits somewhat unstable.

And it's not the only one associated problem to the Darlington pair, since the voltage drop between the base and the emitter is greater due to the existing double junction (equivalent to the sum of both drops of both junctions).

La saturation voltage that they have is also another limitation. On a practical level, that implies greater dissipated power, that is, more heat. And continuing with the disadvantages, the reduction in switching speed is another limiting factor, and it cannot be used on circuits where greater agility is needed. The first transistor cannot actively inhibit the base current of the second, slowing down the shutdown ...

These Darlington transistors can be found both encapsulated separately, that is, just one pair, or in integrated circuits with several Darlington transistors such as the ULN2803.

ULN2803 datasheet and pinout

The operation of the ULN2803 is very simple and its assembly is also very simple. This integrated circuit has a set of 8 reversing gates in its interior implemented by means of a Darlington transistor, in this case using NPN transistors. This makes it possible to connect other devices that have a strong current demand, such as stepper motors by way of driver, relays, etc.

Therefore, the ULN2803 is a cvery versatile circuit that can be seen in a multitude of maker projects as the output of digital circuits to drive actuators, motors of various types, and other components. All of them can be handled with a low current, admitting high demand currents such as 500 mA or 0.5A, which for electronics is a very high value.

Supports supply and digital output voltages of up to 50v, to transform 5v TTL digital signals to any voltage up to 50 volts. This practical function is what is known as a driver, that is, it acts as a type of element that isolates as if it were an electronic barrier, protecting digital logic circuits from others that require higher voltages and intensities.

You can see all the full features and pinout in the datasheet manufacturer. For example, here are two of the most common:

Price and where to buy

It is not too difficult to find if you know where to look. Its price is cheap, and you can even buy in ULN2803 chipgroups if you need several. For example, one of the cheapest is the 10x pack ULN2803A from Amazon that you can buy here for about € 1.

First project with a ULN2803

In the video they have only used 3 of the ULN2803 drivers, but you can use all 8 to get more levels or greater precision from your homemade level meter. Although less could be enough for your project ...

One of the main simple circuits that are usually made with the ULN2803 to show its behavior is a home water level meter. It is very simple, thanks to its 8 inverting gates made up of Darlington and using about 8 10k resistors and another 560 ohms, and also another 8 LEDs, you can have the meter ready. You can add a buzzer, or an Arduino board so that when it reaches a certain level, program the microcontroller to do some action, such as cut a valve, etc. The combinations are very high.

El assembly as you can see is very simple also. The low current demand at its input (high impedance) of the ULN2803 to activate the conduction of the transistors makes it possible to immerse the conductors connected to the chip in a tank of water and that the conductivity of the water itself is sufficient to use the electrical signal to activate them.

This would not work with distilled water, that is, pure, it needs to have dissolved minerals like tap water for it to have some conductivity. Contrary to what many think, water is a poor conductor of electricity, it is its dissolved minerals that conduct. Therefore, the more impure the water, the more it conducts ...

In this way, as the water reaches the different levels of each of its 8 drivers, it will activate the LEDs at its output and the buzzer emitting a beep when the water tank is filled.

How additional idea, you can connect each of the outputs to the input pins of the Arduino, so that when it reaches the first level program a sketch to do X action, when it reaches the second level Y action, and so on. Another option is to use relays instead of LEDs at the output, which would allow to activate or control circuits or devices with greater power depending on the level reaches one level or another, such as electrovalves or electrically controlled valves.

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