Reducing the Number of Parts with Digital Transistors: The First Step of a Simple Circuit

Published by DevicePlus Editorial Team at March 15, 2021

Digital transistor with a built-in resistor

A digital transistor is an electronic component that has a resistor built into the transistor package.
In order to turn on LEDs and operate relays with microcontroller boards, such as Arduinos or Raspberry Pis, a drive circuit (switch) using transistors and resistors is required. The drive circuit is simple, but it requires a resistor that limits the current flowing through the base.
Since the resistor is built in the digital transistor, a drive circuit can be made with one transistor.

ROHM were the first company in the world to develop the electronic component known as the digital transistor. Today, it is sold by other companies as well, referred to as a “resistor built-in transistor”. In abbreviation, it’s sometimes called a “digi-tra”.

Why transistor switches need resistors

Before explaining the appeal of digital transistors, let’s take a quick look at why transistors need resistors. There are several types of transistors, but here, we’ll explain the use of bipolar transistors.

It’s necessary to pass a current through the base to control the ON / OFF of the transistor. If a voltage is applied to the base, current will flow, but if left as is, the load current will increase exponentially as the voltage increases, so even a slight change in voltage will significantly change the operation. In the worst case scenario, the transistor or microcontroller may be damaged, or it may malfunction due to a small amount of static electricity, leading to issues.
Therefore, in order for a bipolar transistor to operate stably, it’s necessary to connect an input resistor to its base to limit the current applied to the base.

Reference: Please tell me the basic ideas behind digital transistors (Digi-tra). ｜ROHM Co., Ltd.

How to use digital transistors

A digital transistor may seem like a simple electronic component that has a resistor built into the transistor, but when you try them for yourself, you’ll begin to see their merits.
Various benefits can be expected when you use digital transistors, including a “reduction of the mounting area (the miniaturization of the circuit)”, a “reduction in the number of parts (the reduction of the mounting cost)”, and an “improvement in reliability”.

For an example of digital transistors in action, the two figures below compare how much the size of the circuit changes when an LED lighting circuit is made.

An example of 4 LED drive circuits using ordinary transistors. The resistance that limits the current in the base complicates the placement and wiring of the electronic components.

If a digital transistor is used, the resistance can be omitted and the circuit scale can be reduced. Consequently, space is saved, and the amount of wiring is reduced, which means that the number of functions in the same area can be increased and the number of wiring issues can be reduced.

With a normal transistor, there are 18 wires and 20 electronic components, but if you replace it with a digital transistor, the number of wires can be reduced to 14 and the number of electronic components to 12. By reducing the number of parts, the amount of work needed to insert components into the breadboard is also reduced by about 30%.
Reduced wiring also reduces problems associated with poor contact and mistakes. This is an example of a breadboard, but in mass-produced circuits that are incorporated into actual products, even a single component reduction can have a significant cost reduction effect. In this way, digital transistors are electronic components that can easily realize cost reduction and increased circuit reliability.

On a circuit diagram, it is common to enclose a digital transistor with a dotted line and use a symbol to indicate that the transistor and resistor are built into one electronic component. Part numbers on the circuit diagram are indicated by DT1, DT2, …, etc.

How to select a digital transistor

There are various variations of digital transistors depending on the collector current and the resistance connected to the base. The selection process for digital transistors is the same as the selection process for determining the resistance to connect to the base when using a normal transistor as a switch. (Refer to the resistance calculation method LED blink on RasPi, Ohm’s law / GPIO / Learn more about transistors｜DevicePlus.)
The digital transistor selection tool is available on ROHM’s digital transistor product page. If you enter the four parameters of polarity, power supply voltage, input voltage, and output current, the optimal transistor will be selected automatically, so if you’re uncertain about the selection of parts, you should try using it.

Digital transistor selection tool｜ROHM Co., Ltd.

Digital transistor summary

When you hear the term “digital transistor”, it may sound more complicated than a normal transistor, but they’re really just simple electronic components used for switch operation, and as you can see, they’re actually very easy and convenient to use.
Even in electronics kits on breadboards, using digital transistors can reduce the number of parts used and issues such as contact defects. Plus, they’re electronic components that provide great benefits, even in small-scale electronic work and operation verification.
Having digital transistors on hand is also recommended for those who often use transistors for switch operation.