Microcontrollers are one of the best ways for adding digital interconnectivity and ‘smart’ features to a product. However, these microcontrollers are not capable of directly controlling anything other than a single LED. This is because most microcontrollers are only able to sink or source directly around 10 mA.
There are many ways in which heavy loads can be switched on the low side by using a simple microcontroller. Using a saturated switch is considered one of the simplest and easiest ways to control large loads running on DC.
Low side switching with BJT
A simple bipolar junction transistor (BJT) can act as a load switch, and it has two types: NPN, which is used for low-side switching, and PNP, which is used for high-side switching. Naming conventions of the respective voltage and current name can be seen in the equation: IE = IC + IB. IB represents base current and it enters from the base of NPN. The same is the case with IE and IC, where IE is leaving the transistor.
Similarly, the voltage between collector and emitter is represented by VCE and it usually has a positive value in the case of NPN transistors. It can be said that the collector voltage is higher than that of emitters in the case of the NPN transistor. In the same way, the voltage between base and emitter is VBE. In NPNs, it is generally positive.
The voltage between the collector and emitter is represented by VCE and it has a positive value in the case of NPN transistors. Source: Vidralta/CC BY 3.0The following equation helps to understand how large load is controlled by the transistor: IC = βIB
In this equation, β represents DC gain, and it varies from 20 to 300 and even more.
In other words, the collector current IC is β times base current IB. For example, IC will be 100 times IB if β is 100. The value of β is somehow dependent on temperature and collector current. The actual value of the β can be seen from the datasheet of the transistor.
Moving to the next part, suppose VCC is 10 V, RL is 10 Ω, and β is 100. When IB is 1 mA then IC is 100 mA as the value of β is 100. The voltage drop across the resistor is calculated by IC x RL, which is 1 V. It means that the value of VC is 9 V as VCC = 10 V, and the voltage across the resistor is 1 V.
Look at an example where IB is 20 mA. As per equations and calculations, IC will be 2000 mA, or 2 A, which is not possible. Since VCC is 10 V and RL is 10Ω, max 1 A of current can flow through RL. It means the max value of IC is 1 A. This state is known as saturation mode. In saturation mode, the transistor has the maximum IC allowed by circuit conditions. In other words, the value of the IC cannot be increased any further even by increasing the IB.
So, the above-mentioned equation IC = βIB is not true when the transistor is in saturation mode. Even in the above-mentioned example, if the value of VCC and RL is changed to 25 V and 1 Ω, then the transistor will not be in saturation mode. It implies that saturation mode has a relationship with external circuit conditions.
In saturation mode, the VCE value of a transistor will be the value of VCEsat. This value can be seen from the datasheet of that particular transistor, and it ranges from 0.2 V to 1 V for smaller to large ones respectively.
Moving toward transistors’ base, suppose VBE = 0.7 V (a very easy and quick way to regulate IB) as for most transistors of this value work.
By Ohm’s Law, IB= (VBB–0.7)/RB, if the given value of IB is needed, then it can calculate RB as: RB= (VBB–0.7)/IB.
To set the transistor in the saturation mode, the minimum value of IB should be the value that can generate maximum IC, where circuit conditions and value of β for that particular transistor is given.
In saturation mode, for maximum IC the value of IB must be 10% to 15% higher than the required minimum IB to compensate β value that varies from device to device.
Driving a BJT from a microcontroller
If the output pin of the microcontroller was VBB and the required load current, β value for that particular transistor, and its logic high value are known, then it is quite easy to calculate the value of RB.
There are some other things that to check to ensure the following:
- The calculated value of the base current must not be greater than the microcontrollers' drive current capability.
- The value of the load current should not be greater than the maximum IC.
- In saturation mode, the power dissipation must not go above the maximum power dissipation of that particular transistor.
- The VCC must not go over the value of the maximum VCE of the transistor.
Driving heavier loads using a Darlington
It is very rare that the value of the drive current of GPIO pin goes over 10 mA, and for a power transistor the minimum value of β hardly goes beyond 50, then a maximum of about 500 mA current can be controlled.
To control higher currents, an arrangement called Darlington can be used. A single package Darlington is available, or a Darlington can be made by using two transistors. In a Darlington arrangement, transistor 1 or Q1 is usually of low power, high gain, whereas transistor 2 or Q2 is of high-power. In a Darlington arrangement, large output IC can be produced by using a small input IB.
Conclusion
Using correct and accurate calculations will increase the lifetime of microcontrollers and transistors. It is always good to have a maximum current limit 20% higher compared to normal running ampere usage of the circuit. However, if motors are being controlled, then it should be six times higher than the normal running current since the starting current of the motor is much more compared to the normal running current of the system.
