Practical circuit Relay Driver Using Bipolar Transistor

Practical circuit Relay Driver Using Bipolar Transistor
How to design a practical relay driver circuit using bipolar transistor? Here is a bit of a description that can be used as a guide in designing tips or relay drivers using bipolar transistors.

Relay is a component consisting of an iron core coil which will produce an electromagnet when the coil is energized by electric current. Electromagnet is then interesting contact mechanism that would link the normally-open contacts (NO) and normally open contacts-Closed (NC). Little explains, normally the word here means the relay in a non-active or non-energized, or relay coil is not energized. So normally-open contacts (NO) is a contact while Normal is not connected, and contact-Normally Closed (NC) is a contact while Normal connect.

symbol of the component SPDT relay (Single-Pole Dual-Totem) which means having a contact NO and a NC contact with a COMMON. When the coil is energized, the NC contacts will be connected to COM. If the coil is energized, the contacts will move from NC to NO,so will be connected to COM NO .


symbol of the component DPDT (Dual-Pole Dual-Totem) which has 2 NO contacts, 2 NC contacts with 2 of COMMON. There are also relay with 3-Pole and 3-Totem, but I do not think I need to show the picture, simply SPDT and DPDT which are often used in practice.

Relay Characteristics

Previously need I say that is not discussed in this paper about the AC relay, although there are some common characteristics, but it would be better if the AC relays are discussed in separate papers. God willing, if there is a chance I will discuss later.

Briefly, the characteristics of relays include voltage coil / coil. There is a 5Vdc, 12VDC, 24VDC, 36Vdc, 48Vdc to. If there is DC voltage relays coil besides the information shared via the comments please. Working voltage is the voltage to be supplied to the coil for the relay to work. In addition there is a characteristic ability to relay contacts. Can 3A, 5A, 10A, or more. That is the maximum current that can flow by the relay contacts are in accordance with its characteristics, so it could 3A, 5A, or 10A. Indeed, despite being forced to drain the larger currents are also not directly damaged. But that is not the right practice.

Here are some sample images DC relays are widely available in the market.


Relay driver circuit

Bipolar transistor is a component that works based on the presence or absence of flow in the foot triggers the base. In the relay driver applications, the transistor works as a switch that at the time did not accept the current triggers, then the transistor will be in the position of the cut-off and does not conduct current, Ic = 0. And when the base receives the flow triggers, then the transistor will turn into a state of saturation and delivers current.

The following image is a practical series of relay drivers that are reliable for use in microcontroler projects. Please listen carefully
Active component in the above series is 2 pieces of NPN transistors arranged in Darlington. Transistor serves as an electronic switch that will drain current if there is bias currents at the foot of its base, and would clog the flow of current if there is no bias on its base leg. Relay that can be used with this circuit is relay with coil voltage between 5Vdc to 45Vdc. If the relay is used above require working voltage 45Vdc, then replace the transistor C828 with transistors that have a greater work stress such as BD139, for example.

For small relays with a voltage of 5V - 24V, to further save costs, TIP31C transistor can be replaced by a C828 or similar NPN. For large relays, the transistor TIP31C is more than enough to activate the relay with a steady.

Here are a few examples of practical calculations (rather than theoretical, such as when a school or college) in the design of the relay driver circuit using a Darlington transistor.

First do the relay coil resistance measurement. For example here I use a SPDT relay 12V with a current capacity of 5A. From the measurement results of the relay coil resistance value amounted to 358 ohms (you probably will get a different value.) Thus, current is drawn at 12V / 358 ohms = 33.5 mA. So that the transistor should be able to generate currents at least 2-3 times larger than 33.5 mA, which is about 100 mA (in this example I use a multiplier factor of 3).

The transistor used was 2 pieces of type C828 NPN transistors are cheap and easily found in the market. Transistor C828 has a strengthening of the DC current (HFE) of approximately 130-520 times depending on the type of group transistor.Tapi than confused, we consider only the strengthening of the current is equal to 100 times. Transistor C828 has a VBE = 0.8 V.

Darlington transistors arranged so that the strengthening of the current is now 100 x 100 = 10,000 times. Furthermore, a minimum base current can be calculated by: Ib = 100 / 10000 = ± 10 Ua. If the VBE is worth 0.8 volts and the output voltage microcontroller logic 1 value 4.8 volts, then RB can be calculated as follows: RB = (4.8 to 0.8 - 0.8) / 10E-6 = 320000 ohms.

In the circuit used in RB with size of 100 kilo ohms, so the value is Ib = Ib (4.8 to 0.8 - 0.8) / 100000 = 32 UA. Installation of 1N4002 diode prevents transient currents generated by the relay coil.

Tips For AT89S51/52/53/8252 and AVR Microcontroller

The series of relay drivers above are very practical and proven success in various series of controls that had been my design. If used on the AVR microcontroller, the circuit will work very well without any problems, because at the time of the RESET, all I / O ports AVR is at the high-impedance state.

Unlike the case when used in microcontroller AT89S. At RESET, all I / O ports of MCS-51 located on the High logic 1, so long as RESET, the relay will be active for a while. To that end, the program must set the initialization of I / O ports that are used to trigger the relay to the state Low. Another solution is to use a better driver is active-low relay.

Another solution if you drive many relay applications, the ICs can be used ULN2003A, which has 7 pieces ready Darlington transistors used to drive relays.

For information, a series of drivers on I used to drive contactors 48Vdc with current capacity of 200A at the application controller electric forklifts.

So in the above series is suitable for use on applications using the microcontroller because the current source port I / O microcontroller typically only 20mA only.