Frank Donald September 16, 4 Comments. IC Working. Thic IC consists of a eight NPN Darlington connected transistors with common Clamp diodes for switching the loads connected to the output. This IC is widely used to drive high loads such Lamps, relays, motors etc. A Darlignton pair is two transistors that act as a single transistor providing high current gain.
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In many situations in your projects with microcontrollers you can use a shift register like 74HC directly with LEDs and displays, but if you check the datasheet you will see that need to take care with the maximum current on each output and also with the total current to not damage the device.
These are important information that we normally do not care in our home projects including me in many times and I would like to explain you this issue in order you go further with more reliable and robust solutions! This overview will help you to understand the differences and the design alternatives using as examples the devices 74HC , ULN , UDN and also the BC transistor, but you can extrapolate all information to another devices configurations.
Did you use this instructable in your classroom? Add a Teacher Note to share how you incorporated it into your lesson. It is very popular for applications on LED displays but my intention here is only discuss its limit of current consumption and not all its funcionality. Let's see the specifications shown at datasheet related to Maximum Ratings :. Within these conditions if you apply one LED for each output port, probably you will go beyond these limits.
In this case, a standard red LED has a consumption of 10 mA what means 80 mA at total when 8 of them are used simultaneouslly in your project. Sure you can put a resistor to limit in 6 mA but certaily you will not like the result of brightness of each LED. Ok, now you are thinking already had seen dozen of projects including books that just use the shift register to drive directely some component.
I confess I did this too in another situations just to keep a more compact design, with less components and with lower costs. Maybe they survive due low duty cicle. But with an equipment you must keep working during all day long for a long time, I am not sure whether it will be robust and reliable enough in terms of its time life.
For these conditions you need to use some interface device that can work with larger currents as we will see in next steps. There are many applications for it such as solenoids, relays, LED display drivers, lamps, inductive load like small motors.
Note that in the images of simplified schematic and in pin configuration you only find the GND but none Vs pin. In fact you do not need any specific Vs pin because the ULN is used as a drain for the current.
When you apply a signal on a input port, the corresponding output port will be able to drain the current from a positive source. Look that LEDs are connected in common anode configuration and the ULN is used to drain the current in its output ports according with the input signals driven by 74HC In this case you can drain up to mA in each output port and keeping the 74HC in safe working conditions.
It is similar to ULN but with an opposite function and you can use it as a interface between 74HC and a device that needs a supply of a higher current to work. Note that in this case you have the Vs and GND pins because you must be connected to the power supply. Now when you apply a signal on a input port, the corresponding output port will be able to supply a higher current to next device.
You can see an example in the schematic shown in attached image. Look that LEDs are connected in common cathode configuration and the UDN is used to supply the current in its output ports according with the input signals driven by 74HC Again in this case you can drain a higher current in output ports and keeping the 74HC in safe working conditions. In the datasheet you can also find an interesting information about the duty cicle versus the output current.
And of course, the current can be higher if you decrease the number of output ports that are being used. This show us how important is to consider the number of outputs conducting simultaneouly, the duty cycle you are using in your application p.
In some applications this transistor can be used as switch for higher currents, p. To work properly as a switch, the transistor BC must be on saturation and you can calculate the resistor in the base as following:.
For example, if you need to drive a current of mA in collector and the gain of transistor is , it means the necessary current in the base is only 1. This presentation show you some ways to improve the robustness and reliability of some devices avoiding they work beyond their limits of design.
Personally, my preferred design is with common anode using ULN because is very simple, easy to find it in different markets and also have a lower cost comparison with UDN By lagsilva Follow. More by the author:. Hello, all!! I hope you enjoy it and that be useful. Add Teacher Note. This is a formal description of 74HC you can find on datasheet: "The 74HC devices contain an 8-bit, serial-in, parallel-out that feeds an 8-bit D-type storage register.
The maximum for each port is 8. In first view it is strange because we expect to see both pins! You can see an example of that in the schematic shown in attached image. The UDN is a 8-channel source drivers.
Typical loads include relays, solenoids, lamps, stepper motors, servo motors, LEDs. But there are Pros and Cons: Pros - Low cost - Availability very easy to find anywhere Cons - Need more space to be assembled on the board. Did you make this project? Share it with us! I Made It! Electronic Affirmation Mirror by bekathwia in Arduino. Reply Upvote.
Add the following snippet to your HTML:. Read up about this project on. In some situations using Arduino and microcontrollers, it is necessary to use a shift register such as the 74HC to directly trigger LEDs, LED arrays or displays, but if you check the datasheet, you will see that you need to be careful with the maximum current in each output and also with the total current so as not to damage any device. According to the datasheet of 74HC, this component contain an 8-bit, serial-in, parallel-out that feeds an 8-bit D-type storage register. This device is very popular for applications on LED arrays, for example, but my intention here is to just discuss your power consumption limit and not all of its functionality. Its Maximum Ratings are:.
ULN2803 current limit
It features common-cathode flyback diodes for switching inductive loads. A Darlington transistor also known as Darlington pair achieves very high current amplification by connecting two bipolar transistors in direct DC coupling so the current amplified by the first transistor is amplified further by the second one. The resultant current gain is the product of those of the two component transistors:. The seven Darlington pairs in ULN can operate independently except the common cathode diodes that connect to their respective collectors. The ULN is known for its high-current, high-voltage capacity. The drivers can be paralleled for even higher current output.