Confused about 74HC273 - Page 1
Forum section: electronics / beginners

author: carl.olson
author_rank: Newbie
timestamp: January 16, 2021, 01:02:49 pm
content: I played around with a 555 timer circuit and wanted to try hooking it up to something. I picked a 74HC273 from my collection, but I have never used one.I'm sure I hooked it up wrong once or twice, but it still seems to work (I'll buy another soon to be sure). A LED I hooked up acted funny, and I realized I needed to tie the unused input pins low.After that, the output seems stable, but it still doesn't work. The input and clock generally don't change the output, and the reset pin seems tied to the output. It also gets quite hot (measured around 50*C before stopping the power). I'm using a 5V USB wall charger for power btw.I searched around for an example circuit and found this:http://tok.hakynda.com/article/detail/59/sn54hc273-sn74hc273-octal-d-type-flip-flops-with-clearThis only makes me more confused. And recreating the circuit didn't work for me.1) Why doesn't he have the Vcc pin hooked up?2) Why is there a resistor on pin 11?3a) How can I prevent it from overheating? b) Do I need to limit the current somehow?4) Should I even be using 74HC chips for learning? Is there another line of basic components (gates, latches, counters, etc.) recommended?Sorry for all the questions.Carl

author: ledtester
author_rank: Super Contributor
timestamp: January 16, 2021, 01:55:05 pm
content: My answers... (corrections and comments welcome)1. Vcc should always be connected. It is possible that the chip is getting power via other pins. This SE answer shows how this can happen:https://electronics.stackexchange.com/a/394409/954882. The resistor is called a pull-down resistor. It keeps pin 11 LOW when the switch is off. When the switch is pressed pin 11 will be connected directly to the Vcc and thus be turned HI.3. Vcc and GND should always be connected. In addition, any unused inputs should not be left floating -- i.e. they should be either tied to Vcc or GND. Floating inputs are easily affected by stray electric fields which can cause them to oscillate and that can cause excessive power dissipation. Moreover, intermediate voltages on digital input pins are also a problem. So, make sure unused input pins are tied to either logic 0 or logic 1.Here's a short article on the perils of floating digital inputs. It also shows the use of a "pull-up" resistor with a switch:https://www.mouser.com/blog/dont-leave-your-pins-floating4. 4000 series CMOS might be better to start out with. It has a wider voltage supply range (up to 15V) and consumes very little power. 74HC also is very power efficient but is limited to 5V and operates at a higher speed which gives it less noise immunity. Being slower, CMOS logic also generates less noise on transitions.Here's a short summary of the differences between the two families:http://www.elecdude.com/2014/07/differences-in-cmos-4000-series-74ls-74hc-74hct.html

author: Terry Bites
author_rank: Super Contributor
timestamp: January 16, 2021, 02:17:34 pm
content: if you don tie all unused inputs to Vdd or groundit may be oscillating like crazy at umpteen MHzthat'll make it get hot.You can pull up or down with a wire or a resistor. Or connect them to Q from a previous stage.You cant have the D inputs flapping in the breeze. Make them 1 or 0..That link leads to the the work of a fool.You can omit the LED series resistors but when you feed the output such a pin to another input it wont work. A raw led will mess up the output logic levels big time. instead of 0 and 5 you'll get 0 and 1.7 which is crap. Stay well under 4mA per output pin.Buy some real books. The interweb is full of crap posted by morons. Start with the ART OF ELECTRONICS read, don't browse!

author: carl.olson
author_rank: Newbie
timestamp: January 17, 2021, 03:19:25 am
content: Thank you both, that is a lot of helpful information!Quote from: Terry Bites on January 16, 2021, 02:17:34 pmThat is good to hear. I've started with Practical Electronics for Inventors, but I'll pick up ART OF ELECTRONICS too for sure.

author: admiralk
author_rank: Regular Contributor
timestamp: January 17, 2021, 04:11:48 am
content: Never looked at it, but it is my understanding that AOE is more of a reference book. You would be better off with a standard text book in that case. If you are working with logic, the CMOS, or TTL, cookbook will help you more. They are not error free, so do not take what they say as gospel, but for the most part they are fine.

author: carl.olson
author_rank: Newbie
timestamp: January 17, 2021, 08:26:29 am
content: Quote from: admiralk onTodayat 04:11:48 amI'll grab all threeI'm in this for the long haul.@Terry BitesI tried again with your diagram and was able to get everything working. Just had to add a pulldown 50k resistor to my push-button clock. I also used my old HP power supply and was able to see the large power usage when inputs were left floating. It was a good learning experience!

author: Circlotron
author_rank: Super Contributor
timestamp: January 17, 2021, 08:36:59 am
content: My favourite for this kid of thing is a 74HC574. Basically the same as a '273 but with much nicer pinout. All the inputs on one side and the outputs on the other. Makes for a cleaner pcb layout. Even on a breadboard it looks better.