Choosing RC circuit components. - Page 1
Forum section: electronics / projects

author: MarkS
author_rank: Unknown
timestamp: August 22, 2024, 05:55:44 pm
content: Is there a functional difference between the two circuits shown in the pic? I need a 1ms POR for the preset and preclear inputs on a 74HC74. Both of these, with the values shown, will do this. However, which is preferable and why?

author: Kim Christensen
author_rank: Super Contributor
timestamp: August 22, 2024, 06:06:31 pm
content: R6/C4 combo is the sane choice since you're driving a CMOS input.R5/C3 will draw extra current on power up. Though 50mA peak is not excessive, the low R makes C3 unnecessarily larger. R5/C3 is something you'd use with a 74xx or 74LSxx TTL series IC, since their input current will influence the RC time constant a lot more than a CMOS chip.

author: MarkS
author_rank: Unknown
timestamp: August 22, 2024, 06:33:11 pm
content: I just saw a mistake. C4 should be 1uf, not 1K.Looking at the datasheet, the input current rating is +-1uA. So, yes, I see your point.

author: PGPG
author_rank: Frequent Contributor
timestamp: August 22, 2024, 06:52:52 pm
content: Quote from: MarkS on August 22, 2024, 05:55:44 pmI am lost with these capacitance specification system.I understood that 10 is 10uF - OK may be someone agreed that unnamed unit is 1uF (1F supercap will be 1M?)But what with 1K?If you want to have 1ms than for 10k capacitor should have 100nF.I can't imagine how 1K could mean 100nF so my answer is: YES - there is functional difference between these two circuits.After your correction that 1K -> 1uf I am more lost.So what is 10 if 1uf needs to get uf than 10 without nothing is?And still 1u*10k=10ms so two circuits have 10 times functional difference between them.

author: MarkS
author_rank: Unknown
timestamp: August 22, 2024, 07:35:20 pm
content: The '10' should be '10u'. Just a brain fart on my part. Same as the '1K' capacitor. And you're correct. The 10K resistor should be 1K, probably the reason why I mistakenly labeled the capacitor as '1K'. I just threw this together really fast in KiCAD and took a screenshot. I didn't put enough thought into it. Sorry about that.

author: Kim Christensen
author_rank: Super Contributor
timestamp: August 22, 2024, 08:39:15 pm
content: I ignored your "1K" capacitor typo and went by the R value and assumed a correct C value for your 1ms time constant. Either way, a higher R value is perfectly OK for driving a CMOS input. Personally, I would use 10K and 100nF rather than 1K and 1uF.

author: bostonman
author_rank: Super Contributor
timestamp: August 23, 2024, 02:28:01 am
content: The 74HC74 is a D flip flop.Not sure if this was mentioned, but believe you need to drive this with a Schmitt trigger because the slow charge/discharge may cause unknown states if going directly to the flip flop.

author: MarkS
author_rank: Unknown
timestamp: August 23, 2024, 04:41:03 pm
content: Quote from: bostonman on August 23, 2024, 02:28:01 amThank you! I hadn't considered that. However, the RC circuit will only be setting the preset and preclear to a known state on power up. Is this an issue in this case?

author: Kim Christensen
author_rank: Super Contributor
timestamp: August 23, 2024, 05:58:32 pm
content: TI does recommend a Schmitt buffer forthe 74HC74if the signal has a slow rise time. They do also make aSN74HCS72which has built in Schmitt buffers, but it is slightly different. (Positive edge trigger vs negative edge)

author: MarkS
author_rank: Unknown
timestamp: August 23, 2024, 09:04:00 pm
content: Thank you. I saw that after I did a crazy thing and read the datasheet.I also have another RC circuit initializing a couple of timers to a start up state through their load pins. As such I added a 74LVC2G17. Kill two birds with one stone.

author: ArdWar
author_rank: Frequent Contributor
timestamp: August 23, 2024, 11:56:47 pm
content: Quote from: Kim Christensen on August 23, 2024, 05:58:32 pmWhy flip the signal if 74HCS74 exists? HCSis HC with Schmitt input for the entire family.

author: MarkS
author_rank: Unknown
timestamp: August 24, 2024, 02:07:20 am
content: Quote from: ArdWar on August 23, 2024, 11:56:47 pmI did not know that was a thing!