N.C. active switch behavior - Page 1
Forum section: electronics / beginners

author: raff5184
author_rank: Regular Contributor
timestamp: November 23, 2020, 02:41:10 pm
content: Hi all,I'm trying to understand how this switch (515, NC) workshttps://www.ti.com/lit/ds/symlink/ts12a4515.pdf?ts=1606141469511&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FTS12A4515I connected the V supply to V+ and COM at the same time, and the load is supposed to go between GND and NC. Control input is between IN and GND.What I expect, for unloaded case, is that when V+=4.5V, then NC≈V+ for IN_low=0 and NC≈0 for IN_high=2.5V.But I have these three situations that confuse me1)V+=4.5VNC≈V+ always, independently of IN.2)If I choose V+lowerthan IN_high then it works as I expect.E.g.: V+= IN_high-0.5V = 2VIN_high = 2.5VNC≈0 when IN_high and NC≈V+ when IN_low3)If I choose V+ slightly higher than IN_high, the tON (time to open the switch) becomes very large, about 500ms and NC doesn't go to 0 completely when IN=0E.g.: V+=IN_high+0.5V=3VIN_high = 2.5VNC=1.4V when IN_high, and NC≈V+ when IN_low

author: jmelson
author_rank: Super Contributor
timestamp: November 23, 2020, 04:20:06 pm
content: In datasheet parlance, N.C. means No Connection.Your "load" need to go between COM and "NC". That's a particularly confusing way to write a datasheet, but that's what they have done.So, that is between pins 1 and 8 or 1 and 2, depending on the package you have.Jon

author: raff5184
author_rank: Regular Contributor
timestamp: November 23, 2020, 04:30:45 pm
content: Quote from: jmelson on November 23, 2020, 04:20:06 pmYes, N.C. vs NC was clear.The load can't go between 1 and 2 (or 8 ) because it would be in parallel with a short circuit. V supply goes to COM, positive of load goes to NC and negative of load to GND, so that load is powered by supply when IN=0

author: m k
author_rank: Super Contributor
timestamp: November 24, 2020, 04:41:43 pm
content: I understand it so that,COM and NC are a switch,V+ and GND are supply to switcher,IN high operates the switcher, 515 (NC) opens, 514 (NO) does the opposite.Manual page 7 also says that better keep V+ higher than IN.

author: raff5184
author_rank: Regular Contributor
timestamp: November 24, 2020, 05:39:19 pm
content: Quote from: m k on November 24, 2020, 04:41:43 pmYes that's correct. I keep V+ higher than IN but it doesn't work well

author: m k
author_rank: Super Contributor
timestamp: November 24, 2020, 06:18:09 pm
content: Are you connecting all at once?My guess is that supply must be ahead, no idea how far.

author: Vovk_Z
author_rank: Super Contributor
timestamp: November 24, 2020, 06:25:02 pm
content: But I have these three situations that confuse me1)V+=4.5VCOM≈V+ always, independently of IN.Couldn't it be that you confused pins (chip orientation)?

author: raff5184
author_rank: Regular Contributor
timestamp: November 25, 2020, 11:21:53 pm
content: ok, so I did few more tests with a new chip. Apparently I had a bad part. This one works better, but it doesn't turn off fast and I think this is the proper behavior. In the picture I changed the frequency of the IN (50Hz, 500Hz, 5kHz). At 500 the tails starts to be long, similar to the figure on pg 8 of the datasheet, and at 5kHz the switch is so slow it can't turn off completely before turning on again.Thanks for the answers they were helpful to understand