Interchanging GND and Power pins - Page 1
Forum section: electronics / beginners

author: Chillance
author_rank: Contributor
timestamp: July 04, 2020, 01:13:26 pm
content: Hey!So, I wanted to expose pins to a header for OLED display modules. I noticed it's common with 8 pins so I decided to go with that.One issue I noticed though is that there really is no standard, but that isn't much of a problem for most pins as that can be fixed in software.However, for GND and VCC (3.3V) I noticed that sometimes it was GND on pin 1 and VCC on pin 2, and sometimes VCC on pin 1 and GND on pin 2.So I thought, maybe I can do a simple solution where one can easily change this around as needed. I came up with the attached schematic.Now, I think this can work. However, I'm really aware of shortcut danger that could happen here. The TMUX154E used here has break-before-make so that should be fine. Also it supports 64mA continuous load so that should be enough for the OLED modules.I'm posting to see what you experts here think about this and what you would do to improve the safety here? Obviously the one connecting the module need to make sure the pins are correct. The idea here is to make it possible to use modules that shift those pins around, but still make it as safe as possible. Since I'm essentially shifting input and outputs I'm not entirely sure what one can do here for safety.Thanks!

author: tggzzz
author_rank: Super Contributor
timestamp: July 04, 2020, 01:19:44 pm
content: What's the impedance of the switches in the TMUX154E? How will that affect the voltages "seen" by the display?What are thefailuremechanisms, e.g. if the jumper falls off or the contacts become poor?

author: Chillance
author_rank: Contributor
timestamp: July 04, 2020, 01:32:21 pm
content: Impedance? Good question. Maybe 6 ohms? That's the ON-state resistance from the datasheet.I don't necessary have to use this particular one. I was also looking at TS3A5223 that has 300mA continues limit on the I/O pins.Failure mechanism of the TMUX you mean? Yeah, that is a very good question.But yes, I certainly want to do something to safe guard shortcuts here.

author: ledtester
author_rank: Super Contributor
timestamp: July 04, 2020, 01:34:20 pm
content: What about using some sort of keyed connecter - i.e. a connector that can only be plugged in one way? They are often used for DC power.https://www.hobbyrc.co.uk/pair-of-jst-2-pin-connectors-with-140mm-of-wire

author: ledtester
author_rank: Super Contributor
timestamp: July 04, 2020, 01:40:28 pm
content: Not a keyed connector, but still a time-saving idea for someone who knows what they're doing...https://www.youtube.com/watch?v=eI3fxTH6f6I&t=2m17s

author: Chillance
author_rank: Contributor
timestamp: July 04, 2020, 01:48:24 pm
content: Hey, thanks ledtester for the response. Well, if someone uses wires they can just exchange those connectors so that should be fine. The problem arises if one want to plugin the LED/OLED module directly into the header I will expose here.

author: tggzzz
author_rank: Super Contributor
timestamp: July 04, 2020, 03:31:56 pm
content: Quote from: Chillance on July 04, 2020, 01:32:21 pm1) and the consequences of 6ohms are?....2) failure of "J?" the jumper

author: Chillance
author_rank: Contributor
timestamp: July 04, 2020, 04:51:23 pm
content: You mean the SEL jumper? I don't see how that could be much of a problem. That is just for either selecting A0/B0 or A1/B1.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 04, 2020, 04:56:24 pm
content: I would use a small PCB mount mechanical DPDT switch instead, such ashttps://www.digikey.com/product-detail/en/c-k/JS202011SCQN/401-2002-2-ND/2094299. Remember to document the hazard of the wrong polarity on the PCB silk screen.Break before make is irrelevant. Changing the polarityduring operationblows up the display anyway, as does choosing the wrong option - and one of them is always wrong. Ithasto be correctly selected before operation, like those 115/230V selector switches on some switch-mode power supplies before the era of active PFC where the choice in 230V countries was quite important to get right. I think most people nailed it.

author: rstofer
author_rank: Super Contributor
timestamp: July 04, 2020, 05:02:02 pm
content: I would just use 2 ea 3-pin headers and a pair of jumpers.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 04, 2020, 05:06:09 pm
content: Quote from: rstofer on July 04, 2020, 05:02:02 pmThis has the added benefit that the product can be shipped with no jumpers in place, or with jumpers in some "neutral" state, so that neither GND nor VCC is connected to the display, forcing whoever connects the display to make the (hopefully informed) choice between the polarities, which would require looking at the manual (or silkscreen explanation).The problem with OP's single jumper, or my DPDT switch suggestion is that it ships in a certain position and having to select the correct polarity is easily missed.

author: tggzzz
author_rank: Super Contributor
timestamp: July 04, 2020, 06:35:38 pm
content: Quote from: Chillance on July 04, 2020, 04:51:23 pmAs someone that fiddles around inside old equipment, I know that any mechanical contact can become a problem over time, or where vibration/shock/stupidity is present.

author: Chillance
author_rank: Contributor
timestamp: July 04, 2020, 11:34:06 pm
content: Thanks everyone for your responses. Seems like the more safer option is just to let it be in one way and if it needs to be changed, they then have to use wires instead. Or just use some other module...I had a feeling this would be hard to make safe, but I thought if anyone knew a solution, it would be here in this forum.

author: Chillance
author_rank: Contributor
timestamp: July 05, 2020, 01:44:23 pm
content: Actually, putting fuses there could help? That way it works until it shortcuts... right?

author: tggzzz
author_rank: Super Contributor
timestamp: July 05, 2020, 01:54:57 pm
content: Quote from: Chillance on July 05, 2020, 01:44:23 pmSemiconductors protect the fuses, not the other way around.

author: Chillance
author_rank: Contributor
timestamp: July 05, 2020, 07:24:45 pm
content: I'm not sure I follow. If I connect a fuse between +3.3V and GND, the fuse would trigger and break the connection (after 100ms)? I'm thinking a PTC reset fuse could help protect the whole thing if somethings is connected in a wrong way. For this example I'm thinking just having it to trip at 100mA.

author: tggzzz
author_rank: Super Contributor
timestamp: July 05, 2020, 07:37:46 pm
content: Quote from: Chillance on July 05, 2020, 07:24:45 pmAnd what happens to a semiconductor in that time?Don't forget you would also have to account for the effect of the fuse's resistance in normal operation.

author: tkamiya
author_rank: Super Contributor
timestamp: July 06, 2020, 08:24:26 pm
content: You can simply use a diode bridge. Then whatever comes in will be directed accordingly. Of course, you will have to account for voltage drop of twice the 0.7 volt.

author: Chillance
author_rank: Contributor
timestamp: July 07, 2020, 01:28:38 pm
content: Oww, diode bridge seems promising! And I'm already using diode IC with low voltage drop. That could work maybe!

author: Chillance
author_rank: Contributor
timestamp: July 07, 2020, 07:55:39 pm
content: Hmm, or maybe not as the diode bridge is for having the VCC and GND switchable, but coming out the same. That means that the header pins wouldn't switch...