Connecting Power then Ground or Ground then Power? - Page 1
Forum section: electronics / general

author: logictom
author_rank: Unknown
timestamp: November 07, 2015, 05:11:07 am
content: Does this make a difference, and if so, why?I was reading Adafruit's page about the neopixels that said if you had to connect them whilst powered you should connect ground then power and started wondering if I should be paying any attention.

author: Richard Crowley
author_rank: Super Contributor
timestamp: November 07, 2015, 05:17:59 am
content: I cannot think of ANY situation where you would NOT connect ground (and/or "return") FIRST.And if you are using any voltage > 50V, it is downright dangerous (and potentially fatal) to connect the power first.Well, I take that back. The ONE situation where the recommendation is to connect the power first is when "jumping" a vehicle battery.You would connect the +12 (red) to each side FIRST, and THEN connect the ground clips.That prevents "arc-welding" with the +12V clip if you accidentally touch the chassis of the vehicle.

author: retrolefty
author_rank: Super Contributor
timestamp: November 07, 2015, 05:41:16 am
content: Quote from: logictom on November 07, 2015, 05:11:07 amI think the key part is the 'whilst powered' part. That is termed 'hot swapping' by some meaning a circuit is being either unplugged/un-powered/ or plugged/powered into a larger system that is already powered on.I worked on several industrial control systems. Some did not allow hot swapping at all, meaning the whole system was required to be powered off before removing and replacing boards or subsystems and of course also means there is no issue with which power connections are made first or last or at the same time.Those systems that did support hot swapping, one could usually find that the card edge ground connector pads or pins would be a little longer then all the rest of the other connector pads or pins so the power ground would be first to make and last to break on insertion and removal respectively.Keep in mind that this deals with more then just the ground and power pins/pads but also all the input and output signals of a module that is being 'hot swapped' into a live system.Our rule at work was unless the OEM manual specifically stated that 'hot swapping' was allowed that we would never attempt it and I saw equipment damaged and/or at halted by a few that failed to follow that rule.

author: helius
author_rank: Super Contributor
timestamp: November 07, 2015, 06:11:47 am
content: Correct, when components are designed to be hot-plugged or hot-swapped, they need to make contact with ground first, and disconnect ground last. Otherwise the stray voltages on isolated devices could force high currents to flow through data pins, damaging their transistors. Incorrect power sequencing can also cause latch-up. Does it matter if the power pin makes contact before ground, after all, no current can flow in a circuit without a return path? I think the reasoning for not doing that is that parts of the chassis could be grounded, so it could already be rubbing against something when the pins are in position to connect. You don't want any devices to start turning on with a current path through the EMI shielding.

author: logictom
author_rank: Unknown
timestamp: November 07, 2015, 07:24:21 am
content: I hadn't thought of some of these things, but I realise now that I assumed some things about the Adafruit statement and that I didn't really specify the case I was thinking of when I posted the question.Quote from: Richard Crowley on November 07, 2015, 05:17:59 amI work with embedded systems so 5V is usually as far as I go, I used to work with EVs with 150V battery packs but like above that was positive first.My assumptions were this is a low voltage isolated system without any data connections - so I assumed the Adafruit page was describing connecting a neopixel to a power source without the data pins connected - in this case does it matter? Assuming no shorts to any pins/connections etc.

author: rs20
author_rank: Super Contributor
timestamp: November 07, 2015, 08:32:17 am
content: Quote from: logictom on November 07, 2015, 07:24:21 amThere's an IC in the neopixel that has 3 connections: VCC, Ground, and Data. Your question boils down to: can something bad happen if I have data and VCC plugged in, but not ground? The answer: yes. Here's why:IC's typically have parasitic or ESD protection diodes that sit between the IC pins, and power and ground. So, a diode running from every IC pin up to the Vcc rail, and a diode running from the ground rail up to every IC pin. Normally, these pins are kept reverse-biased, since there's 5V between Vcc and ground, and the data pin is some intermediate voltage.Now, suppose that VCC is 5V and Data has 0V. The ground line in the neopixel can't sit at 5V, since there's a diode running from the ground line up to the data line. So the ground line is pulled down to 0.7V (diode drop) by the data line, and the neopixel will attempt to power itself through that one tiny diode that was never designed to carry any current at all. The diode gets very hot, melts into a short circuit, and now your neopixel is dead, the data pin is permanently crowbarred to ground.(Red represents current flow)EDIT: Fixed broken image link.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 07, 2015, 09:58:48 am
content: Also ESD, a strike to VCC of which might be tolerable (depends on how it's routed and bypassed near the connector), but a strike to any data/signal pin will be somewhere between risky and deadly, unless also protected for that kind of service. (IC pins are not usually rated for that kind of stress: a typical rating is 2kV HBM, whereas the most common test is 5kV (direct) IEC 61000-4-2, much more dangerous.)USB, for example, cannot be made to withstand ESD strikes directly -- sufficient protection would spoil the bandwidth (USB 2/3 speeds). The connectors are very carefully designed to have exposed metal shielding as the most prominent feature, preventing strikes to the sensitive data lines. Ditto HDMI, IEEE1394 and other similar signal standards.Tim

author: logictom
author_rank: Unknown
timestamp: November 07, 2015, 11:22:57 am
content: Quote from: rs20 on November 07, 2015, 08:32:17 amActually I was asking the opposite, without the data lines. I was aware of the issue with power and data but never looked at the why.Is the orange link between 5V and the gnd the current path through the workings of neopixel to ground back through the diode? If so, I've got it now, if not could you elaborate?Quote from: T3sl4co1l on November 07, 2015, 09:58:48 amIs this in the case of power and data lines being driven or is this an issue with just power?Quote from: T3sl4co1l on November 07, 2015, 09:58:48 amWhat about protection like thesehttp://www.semtech.com/cn/esd-protection/USB-ESD-Protection/index.htmlor do you mean at the chip level?

author: rs20
author_rank: Super Contributor
timestamp: November 07, 2015, 12:09:46 pm
content: Quote from: logictom on November 07, 2015, 11:22:57 amWhat? Just the VCC line connected and nothing else? Kirchoff's current law states that a closed circuit must exist for current to flow; with only one line no current flow or damage can occur. Having said that, ESD doesn't follow those rules.QuoteCorrect!

author: logictom
author_rank: Unknown
timestamp: November 07, 2015, 12:20:29 pm
content: Quote from: rs20 on November 07, 2015, 12:09:46 pmWell that's what I thought but I started wondering if there was something obvious I was missing because why would someone plug in the data lines before power and ground if they were plugging into a live system... If I were plugging in hot it would go ground, power then data, but usually it's a power down if there's anything that could cause problems.You know that worrying feeling when you think you're missing something blindingly obvious, that's what was kicking in after reading that with the above also going around my head.Quote from: rs20 on November 07, 2015, 12:09:46 pmGotcha

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 07, 2015, 03:36:20 pm
content: Quote from: logictom on November 07, 2015, 11:22:57 amESD is a separate issue from lines being driven. ESD is a property of the unplugged component (as a whole) being charged to a randomly high voltage. You don't want the device's static charge being dumped through signal lines. Once that energy has dissipated, everything is fine. So, it's best to allow it to dissipate through a solid ground shield.QuoteEither. A device which does not use additional protection devices (like those) is at risk, and has all the more reason for good grounding and shielded connectors that shunt ESD.Even with, those diodes are so small that they really don't withstand too many strikes before being questionable (their survival might be in the 10s to 100s of strikes). Higher bandwidth signals (PCIe, perhaps?) simply can't tolerate even that little capacitance, and therefore must be kept internal (as far as I know, PCIe can actually be brought out on a ribbon cable for external use, except for this hazard).Tim