What is the easiest way to protect PCB input/output? - Page 1
Forum section: electronics / beginners

author: crgarcia
author_rank: Regular Contributor
timestamp: May 08, 2023, 05:00:42 pm
content: Hi all!First of all, I am a very very beginnerAs a hobby project, I want to build a control panel to turn on/off the devices in my lab desk (Oscilloscope, PS, LED lights, etc).The panel will have 10 buttons with LED, and some relays:I am trying to learn what protections I should add to my PCB at every input/outputSearching the web, I found many examples like this one (link)Is there a simpler way to do that? Like a multi-channel IC with all those components in place?Thanks!

author: wasedadoc
author_rank: Super Contributor
timestamp: May 08, 2023, 05:08:39 pm
content: A simpler way is to not use any ICs. Latching push buttons switching power on-off to the relays and LEDs.

author: ataradov
author_rank: Super Contributor
timestamp: May 08, 2023, 05:28:42 pm
content: This circuit is an overkill in most cases, especially if you are not in an electrically noisy environment.In most regular cases, the series resistors you already have will be sufficient. They will limit the current though the clamping diodes inside the device.There are dedicated multi-channel ESD protection ICs, but they usually have 4 channels at most, so you would still need a few of those.You also need to take into account your application. If you are working on a mass market product you are going to ship around the world, then sure, take all the precautions you can.But if you are working on one-off device for your lab, there is no need to worry. In an unlikely event that you actually manage to damage the IC, you can always modify the circuit later.

author: crgarcia
author_rank: Regular Contributor
timestamp: May 08, 2023, 06:06:39 pm
content: Quote from: wasedadoc on May 08, 2023, 05:08:39 pmyou are very right! ... just between you and I... if I am honest with myself, I don't think I really need this device... but please don't tell anyoneMy excuse to convince myself is that I can turn everything off remotely, or that I can build scenes (like, "Soldering" that will turn on the solder, the fumes extractor, etc, or "Debugging" that will turn on the PS, Oscilloscope)Quote from: ataradov on May 08, 2023, 05:28:42 pmThank you for the tips! I fully agree... I think it is more of a learning exercise for me... I trend to over complicate all my PCBs on the name of learningIf you would have to build a mass production industrial PCB. What would you use?Thanks a lot!

author: ataradov
author_rank: Super Contributor
timestamp: May 08, 2023, 06:13:55 pm
content: Quote from: crgarcia on May 08, 2023, 06:06:39 pmDepends on the requirements. You don't just build something out of the blue. You define the requirements on what tests your device should pass and build accordingly.The circuit shown is not a bad start. If you need more integrated solution, then look herehttps://www.mouser.com/c/circuit-protection/esd-suppressors-tvs-diodesBut this would still only provide an array of diodes, any current limiting and filtering would still have to happen outside. You will save some board space, but not much.

author: srb1954
author_rank: Super Contributor
timestamp: May 08, 2023, 10:28:44 pm
content: Quote from: crgarcia on May 08, 2023, 05:00:42 pmThe most likely problem you will have is ESD discharge to the push-buttons. The simplest and most effective protection against this is to mount your push buttons in a grounded metal panel so that any ESD discharges are direct to the metal panel and any discharge currents are diverted away from the circuitry and into the ground. You need to ensure that the ground connection wiring to the metal panel does not share any common path with the logic circuit grounds and is wired directly back to mains earth or circuit common. ESD pulse have very fast rise times and moderately high currents so any shared circuit path might have sufficient resistance and/or inductance where sufficiently high transient voltages can be developed to damage your logic circuitry.A couple of other points on your circuit design:(1) The switch commons should be wired to ground so that the MCP23017 I/P is pulled low when a switch is activated. The MCP23017 has internal pull-ups so, with the current circuit, when the switch is open there is no way for the I/P to be pulled low. Reversing the sense of the switches pulls the I/Ps low when the switch is activated and the internal pull-ups will return the I/P high again when the switch is deactivated.(2) The internal pull-ups in the MCP23017 have to be S/W enabled so remember to incorporate this in your start-up S/W. Even with the internal pull-ups activated the pull-up current is relatively low (40 - 115uA) and this may not provide enough current to ensure reliable long-term operation of the switches. Switches need a reasonable "wetting current" to ensure that any contaminants on the contacts are burnt off the switch contacts when activated and released. Check the switch data sheet to see if it has any recommendations for minimum operating current and add an external pull-up resistor to supply this. In the absence of any specific information on the switch data sheet I would recommend a 4k7 pull-up providing approx 1mA of wetting current.(3) You need to have current limiting resistors between the O/Ps of the ULN2803 and the LEDs if such resistors are not built into the switch assembly. If your desired LED current is less than 6mA you can dispense with the ULN2803 altogether and drive the LEDs direct from the 74HC595 O/Ps. You will still need current limiting resistors though.