TVS Input protection and grounding - Page 1
Forum section: electronics / beginners

author: Sudo_apt-get_install_yum
author_rank: Regular Contributor
timestamp: October 29, 2020, 09:06:23 am
content: Hey everyone!I’m working on a small outdoors project that will collect sensor data from different outdoors locations. The enclosure is an of the shelf weatherproof metal box so I will earth the enclosure for safety reasons.The sensor board I have made uses 20VDC as supply and steps it down to different voltages on the board, the internal voltages are not isolated.I’ve had some issues so I added some protection (and minor filtering) to the input and have tested it and it seems to work, but I’m worried it will cause problems since some components are referenced to earth instead of DC-ground.I would appreciate any feedback!Below is the protection schematic.SMBJ25CAhttps://www.digikey.com/en/products/detail/semtech-corporation/SMBJ36CA/10257787SMBJ300ATRhttps://www.digikey.com/en/products/detail/smc-diode-solutions/SMBJ300ATR/5993641

author: brabus
author_rank: Frequent Contributor
timestamp: October 29, 2020, 10:03:34 am
content: If the 20V are isolated from earth, I would keep them isolated all the way inside the cabinet, and connect only the cabinet to earth.Alternatively, you can connect the ground potential of the 20V power supply to earth and ditch the protection circuit altogether: you wouldn't need it anymore.The schematic you drew basically captures any AC voltage present between the power supply and the earth, and transforms it into a common mode voltage superimposed on the DC. This can cause functional problems on the supplied circuits, especially if they need to communicate with other modules.The TVS diodes could even find themselves on permanent conduction (and fail), in case the potential difference between earth and the power supply output is permanently over their threshold voltage.Golden rule with the TVS: keep in mind that they fail short. You can mitigate the short circuit using a fuse or, a thermo-fuse or even just a resistor.Can you give us more details regarding the power supply and the powered circuit? Which kind of issues have you had in the past, without the protection?The topic is actually complex also in the industry, it is very interesting to analyze every bit and formulate a good solution together.

author: Sudo_apt-get_install_yum
author_rank: Regular Contributor
timestamp: October 29, 2020, 12:52:34 pm
content: One of the problems is that different power-supplies should be compatible with the circuit. So in short it should be compatible with isolated and non-isolated power supplies ranging from regular "household" power supplies to central DC supplies.The clamping voltage of the bidirectional (D3) is 36V while the clamping voltage of the diodes connected to earth is 300V (D1 &D2).The though being that D3 would control the majority of the voltage spikes on the power rail and D1 & D2 would take sudden high voltage spikes on either the power or ground rail.The powered circuit is a microcontroller (STM32) and sensors that are powered by a switch mode DC/DC and some linear regulators for the other low-power power rails. The main power draw is supplied by the switch mode DC/DC.Some issues have been high frequency noise and in one case "too" high voltage spikes. Not sure what voltage it was but at least >60V.The DC/DC used can handle up to 48VDC and has built in protection for high voltage peaks adn reverse polarity.I’ve done some reading online and it seems to be a complex question with lots of solutions like you say!Thanks for the input!

author: Sudo_apt-get_install_yum
author_rank: Regular Contributor
timestamp: October 29, 2020, 03:57:18 pm
content: I’m not sure if this is a better solution for my problem but eliminating D1 & D2 would remove the problem that would "short" the supply to ground if the DC supply was floating above the threshold voltage of the TVS diode.Implementing an LCR low pass filter with a high "R" value would still pass low frequency and filter out the unwanted high and reference the DC ground to the earth.The problem with this circuit is that there would be no protection against a high voltage spike on the ground pin and no return to earth, but I’m not sure if it is needed. D3 still protects against voltage spikes on the DC power rail.Below is the circuit and filter results.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 30, 2020, 10:37:52 pm
content: Start by thinking what you want to protect against.For outdoors, and especially buried cables, induced currents from nearby lightning strikes are a mayor factor.For something like that your TVS diodes will get blown to bits because of sever overload and your inductors will just arc over.Effective protection for such requires careful design, though the parts do not have to be expensive.Recently I read a tip about putting a few knots in the cables and use that as inductors.I have not used this method myself though.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: November 02, 2020, 02:42:55 am
content: Quote from: Sudo_apt-get_install_yum on October 29, 2020, 03:57:18 pmIf you have a flying power supply or have a ground connection, the best solution is still to use an optocoupler to decouple to external discrete sensors. If you use sensors that supply the protocol, such as the 1-wire in the DS18B20, the sensor GND can be linked to the power supply GND, the 1-wire via a resistor is fed to the MCU input, and the GND via a varistor to limit bursts. But at the same time, you need to supply the GND circuit via a 1MOhm resistor and a small capacitor to the earth (PE).

author: Sudo_apt-get_install_yum
author_rank: Regular Contributor
timestamp: November 04, 2020, 11:31:22 am
content: Quote from: Doctorandus_P on October 30, 2020, 10:37:52 pmI'm not concerned about protecting against lightning, but will look into the "knot as inductor"!The voltage spikes I have noticed most likely come from "high power" pumps/ motors, I am aware that pumps in some cases are connected to the same supply as the sensor box I'm building.I'm just concerned that the TVS diodes are going to do more harm than good and just result in a short circuit if the DC ground is floating high enough. The solution inReply #3might possibly be better in regards to "compatibility". It would help prevent a large potential between earth (case) and the DC ground while still passing the high frequency components to earth while maintaining some protection between the DC supply and DC ground.The issues haven't been major but would still like to address them, I just don't want to cause more potential harm than good!