Will the TVS diode protect the DC line from AC power? - Page 1
Forum section: electronics / beginners

author: Glebiys
author_rank: Newbie
timestamp: October 13, 2019, 07:28:23 am
content: Hi,I have a line with a voltage of 5V DC. It is possible to supply high voltage AC (230 V) to this line.Will the TVS diode protect the DC line from AC?My TVS:https://datasheet.lcsc.com/szlcsc/Littelfuse-SMBJ5-0A_C83333.pdfThank you!

author: SeanB
author_rank: Super Contributor
timestamp: October 13, 2019, 08:21:01 am
content: Yes and no. If the impedance of the supply is high, the diode will provide quite effective protection, but for a direct connection the diode will rapidly exceed it's SOA and will explode, no longer providing protection. Best solution is to have enough series resistance on the input ( if it is an input) to provide that limiting, preferably with a resistor that is fusible or is capable of dissipating the power it will have to handle.

author: Glebiys
author_rank: Newbie
timestamp: October 13, 2019, 08:57:23 am
content: @SeanB,Thanks for the answer!My circuit:here is a series resistor with a resistance of 330 ohms and a power of 1/8 W (SMD size: 805). I want to increase power to 1/4 watts.Space for large resistors is not enough.330 kOhm is this normal?It is also necessary to make a small track width from the relay to the TVS diode so that it acts as a fuse.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 13, 2019, 10:52:29 am
content: It's just an LED. No need to baby it!Split the series resistor in half. Make the first half really big (pulse rated, preferably a larger size, maybe as big as a wirewound power resistor). Put a TVS in the middle, or maybe even an MOV.This gets a survivable LED and TVS, but the front resistor may not handle it. If it blows out instantly, the circuit's dead either way. Current limiting would be nice.Consider using a PTC for the first resistor. These are available in ceramic types that handle much more voltage than the polymer fuse types. It's possible that it can heat up and partially open the circuit before the TVS or PTC fails.Note that you can offset some of the PTC's voltage drop by choosing a higher TVS rating. If your resistors are sized for nominal, 20mA or whatever, at 5V, then the LED is straining at maybe 20-30V (~100mA). Not that that's much savings out of a 230VAC supply, but 10% isn't nothing.To get really fancy, you might consider a current limiter using depletion mode MOSFETs like DN3545 or LND150; there is a two-transistor connection that allows bidirectional current limiting with one resistor to set the current. Then you can use a bigger MOV at the front end to protect against not only unreasonable voltages like mains, but surge as well. (In this case, you could remove the inside TVS and series resistor, and use an antiparallel diode (say 1N4148) across the LED to protect it against reverse voltage. The MOSFETs will limit current to a ~fixed value, from just a few volts all the way up to their rating.)The last option of course is the most expensive, but also the most reliable. Shop around.Tim

author: Glebiys
author_rank: Newbie
timestamp: October 20, 2019, 12:46:48 pm
content: Tim, thanks for the answer!I'm sorry for the late reply.I really like the last option, in the near future I will consider its implementation