High Voltage MUX (conceptually) - Page 1
Forum section: electronics / beginners

author: LoveLaika
author_rank: Frequent Contributor
timestamp: December 15, 2020, 07:08:04 pm
content: I'm designing a new version of a circuit, and one part of the circuit baffles me. Below is a schematic of the part, using just simple parts to showcase the problem.The idea behind the circuit is that you have six voltages, and you select one of them (or ground) by jumping the appropriate pins physically on a 2x7 header. This doesn't seem like a big deal when the voltages are small, but the problem here is that the voltages are high voltages (ranging from 100 to 500 volts max). The original design used a 2.54mm pitch 2x7 header, with the rows and pins separated by 2.54mm. According to the website below, for a max of 500 volts, 2.5 mm is just at the limit (yes, the traces are in open air, not on an inner layer, and uncoated). Is it safe to continue with this design? I can't find another header with a higher pitch (unless I go for individual rows and jump them that way rather than getting a single 2x7 header).https://www.smps.us/pcbtracespacing.html

author: PartialDischarge
author_rank: Super Contributor
timestamp: December 15, 2020, 07:54:31 pm
content: Is it installed in a clean environment? If yes, then it seems ok to me, specially beacuse of the 1meg resistors.Also it is ac or dc?Where does the source of this voltage come from?

author: LoveLaika
author_rank: Frequent Contributor
timestamp: December 15, 2020, 08:36:02 pm
content: Thanks for replying. I'm not sure what you mean by a 'clean environment', but I would assume so. At the least, the board is enclosed.The voltage is DC, and it's coming off-board from external power supplies. The high voltages are not generated on the board itself.

author: PartialDischarge
author_rank: Super Contributor
timestamp: December 15, 2020, 08:49:51 pm
content: I’m referring to the pollution degree. Not everything is creepage or clearance. Dirt or dust in the surfaces affects safety distances a lot. If dust is expected then distances should be higher. But in this case overall seem fine.

author: LoveLaika
author_rank: Frequent Contributor
timestamp: December 15, 2020, 09:13:49 pm
content: Thanks for replying. Given the application, there should ideally be no dust or dirt or debris on the surface. Glad to know that 2.54 mm is okay.Considering the resistors, if the max is 500 volts, then with the 1M resistor, the HV can only supply a max of 0.5 mA. Given that amount of current, would 10 mil traces be adequate?

author: PartialDischarge
author_rank: Super Contributor
timestamp: December 16, 2020, 01:39:52 am
content: Quote from: LoveLaika on December 15, 2020, 09:13:49 pmYou can even do 3 mil traces, youd be amazed at how much current a long 3 mil line can carry. Don’t forget to properly size the resistor in power rating

author: amyk
author_rank: Super Contributor
timestamp: December 16, 2020, 02:03:07 am
content: Dielectric strength of air is 3kV/mm

author: bdunham7
author_rank: Super Contributor
timestamp: December 16, 2020, 02:10:25 am
content: I don't think 500V is going to leap across a 0.100" gap, but given the low currents involved, any surface dirt and humidity could result in significant leakage. Do the non-connected HV sources mind being grounded via their 1M resistors? You could use a 3-row header and have one row grounded, then have jumpers on all 7 circuits, with only the selected one connected to your bias and the other 6 to ground.

author: westfw
author_rank: Super Contributor
timestamp: December 24, 2020, 09:02:07 am
content: Aren’t the pads used to mount 2.54mm headers going to end up much closer together that 2.54mm?

author: Siwastaja
author_rank: Super Contributor
timestamp: December 24, 2020, 09:28:14 am
content: Do not use 2.54mm header for 500V.You can always remove pins from the header (and the mating connector as well!), just pull with needle pliers. Remove half of the pins and it's good.