wierd resistor - Page 1
Forum section: electronics / beginners

author: boy2018
author_rank: Newbie
timestamp: August 30, 2018, 11:09:27 pm
content: i would be very happy if someone could help me with this one to find out what is the fuction of this resistor

author: ArthurDent
author_rank: Super Contributor
timestamp: August 30, 2018, 11:25:40 pm
content: There is capacitance coupling between the primary and secondary of the transformer and the secondary will float at some voltage unless it is actually referenced to ground. In this case the neutral is ground (earth) potential so a high value resistor, typically 1 to 3 Mohm, will drain this voltage to ground. Often used in laptop or other linear power supplies with no 3rd ground pin on the line cord.

author: JS
author_rank: Frequent Contributor
timestamp: August 31, 2018, 05:44:40 am
content: Bad if something is wired backwards in the mains line, won't hurt you but you will feel it. Luckily for living organisms, resistors like to fail open and not shorted.JS

author: boy2018
author_rank: Newbie
timestamp: August 31, 2018, 10:46:17 am
content: Quote from: ArthurDent on August 30, 2018, 11:25:40 pmMy only issue with that is that the plug terminals weren't polarized, so if you plugged it in upside-down, you'd make everything live at mains potential . So i request someones help with this one

author: Zero999
author_rank: Super Contributor
timestamp: August 31, 2018, 12:37:34 pm
content: The resistor should be a high so the leakage current will be too low to cause a shock, even if the polarity of the mains is reversed.This resistor is probably used for ESD purposes, i.e. to stop static electricity from building up on something.What's the power supply from?

author: boy2018
author_rank: Newbie
timestamp: August 31, 2018, 12:52:58 pm
content: Quote from: Hero999 on August 31, 2018, 12:37:34 pmits just a very old power supply that i found laying in my home (thanks for your help )

author: ArthurDent
author_rank: Super Contributor
timestamp: August 31, 2018, 01:31:20 pm
content: I'm not saying I agree with wiring the resistor that way, I'm just explaining what I see and as presented with the resistor going to neutral (earth), the drawing is correct and fairly safe. In the U.S. even two prong plugs are polarized by having one prong wider than the other so the plug cannot be put in backwards. A 3-wire plug for grounding is much better for permanent installations.What you show is a hand drawn schematic of a linear power supply and it doesn't show a switch or a fuse (which should be included) so we can only go with what you have shown us. You don't say whether it is commercial or home built but with the capacitors across the diodes and the resistor connected to neutral I suspect commercial. It looks similar to what would have been done in older stereo equipment.Your drawing may be correct but it doesn't mean it is good practice. Here is a schematic of a similar supply setup that I found online. You can see the cord is 3-wire, there is a switch and a fuse, and the secondary is grounded. Every piece of equipment I have in my workshop has a 3-wire line cord so every ground terminal on every piece of equipment is at the same ground potential to protect me and the equipment. I have an isolation transformer if there is some short term testing situation where I need it for some weird situation but not for long term use.

author: boy2018
author_rank: Newbie
timestamp: August 31, 2018, 02:02:55 pm
content: Quote from: ArthurDent on August 31, 2018, 01:31:20 pmi dont agree neither disagree i just found this old power supply and i start asking my self what was that resistor doing there (thats tiltle after all "wierd resistor") but i knda said that was comercial when i aswered "i found laying in my home " thought thanks for your help and everyone that help me finding what was the fuction of that "wierd resistor "