Questioning my assumptions of AC coupling I observed - Page 1
Forum section: electronics / general

author: DarkPrince
author_rank: Regular Contributor
timestamp: March 11, 2013, 09:25:54 pm
content: Hey guys I just want to double check something to be sure there is no dangerous/lethal issue I am not aware of. I am 90% confident that things are fine but it never hurts to check.So what I observed today I wasn't expecting. I have a strip of Cree LEDs for lighting and noticed that when a terminal is disconnected (only tested positive side not negative), I touch it and a grounded device[1,2] the LEDs light up ever so dimly. I got a moderate idea what is going on but just want to verify. I presume the ground line is 'OK' but this is a very old house and I obtain the ground from the box itself and not a outlet with a ground terminal. Machine in this case was a drill press sitting on my work bench.What I believe is happening... somewhere... is AC coupling between the power supply and the AC line through either parasitic or ESD protection capacitance. I attempted to measure the voltage but it fluctuates significantly, so an extremely high impedance loop I am experiencing. I attempted to measure the current using my UT-61E but it is quite small (50uA or less). Amazing these LEDs are visible at such low currents.Anyway, just want to make sure my assumptions are correct... can't say I am not afraid of line voltages.Power Supply: Mastech HY3005F-3 (most likely not a clone..)* All equipment on a beefy power strip so at least all equipment share ground..Random photo attached because... I can.

author: smashedProton
author_rank: Frequent Contributor
timestamp: March 12, 2013, 05:26:57 am
content: This should NOT be in general chat_____________there are a number of things that you can do. You can use a mains sense stick (no idea what they are called) to sense if it is at mains potential. You could use a scope to do the same thing. It is also possible that your body is charged because it is acting as an antenna. Do you live in an industrial area? Is your home wiring horse shit? Also try shunting it off with an rc filter if you are so inclined

author: amyk
author_rank: Super Contributor
timestamp: March 12, 2013, 06:51:15 am
content: https://www.eevblog.com/forum/chat/led's-light-when-soldering/https://www.eevblog.com/forum/beginners/soldering-iron-is-giving-off-usable-power/Doesn't take much to make one light. Your body is also a huge antenna.

author: DarkPrince
author_rank: Regular Contributor
timestamp: March 12, 2013, 07:14:55 am
content: smashedProton:Quote from: smashedProton on March 12, 2013, 05:26:57 amI believe this forum is just fine.The topic is sort of broad enough not to be placed in a specialized category.Quote from: smashedProton on March 12, 2013, 05:26:57 amYeah I would not put any confidence in the wiring of the house (it is from the knob and tube era). Suburb, away from any large plants. It wasn't much of a concern... granted I could predict something is possible it is more of the concern of possible mains infultration on the circuit. I hardly believed it but it couldn't hurt to post.Quote from: amyk on March 12, 2013, 06:51:15 amYeah I think I will mark it to stray sources. I really figured that much since:1) I didn't get electrocuted from it.2) Even bypassing myself it was very minimal currents.Once again I am not even sure why I brought it up as a concern. I suppose I never really dealt with much where I could actually witness the stray charges. None the less things seem to be in order and I got to confirm things are where they should be in the power supply. Back to work.

author: OLD EE
author_rank: Newbie
timestamp: March 20, 2013, 03:55:53 am
content: If you have knob and tube wiring and are messing about with electrical stuff I'd suggest that you consider having a seperate 20A circuit (three wire) run from the main house electrical box to your work bench. I would also recomend working off a ground fault outlet as well. If you do this have the electrician check your 'house' ground while he's there.Knob and tube often doesn't have the third 'earth' ground wire so you could very well be working off a floating outlet. It could be why you are getting stray currents. I live in and old house as well and have had experience with this problem. You don't want to blow up you project not to mention your self.Just a thought.Old EE