power supply common ground connection - Page 1
Forum section: electronics / beginners

author: cosminnci
author_rank: Regular Contributor
timestamp: July 08, 2014, 11:57:22 am
content: Hello,I have an mosfet I drive with an arduino like in the schematic. the mosfet drives a led strip powered from a rectified 220vfor this mosfet I need to use a 12v VGS to turn it on, and an arduino to drive the optocoupler.the arduino 5v the 12v are switching supply and powered from mains, like the rectifier.separating the gnd for arduino and optocoupler form the 12v gnd and rectified 220v gnd provide an optical isolation?is it safe in this configuration ?thank you,regards,Cosmin

author: con-f-use
author_rank: Unknown
timestamp: July 08, 2014, 12:42:30 pm
content: First of all:I hope you mean NEUTRAL_HV by GND_HV. Because if it really is mains ground as in EARTH, current will flow from your hot line (VCC_HV) to earth ground. That will either trip you GFCI or blow up your little circuit.Second:You have connected the bridge rectifier wrong! I corrected it in my schematic.Then:I assume that GND_12V and GND_HV are connected together?GND_ARDUINO (or just GND in your schematic) is not connected to either, right?Further I assume you want to connect the led strip like in my schematic?If all that is correct, you might still have a problem if someone connected to earth ground, i.e. standing on the floor could touch anything connected to the outer loop: rectifier, led strip transistor or if your 12V supply output is not isolated from its input.You can do away with the 12V supply by using a solid state relay (SSR) like the Sharp S202-02 or the like.

author: mij59
author_rank: Frequent Contributor
timestamp: July 08, 2014, 12:59:59 pm
content: Led strip powered by 220Vac main sounds dangerous, if you touch the led strip you'll could be in for a nasty shock.Wondering if the 100Hz flicker will be a problem.

author: cosminnci
author_rank: Regular Contributor
timestamp: July 08, 2014, 09:01:53 pm
content: the led strip will be fixed on the ceiling, so no one touches it.the circuit would be on the ceiling also, and an utp cable would provide 5v for arduino 12v for the mosfet.the supply both 5 and 12V should be isolated being switching power supply.this is correct"I assume that GND_12V and GND_HV are connected together?GND_ARDUINO (or just GND in your schematic) is not connected to either, right? "I did not understand the solid state relay part.I am a little worried about vcc_hv negative as you draw ed it.and the fact that I have the posibility of hv_gnd go on the utp cable, the cable could be a few meters long.

author: con-f-use
author_rank: Unknown
timestamp: July 08, 2014, 10:09:14 pm
content: Quote from: cosminnci on July 08, 2014, 09:01:53 pmYou can use a SSR instead of the optocoupler, the 12V supply, the two resistors and the transistor. Less parts, smaller, less prone to errors. See thedatasheet for the S202S01.The rest should be fine.

author: mij59
author_rank: Frequent Contributor
timestamp: July 09, 2014, 03:59:43 am
content: Quote from: cosminnci on July 08, 2014, 09:01:53 pmIf you can touch bare metal parts it is still dangerous.An utp cable is a low voltage cable it is not designed for mains voltage.The 12V is mains referenced, in case of a failure the 5V will become also mains referenced, it is better to separate cables.

author: cosminnci
author_rank: Regular Contributor
timestamp: July 09, 2014, 05:52:26 am
content: The ssr solution would be nice, but the circuit is an led dimmer and would need to use pwm to drive the ssr.Quotecould I use a different mosfet with ttl level VGS, but there still is the problem of isolating Hv from Lv, the optocoupler does nothing if I use common gnd