please help with relay circuit - Page 1
Forum section: electronics / projects

author: teleporturtle
author_rank: Newbie
timestamp: September 09, 2015, 02:31:21 am
content: i have been recently looking into arduino control of electrical outlets. unfortunately the popular relay boards all use a relay that uses a constant power in the coil to activate the switch. it seems even sellers on ebay are uninformed and selling some of these as latching switches.i was looking into reed relays and saw that the BESTAR ELECTRIC LTD. BR-1050 only can run 10W total, so it isn't suitable for mains power of 200W.Mercury switches and Mercury wetted relays seem to be the correct peice of reliable equipment, especially for durability.so far i have only seen the hgrm-55211-p00 which only can run 100W, 500V, 2A max.also i am fuzzy/not totally comprehending about how to set up the circuit.if you will please review my concept and advise me on what i may use/need/and need to knowi figured from a video called "Using NPN Biploar Transistors with Arduino, PIC " by Lewis Loflin on youtube that it will need a darlington transistor configuration.if i need to run a coil or something that may feed back it will seemingly need diodes to protect the transistors and arduino pin which can handle only 20mA.arduino grounds to negative and the exact setup of the transistor still confuses me.the concept i am wondering about is two leads per battery terminal one with a diode out and another with a diode in. also the arduino would connect into the coil from a darlington for one out and ground and another out would run the coil the other way and either ground to another ground pin or the same (idk, i am a beginner and this forum seems more appropriate than the beginning forum as it only seems to need touching up and is about building a circuit/project design)i got the equation from the video about calculating the resistor value so that the arduino doesn't get too much feedback. if possible please give me other equations to calculate with.i am a bit confused about something. i have heard some confusing stuff so i will just write what i got so far. electrons flow from negative to positive. the actual negative terminal is where they flow to the positive one. i don't know if this is the same direction as the current and when grounding something to negative, i am confused.i can imitate to some degree and i got more puzzled than breaking through, so if you can help me, please dothanks

author: max666
author_rank: Frequent Contributor
timestamp: September 09, 2015, 12:40:41 pm
content: Why would you need mercury wetted switches, what exactly is it you are switching?And since you are intending to switch mains power, why do you care about half a watt off power dissipation in a relay?And may I say be careful doing anything at mains voltage! You appear to be rather young and inexperienced, better leave mains power stuff alone until you better understand the dangers and risks involved. Or have and adult, who knows this stuff, supervise you.

author: calzap
author_rank: Frequent Contributor
timestamp: September 09, 2015, 03:02:09 pm
content: I support the previous warning about working with mains voltage, especially switching high current loads. Fire, shock and melted equipment are easily achieved. There are a lot aspects to mains wiring and switching including phase versus neutral versus ground, unintended inductance, contact arcing, heavier insulation, inductive versus resistive loads, spikes from relay coils, multiple phases, codes, etc. Hopefully, if you continue with the project and I'm not sure that you should, you'll be working only with single phase mains. I recommend switching both the phase and neutral, i.e. use a double pole relay. A common approach is to use a small relay with low current and voltage to switch mains or low voltage into the coil of a larger relay or contactor. Opto-isolation can be used to isolate the coil of the small relay from control circuitry if deemed advisable.It's possible to setup a pair of small, cheap regular relays in a self-latching arrangement without the need to use latching relays per se. One of the small relays needs to be double pole (one pole for the latching circuit, the other to switch the coil of the large relay). Consider using flyback diodes on the coils of the small relays (assuming they have DC coils) and spark/surge suppression on the coil and contacts of large relay.The reason electrons flow oppositely from the convention for current flow is historical. Early workers with electricity did not know what actually carried the charge through conductors, but they did distinguish between positive and negative. For uniformity, it was decided that current was to flow from positive to negative. Only later was it discovered that the carrier was the negatively charged electron, which moves from negative to positive.Mike in California

author: Isad
author_rank: Frequent Contributor
timestamp: September 09, 2015, 03:30:46 pm
content: I have tryed this and yes i have tryed it with relay but only 2pc so i had to useled.The relays would turn on as the arduino booted as for the LED i had to switch thenegativ for the relay i used those opto coupled arduino realy module u finde in ebay.But pls dont do this as a long term thing i tryed this on a 220v lamp for about 1h anddidnt see any heat or anything but this could get dangerus.And yes you need diode for the realy.I havent put any circuit or anything but u will get some info on the blog site and the code.Here is the youtube video:https://www.youtube.com/watch?v=T-7LF_k0O7sPls be carfull.

author: tron9000
author_rank: Frequent Contributor
timestamp: September 09, 2015, 04:27:06 pm
content: Mercury wetted relays appear to be between £10 to £50! that's probably the same price as an arduino relay shield or 5!But as max666 says, what are you switching? There kinda only used in really high power, repetitive switching applications where a standard relay would fail.

author: jlmoon
author_rank: Unknown
timestamp: September 09, 2015, 04:54:57 pm
content: Might I suggest this,Please be careful on the business end (Mains side) when working with these.

author: tron9000
author_rank: Frequent Contributor
timestamp: September 10, 2015, 08:22:55 am
content: Quote from: jlmoon on September 09, 2015, 04:54:57 pm+1, and to restate, please be careful, if in doubt its wrong/ask!

author: Rick Law
author_rank: Super Contributor
timestamp: September 11, 2015, 12:47:39 am
content: Quote from: teleporturtle on September 09, 2015, 02:31:21 amMost of the eBay cheap Arduino relay boards allows the coil to be supplied separately. Typically it takes around 80mA (each relay) to drive the coil side of the Opto coupler, and less than 10mA to drive the logic side of the Opto coupler.The Arduino pin can handle 40mA absolute max, but 20mA is the recommended. So, with Arduino, you must supply the relay board's coil side separately (ie: not from an ATMega pin) to get the 80mA needed. As to the logic ( ie:digitalWrite ), 20mA is more than enough to drive the logic of the relay board.