Why RELAYs go BOOM!!! And How to Use Them

hi today i was going to make logic gates and circuits for you with relays but then i realized you know nothing about relays to start with so today is relay day let me make one for you [Music] here this is similar to a relay which is basically a switch the spoon acts as the switch lever constantly pulled by a spring that connects this terminal called common to either normally closed terminal when unactuated or normally open when actuated see with such switch i can turn on say a light bulb see when the contact moves when there is live voltage exposed you should stay away from live exposed metal at all costs you can use rubber gloves or non-conductive material like a piece of stick to move things around [Music] what is this circuit a switch goes in series with the power lines not parallel maybe i should have just made relay logic but no today's relay then you can learn the logic itself from my sponsor brilliant brilliant is a website and app to learn many things about math science and computing like logic with highly interactive courses where you learn it by doing it so become way more productive using brilliant and get 20 percent off of a year of brilliant premium membership using brilliant.org electro boom now with the proper arrangement it should turn on [Music] it should have been easier this is a single pole because it has one common terminal dual throw switch because it can throw that one common over to two different terminals there can be other types of switches like single pole single troll single pole dual throw dual pole single throw dual pole dual throw in a relay the switch actuation is done by magnetic force that pulls the lever up closing the magnets can be quite conductive let's try it again with a layer of insulation there you go and when i let go of the magnet the spring returns it back to the normally closed position in the actual relay an electromagnet like this is used to actuate the relay like this this is good because now the coil is completely isolated from the circuit it's switching that's all and this is an actual relay that if i put 12 volts across its coil contacts you would be able to hear it click like this well apparently you are not done because it shocks for a simple reason that it is a coil i talked about inductors in my electro boom 101 but basically when you place a voltage across a coil its current rises to some level and it stores electric energy in form of magnetic fields when you disconnect the voltage the coil will want to release its energy the polarity across it flips and it tries to push the current through but now there is no low resistance path for the current to flow so the voltage across the coil rises so much that it arcs across the gap shortening it so that the current can flow like this do you see the little arcs so if i touch across the coil when i disconnect the voltage i receive a shock if the coil is large enough like in this relay you see i can make a novelty zapper with this quite simple what i'll do is i connect the relay coil to the supply voltage on one side and to the ground through the normally closed contact of the same relay on the other side what happens initially is that the coil is energized and so the relay switches disconnecting the coil and creating high voltage across it and the coil is de-energized so the relay switches back so the relay oscillates like this and in every cycle it creates a high voltage spike ow yes it's shocks oh it's a harmless prank so let's not go higher inductance than this oh it seems like i'm getting short pulses of around 300 volts not as shocking as a thousand volt but whatever but in our circuits we typically use a transistor to turn the relay coil on and off which means when it turns it off the spike of high voltage will fall across the transistor and kills it that's why we always provide a path for the current to flow like a freewheeling or flyback diode when we turn off the transistor the coil current will flow through the diode which will clamp the voltage and eliminate the high voltage spike and discharges the coil energy so here i added a flyback diode across the coil that will clamp the voltage and make it absolutely safe for me to touch them the hell what the hell is going on oh yeah if you have noticed this diode must be reverse of the supply so it's normally off until the coil pushes current through it in my setup i put it the other way so it shorts the supply trying again with correct polarity there you go and now it won't shock me at all but as you can also hear the switch is switching slower now of course with such a diode when you turn off the coil the coil current doesn't die right away and it flows through the coil for longer keeping it on for longer and so the relay switches are slower but there is a way to make the relays reach faster we could add a zener diode like this in series like a 5 volt zener diode so when we turn off the coil and current starts flowing there will be much larger voltage drop across these components times the flowing current will result in a much higher power loss that diffuses the coil energy much faster and the relay switch is quicker here's a test with a single flyback diode there from the yellow line where the coil turns off to when the switch switches it takes around 10 milliseconds and here's the test with an addition of a 15 volt zener see now it takes less than four milliseconds to switch the pulses you see on the relay switch is because of the relay contacts bouncing against each other when they switch fun now there's of course more stuff to consider about relays like for example there is a maximum switching current because like i showed when the relay switches the contacts bounce against each other and don't make proper contact right away or the contacts might have been oxidized and are initially higher resistance so the contacts could overheat either by arcing when it's bouncing or through the contact resistance and they could melt and fuse together like this imagine your relay wants to connect a power supply to a circuit that has a large capacitance see the relay switches now and if i connect the capacitor and try to switch the relay see the relay is stuck because the contacts are melted yeah typically some good slapping disconnects the fused contacts until next time so it's best to keep the current limited when you are switching the relay and that's not all you can only switch a voltage below a certain level for reliable switching of course if the voltage is too high the gap between the contacts will break down and an arc will short them but that's not the only issue the voltage may not be high enough for an initial arc but if it is too high and powerful when you switch to it and try to switch away the arc within the contacts will not go away and will keep the contacts short here's my zvs circuit that can create tens of kilovolts but i can put the contacts at a distance that no arc jumps like this if i close between the contacts i would be able to sh if i short between the contacts carefully there it's maintaining the arc same thing happens to a relay i'm gonna put two thousand volts across my man-made relay very dangerous never try this at home see the voltage is not high enough to jump across the contacts but if i close and open the relay close and open the arc doesn't go away anymore the breaker popped so if the high voltage and power is available within your contacts the arcs may never go away and keep your contacts shorted or open after a long time which can melt your contacts and damage your relay and short your circuits such high voltages can happen when you switch inductive loads like i showed so you must keep the switching voltage and current below the relay rating for reliable switching fun but that's not all every relay coil is designed to switch the relay at a nominal voltage like 5 volts or 12 volts but there is a minimum voltage called pickup voltage above which the relay is guaranteed to switch after you turn on the relay though you can maintain its state at even lower coil voltages so to reliably turn off the relay the coil voltage must go below the release voltage and of course there is a maximum voltage above which you will over current and melt your coil see this is a 12 volt relay but it will turn on around 6 volts and turns off around 3 volts but what if you have a supply voltage much higher than what the coil can handle easy instead of a continuous on signal you give it a pwm to turn the coil voltage on and off so fast that the coil doesn't have time to de-energize something above one kilohertz should be enough with a proper pwm duty cycle you can create an rms voltage and current no more than the maximum power rating allowed by the data sheet you can even regulate these by adjusting the pwm duty cycle for different supply voltages and that's not all for some weak and small relays there needs to be a minimum contact voltage and current present like when you switch them there is a recommended minimum load for reliable switching why is that you ask let me show you if you don't use your relay for long periods there can be a very thin layer of oxide on the contacts that don't let it connect well or in my case it's suit suit here i'm trying to turn on a low voltage led circuit and because the contacts are covered in suit it won't turn on see is suits conductive it's made of carbon it could be graphite so maybe if i melt the contacts with my capacitor a bit again again again again does it conduct now it doesn't but if the voltage is high enough like 120 volt ac it is guaranteed to break through the oxidized layer see and that's why there is sometimes a minimum required switching voltage and current and there you have it of course there are some other parameters that are easy to understand like the maximum operating temperature and there are other type of relays to like the latching relay or polarized coil or even the solid state relays that have their own benefits and disadvantages but these relays will never go away or at least as long as i leave next lesson i guess at some point digital circuits with relays but not before you learn logic which you can learn from my sponsor brilliant my daughter and i spent hours solving the fun and challenging logic puzzles at brilliant which really teach proper and logical thinking and lead into arithmetic and computing logic i just didn't expect her to solve those quicker than me and brilliant keeps releasing new courses that are even more interactive like the newly updated scientific thinking course where you learn how to think like a scientist learning a ton of science by doing scientific puzzles and reading their detailed explanations can you imagine being entertained by learning that's what brilliant is brilliant at they understand that hands-on active problem solving makes it fun and is essential in learning complex concepts if you're really looking into refreshing improving or 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Channel: ElectroBOOM

Views: 1,386,192

Keywords: educational, electrical, ElectroBOOM, science, electronics, engineering, entertainment, equipment, measurement, experiment, mehdi, mehdi sadaghdar, arc, mishap, physics, Sadaghdar, test, tools, circuit, funny, learn, shock, spark, discharge, relays, STDT, STST, DPST, DPDT, Switches, Coil

Id: PHcCSZ5nScM

Length: 13min 58sec (838 seconds)

Published: Wed Aug 18 2021