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*Beginners:* While rare in consumer electronics products, there are also 2 additional types of relay coils single and dual polarity latching coils. There is also a subcategory of traditional relays generally referred to as "signal" or "small signal" relays. These small signal relays have much higher coil resistance. Why is the coil resistance important? With Ohms Law you'll recall that, Current(mA) = Voltage / Resistance This means that a Relay with a resistance of 78ohms at 5 volts will draw around 64mA of current. If you directly connect this to something like an Arduino I/O pin you will quickly find yourself buying another Arduino replacement. It isn't a good idea to connect a microcontroller directly to any relay coil due to the voltage spike that happens when voltage is switched off an inductor (coil of wire) as the magnetic field collapses. Next point, pay very close attention to the contact rating for any relay. If the relay doesn't have a DC contact rating, do not use it to switch DC voltages. If you want to see a real demonstration of why the contacts rating is important John Ward posted a video demonstrating how and why AC rated switches are not compatible with/as DC switches. The physical contacts and structure inside a relay is exactly the same as all manual switches like toggles, rockers, push on/push off, etc.. John Ward's RU-vid channel is just his name (I have no affiliation with him). This is just a reference to where I learned the difference myself beyond just an awareness of the numbers printed on relays. If anyone is too lazy to look it up, basically (IIRC), an AC contact is easier to manufactur because the voltage and current are always crossing zero as the wave cycles. This means the contacts are easier to pull apart without arcing. The contacts can be mounted closer together and the spring is lighter because the contacts can come apart much slower. In a DC switch contact the switching action must move farther with a wider separation between the contacts, and the spring/pivot structure must break the connection much faster. This design is specifically intended to prevent arcing at the contacts. Arcing in a switch is an extreme fire and shock hazard as John Ward clearly demonstrates in the aforementioned reference. Lastly always check that the switching contact PCB traces have a large amount of clearance on any application where you intend on switching AC mains voltages. There were some awful hobbyist relay modules coming from China a few years ago that were marketed for use with Arduino's, etc.. The PCBs on these modules had less than 1mm of trace clearance between a ground plane and the switch contacts. It can be really tricky to design a safe PCB layout for some of these relays because a switch contact pin is sometimes located in a really bad spot that is difficult to keep isolated from other parts of the circuit. This is very very important to understand before using a relay in a AC mains switching circuit. Ultimately, it's much safer to try to use a DC rated relay contact design as a beginner. You will find that the vast majority of cheap relays are not DC contact rated. Always remember *if it doesn't say it's DC rated it is not DC rated.* -Jake
Can i use a rellay as a AC boost convertor.if yes,what kind of transistor should i use.NPN or PNP.i just thougt because it has lots of magnetic coils in a rellay it can store and boost energy.please i need your advice.your video tutorials are the shortest and most detail then other.i always learn new things from you.thank you
@5:35 What equipment do you use to supply the voltage to the coils of the relay? I need to test my relay using 120V. What can I use that may not be too costly? Thank you.
This was a bad explanation and probably the worst video I have seen from Ludic Science ... You could have explained the difference in resistance readings between the relay coil and contacts. Using the resistance readings you could explain if you are reading across switch contacts or energiser coil. It is not hard as most low voltage relays typically have a few hundred Ohms across the coil and less than one Ohm across the switch contacts....
