Understanding the wiring diagram of a reed relay and more - Page 1
Forum section: electronics / projects

author: Sariel
author_rank: Regular Contributor
timestamp: December 07, 2023, 01:29:44 pm
content: Hello,I'm in the process of incorporating the 8L61-05-111 reed relay into my upcoming design. You can find the datasheet here:https://www.cotorelay.com/wp-content/uploads/2014/09/8L_series_reed_relay_datasheet1.pdfRegarding the wiring diagram for this 1 Form C device, as shown in the attached picture, I'm seeking guidance on how to interpret and understand it. Specifically, I'd like to know which pins are normally connected when there isnocurrent flowing through the coil.Furthermore, the datasheet mentions that the relay has an internal diode. Does this imply that I can confidently omit the external diode for kickback protection?Lastly, when connecting the relay to a 5V source, can I rely solely on the coil resistance to generate the necessary current for activation, or should I include a series resistor in the coil circuit?Thanks for your assistance.

author: CountChocula
author_rank: Unknown
timestamp: December 07, 2023, 02:26:46 pm
content: Looks like pins 8 and 14 are normally open, and pins 1 and 8 are normally closed (you can figure out the symbology by comparing the diagram with the corresponding diagram for the class A relay). When the coil is energized, the state reverses using a break-before-make sequence (this is implied by the fact that the relay is of Form C type).It does look like the model you picked has an internal flywheel diode, so you should be able to omit it from your circuit.AFAIK, you don't need to protect the coilunless you're driving it a voltage higher than its rated voltage, but I would perhaps consider protecting the circuit overall (e.g. with a fuse at the input) in case things go sideways. You should also make sure that whatever is powering the coil can source the required amount of current (looks like about 30mA).—CC

author: Sariel
author_rank: Regular Contributor
timestamp: December 07, 2023, 03:34:55 pm
content: Quote from: CountChocula on December 07, 2023, 02:26:46 pmThanks for the elaborate answer.It truly helps

author: ajb
author_rank: Super Contributor
timestamp: December 07, 2023, 03:58:42 pm
content: Aside from some relay manufacturers, that style of contact symbol seems to be most common in automation diagrams, like PLC ladder diagrams. I think they're part of an IEC standard in that area. But CountChocula is correct, the parallel lines with the slash across indicate a normally closed contact, and the ones without the slash normally open.Quote from: Sariel on December 07, 2023, 01:29:44 pmYes, but if the relay comes in variants without a built-in diode, it may be worth including a footprint for an external one anyway. Makes it easier to fit an alternate part if the one with the diode goes out of stockQuote from: CountChocula on December 07, 2023, 02:26:46 pmRelays often have a fairly generous amount of operating margin above and below their nominal voltage, at least in normal operating conditions. Adding overvoltage protection to the circuit isn't a bad idea, but it should probably prioritize protecting other components more sensitive than the relay coil.

author: wasedadoc
author_rank: Super Contributor
timestamp: December 07, 2023, 03:59:27 pm
content: 1.5 Volts and 200 Ohm is closer to 40mA than 30mA.Incorrect. See below.2. The diode unidirectional. It is essential that you connect the coil polarity correctly. The datasheet is slightly ambiguous. The + and - in Note 3 can be interpreted as the anode and cathode of the diode or the polarity of the voltage applied to the coil. The two are opposites! Best to measure to ascertain.

author: CountChocula
author_rank: Unknown
timestamp: December 07, 2023, 04:22:32 pm
content: Quote from: wasedadoc on December 07, 2023, 03:59:27 pmHow did you calculate that? Isn't 5V / 180Ω ≅ 28mA?

author: TimFox
author_rank: Super Contributor
timestamp: December 07, 2023, 05:03:40 pm
content: It is often a good idea to put a diode across the relay coil to avoid damage to other devices running from the same DC supply.An interesting problem to watch for:I have an -hp- 8013B pulse generator.It uses DIP TH reed DPST relays to switch the internal 50\$\Omega\$ termination resistors on the outputs.The footprint for the relays allows it to be installed rotated 180oand still function, but the internal-diode option used in the 8013B will end up with a rotated diode (not a good thing), that gives "flaky" performance (not totally dead) with a very low coil voltage in the ON state (there is a series resistor feeding the coil, and each end of the coil has two pins).When I found the rotated relay, I could not see any sign of previous rework, so that may have been a factory error.Replacing the relay with a new one solved the problem.

author: nigelwright7557
author_rank: Frequent Contributor
timestamp: December 07, 2023, 05:38:21 pm
content: Quote from: TimFox on December 07, 2023, 05:03:40 pmWhile adding a diode provides protection against high voltage discharge of coil it can slow down relay release and this can cause arcing.I put a 12 volt zener in series with diode.

author: wasedadoc
author_rank: Super Contributor
timestamp: December 07, 2023, 11:41:14 pm
content: Quote from: CountChocula on December 07, 2023, 04:22:32 pmI calculated that incorrectly. 5 Volts and 200 Ohms gives 25mA.Glad to see someone is paying attention.