Relay not turning off - Page 1
Forum section: electronics / beginners

author: dansku
author_rank: Regular Contributor
timestamp: January 19, 2016, 04:48:37 pm
content: I am switching a 12v load with a relay but I am having a problem where the relay turns on but doesn't turns off.I am using this relay:http://uk.rs-online.com/web/p/non-latching-relays/6839829/and switching with a 3v3 input.What could be the reason for it not to turn back off? Ideas?Thank you!

author: Supercharged
author_rank: Regular Contributor
timestamp: January 19, 2016, 05:30:48 pm
content: Maybe try to put a pull-down resistor on the input

author: Tomorokoshi
author_rank: Super Contributor
timestamp: January 19, 2016, 05:51:15 pm
content: 1. That datasheet shows a coil voltage of 3V. Perhaps 12V is magnetizing it to make it stick open.2. What kind of circuit is driving it?

author: wraper
author_rank: Unknown
timestamp: January 19, 2016, 06:31:45 pm
content: Probably an inrush current is so high that contacts get welded together. Probably you have a capacitor in the load and capacitor in the power supply too. That can result in the inrush current which is limited only by the ESR of capacitors and the interconnect resistance.

author: handaza
author_rank: Newbie
timestamp: January 19, 2016, 06:53:31 pm
content: how you control the relay?

author: dansku
author_rank: Regular Contributor
timestamp: January 19, 2016, 11:09:19 pm
content: Actually this circuit is pretty simple, it has a pull-down on the input already.The relay is controlled by a 3v3/0v input.Maybe the current can be high, I'm not 100% sure.Any ideas on things I can test to fix this issue?Thank you

author: SL4P
author_rank: Super Contributor
timestamp: January 20, 2016, 02:06:56 am
content: Where doesIGNITION_OUTcome from?Have you verified - with a LED etc - that it actually turns on and off ?You need to establish that before chasing shadows.

author: sleemanj
author_rank: Super Contributor
timestamp: January 20, 2016, 06:38:48 am
content: MEASURE across the coil when it should beoff, and when it should be on. Confim that when it's on it really is 3v3, and when it's off it really is 0v.

author: LA7SJA
author_rank: Regular Contributor
timestamp: January 20, 2016, 11:55:00 am
content: Have you try'd to paralell pin (3 and 6) and (4 and 5) on the relay to reduce the posibility of the contacts get welded together. What is the voltage over the relay contact, how much current are going true the relay and is the current AC or DC?Johan-Fredrik

author: macboy
author_rank: Super Contributor
timestamp: January 20, 2016, 05:22:41 pm
content: This is a signal relay; you should not use a signal relay to switch a high current load, even if the load current is within the maximum current of the relay. These are designed for signals on high impedance circuits, not high current loads.Get a proper power relay rated for the job (not too overrated, since they do require a certain minimum load current to keep the contacts clean over time).Please determine: what is the voltage at Ignition_OUT when the relay should be off (and when on), and what is the load current?This is a 3 V relay, so it probably won't drop out (turn off) until the voltage across the coil is less than ~1 V.

author: dansku
author_rank: Regular Contributor
timestamp: January 20, 2016, 05:30:02 pm
content: Just realized I am not switching a 12v circuit, but a 52v one.But it's weird as in the datasheet it say it can use a way higher voltage.I measured the current on the load and it's only 50mA.Any 3v3 relay to suggest? I guess that's the way to go.Thank you

author: LA7SJA
author_rank: Regular Contributor
timestamp: January 20, 2016, 07:38:56 pm
content: Check the following:1. Can Ignition_Out supply 30mA+ the relay needs (witout latching the output gate or transistor)?2. Is the coil messuring the correct voltage (3.3V on and 0V off)?3. Can you hear the mecanical switching sound in the relay?4. Why don't you use a 12V relay from 12V to the collector of a NPN transistor with grounded emitter and where basis is connected to Ignition_Out via a small resistor 120Ohm?If bye any chance the relay has been testet with a higer voltage than 3.3V the electormagnet in the relay can be a partly permanent magnet and the relay sticks also check carefully the coil polarity of the relay (+V on pin 1). It could be that you just have a defective relay.Johan-Fredrik

author: dansku
author_rank: Regular Contributor
timestamp: January 21, 2016, 08:56:37 am
content: What about a 3V3 relay designed for power?The problem is that this relay would turn on the 12v circuit.

author: dansku
author_rank: Regular Contributor
timestamp: January 21, 2016, 09:01:28 am
content: What about this one:http://uk.rs-online.com/web/p/latching-relays/7181825/

author: grumpydoc
author_rank: Super Contributor
timestamp: January 21, 2016, 09:40:47 am
content: I'm curious - you said that the relay turned on but not off - did it ever turn off or do you now have a small pile of permanently "on" relays?If it turned off did it do so- when you removed power from the circuit driving the coil?- when you removed power from the circuit driving the load?Otherwise you need to look at the things that have already been suggested.

author: dansku
author_rank: Regular Contributor
timestamp: January 21, 2016, 10:53:17 am
content: Yeah, in the end I believe it's the in-rush current.I will be building a protective circuit for it, and I will keep you guys updated

author: sleemanj
author_rank: Super Contributor
timestamp: January 21, 2016, 11:16:40 am
content: Dansku, we can not see what you are doing, we can not read your mind, when somebody asks you a question like...> "I'm curious - you said that the relay turned on but not off - did it ever turn off or do you now have a small pile of permanently "on" relays?"an answer of> "Yeah, in the end I believe it's the in-rush current."is not a useful answer.You've been given lots of helpful suggestions in this thread for you to check, but you either haven't followed them, or think we can read your mind about what the results were.

author: dansku
author_rank: Regular Contributor
timestamp: January 21, 2016, 11:18:40 am
content: You are right, I am sorry for that.I have tested most of the answers, my fault is to not reply correctly, but I will do it pretty soon!Thank you everyone for your help and cooperation <3Quote from: sleemanj on January 21, 2016, 11:16:40 am

author: johnkenyon
author_rank: Regular Contributor
timestamp: January 21, 2016, 03:23:05 pm
content: Quote from: dansku on January 21, 2016, 10:53:17 amA google for "Snubber" (specifically "RC snubber") would be a good place to start.As simple as a capacitor+resistor in series across the contacts.

author: wraper
author_rank: Unknown
timestamp: January 21, 2016, 03:36:31 pm
content: Quote from: johnkenyon on January 21, 2016, 03:23:05 pmIf the relay welds it's contacts right away, no snubber will save it as the relay is underrated as minimum. Very useful for ensuring long contact life though.