Relay coil voltage question - Page 1
Forum section: electronics / beginners

author: made2hack
author_rank: Frequent Contributor
timestamp: November 29, 2014, 06:26:31 pm
content: Hello all,This is the relay in question, it is theSONGLE SLA-48VDC, which has a coil voltage of 48V.If the coil voltage is 48V, does that mean it cannot activate the relay (trip the relay) if it doesn't get 48VDC? What if it is lower? Anything? If so, how low can it go? Coil sensitivity is 0.93W as per datasheet.What if more than 48VDC is put on the coil? (Solenoid?). What happens then?Thanks

author: Zero999
author_rank: Super Contributor
timestamp: November 29, 2014, 06:36:33 pm
content: See table 6 on page 2 of the datasheet.Hint:Pull-in = activateDrop-out = deactivate

author: Seekonk
author_rank: Super Contributor
timestamp: November 29, 2014, 07:08:35 pm
content: Typically they are guaranteed to pull in at about 80% and drop out at about 25%. I use a 110V DC relay in my solar system and that pulls in at about 50V. We produced a product that required a very sensitive relay and we hand selected them. Some batches I could get 90% acceptance and some times it was down to 50%. The story here is your results can vary if you have very good success with just one relay. At lower voltages a relay may drop out from mechanical shock. In my micro projects I generally PWM a relay to about 50% to save power and lower heat.

author: fubar.gr
author_rank: Unknown
timestamp: November 29, 2014, 07:09:53 pm
content: It is possible to operate a relay with a much lower than rated voltage, even lower than the drop-out voltage.Of course there will be too little pressure on the contacts, and if the current is too high they will overheat and burn. Basically the drop-out voltage is the absolute minimum safe voltage to operate the relay.And there's a voltage range where the common is floating between the NO and NC contacts and makes contact with neither.

author: madires
author_rank: Super Contributor
timestamp: November 29, 2014, 08:00:53 pm
content: Quote from: fubar.gr on November 29, 2014, 07:09:53 pmTo switch on the relay you have to use the pull-in voltage, which is the voltage the relay is rated for. The drop-out voltage is the voltage at which the relay switches off again. You could call it also hold voltage. After the relay is switched on you could reduce the voltage to the hold voltage and save some power. But you can't switch the relay on with the hold voltage.

author: Andreas
author_rank: Super Contributor
timestamp: November 29, 2014, 08:18:28 pm
content: Quote from: Seekonk on November 29, 2014, 07:08:35 pmHello,Of course the voltage behaviour is temperature sensitive.So in winter when the winding is cold (has lower resistance) the acceptance might be better.In reality only the current (and not the voltage) counts for operation.With best regardsAndreas

author: IanB
author_rank: Super Contributor
timestamp: November 29, 2014, 08:19:55 pm
content: I think the drop-out voltage is the voltage where the relay is guaranteed to open. So it is not the minimum safe voltage to hold the relay, it is the maximum safe voltage to ensure the relay doesn't hold.Basically, you need to go above the pull-in voltage plus a safety margin to ensure the relay closes, and you need to go below the drop-out voltage minus a safety margin to ensure the relay opens. The relay will generally hold closed between these two limits but it would be a bad idea to go too close to the drop-out voltage if you don't want the relay to drop out unexpectedly.The nature of the drop-out voltage is highlighted by the datasheet referenced, where the drop-out voltage is quoted as 10% of 24 V, i.e. 2.4 V. They are saying if you drop the coil voltage below 2.4 V you are guaranteed the relay will drop out.

author: Seekonk
author_rank: Super Contributor
timestamp: November 29, 2014, 09:56:22 pm
content: Quote from: Andreas on November 29, 2014, 08:18:28 pmWe actually use the resistance of the coil to calculate operating temperature of the relay. Beats sticking thermocouples inside.

author: made2hack
author_rank: Frequent Contributor
timestamp: November 30, 2014, 12:21:59 am
content: Ok, I think I get it.But, in order for the relay to be "pulled in", there has to always be voltage to it? You are always powering the relay? And then, once you stop and it goes below drop out voltage, it "switches off" so to speak?

author: IanB
author_rank: Super Contributor
timestamp: November 30, 2014, 01:12:05 am
content: Quote from: trs80 on November 30, 2014, 12:21:59 amYes, normal relays rely on the magnetic force from the solenoid to hold the contacts closed. If you remove power to the solenoid the contacts spring back to the open state. This might be considered a "fail safe" behavior, and is often advantageous.I think there may be special bistable relays that hold either state without power and only require a voltage pulse to switch them. However, these are not typical.

author: rolycat
author_rank: Super Contributor
timestamp: November 30, 2014, 03:30:19 am
content: Quote from: IanB on November 30, 2014, 01:12:05 amAKA latching relays. Very useful for low energy circuits.