Warm Relays - Page 1
Forum section: electronics / projects

author: German_EE
author_rank: Super Contributor
timestamp: July 24, 2015, 07:57:44 pm
content: I've just completed a project that uses five DIP relays on a small PCB, these devices have a 12V coil and can switch up to 8A @ 230VAC. Each relay switches a 2A load and when activated the coil is fed with just under 12V (I measured 11,8V).The relays run warm, not hot but warm to the touch, enough to notice if you touch one. Is this normal and, if so, what sort of temperature would be considered excessive? The devices are these:http://www.mouser.com/ds/2/316/st-catalog-461843.pdf

author: c4757p
author_rank: Super Contributor
timestamp: July 24, 2015, 08:01:33 pm
content: A 600-ohm coil at 12V? That's 240mW power dissipated in the coil - definitely going to make them warm. It's fine.

author: McBryce
author_rank: Super Contributor
timestamp: July 24, 2015, 08:04:39 pm
content: Are the relays being held on for a long time? Or are they constantly being quickly switched on/off?Some warmth is always normal from relays when they are actuated. Permanent or high frequency actuation will increase this.McBryce.

author: fivefish
author_rank: Frequent Contributor
timestamp: July 24, 2015, 08:52:17 pm
content: I've tested some 12V rated DIP relays (Fujitsu) when their 18V version got temporarily discontinued by Mouser.Anyways, I wanted to see how robust the 12V units were, so fed them 18V and continuously run them for 30 days straight powered on. Testing every few days if they're still clicking on/off. They got slightly warm, but never hot, and never failed.

author: calexanian
author_rank: Super Contributor
timestamp: July 24, 2015, 09:50:41 pm
content: Agreed. In my experience with the relays in our products being warm to the touch is normal, especially when they are on continuously.

author: TheMG
author_rank: Frequent Contributor
timestamp: July 25, 2015, 01:09:00 am
content: Yes perfectly normal for mechanical relays to get warm when the coil is energized continuously. They draw quite a bit of power (relatively speaking).If power consumption is an issue, latching relays are another option.

author: German_EE
author_rank: Super Contributor
timestamp: July 25, 2015, 06:47:03 am
content: OK, thank you for all of the replies. I'm used to using open frame relays (there's a box of them in the basement) where heat is not an issue but I was just a little worried as this project will be used in a domestic environment.

author: bookaboo
author_rank: Frequent Contributor
timestamp: July 25, 2015, 10:16:58 am
content: Totally normal, One of the nice uses for the Flir cameras is to immeadiately see which relay coils are energised, all get hot many get quite hot. I recall some Omron automotive 12v relays we used were almost uncomfortable to touch but this was confirmed as normal.Just thinking about it, in all my time in electronics I don't think I've ever seen a relay coil fail in normal use.

author: fcb
author_rank: Super Contributor
timestamp: July 25, 2015, 10:38:47 am
content: It's quite wasteful of power.For many years, when I've designed something uses 'bigger' relays (i.e. bigger than BT47's) and expects to keep them energised for a ling time I've tended to build in an "economiser" circuit.Most relays will can cope with a MUCH lower voltage to hold them open once they've been switched. These circuits can take the form of: PWM'ing the coil from a micro, using a zener and another transistor or even reducing the PSU voltage (I did this one recently on a project with buck-converter supplying 12V for the relays, just used a 2N7002 and an extra resistor on the FB to the buck - when the micro wanted to energise a relay it would switch on the 2N7002 (hence increasing the switched rail to 12V), turn on the relay, wait 250ms and then drop the rail back to 7V.

author: SeanB
author_rank: Super Contributor
timestamp: July 25, 2015, 11:03:36 am
content: Most extreme common example is the old starter relay, used on a vehicle with a bendix starter. That has 2 coils, a very low resistance one to pull it in, and then after it is closed fully a separate contact breaks that pull in coil and then a holding coil holds the relay on until you release the start switch. With the holding coil only the relay will not pull in, and if the pull in coil operated for over 2 seconds it will melt.With modern pre engaged starters the 2 coil system is still there, but the pull in coil is controlled via the starter motor itself, with the coil being connected to the start terminal and the output terminal, so that the coil will only pull in if the starter motor is electrically working to complete the ground connection. The hold coil operates between the start terminal and the grounded case. The other terminal on the back ( often now not connected) is a small auxiliary contact, which is there for older ballasted coil point systems, which used a coil running at 8V in normal operation, with 4V being dropped by a ceramic resistor in the supply wire. This terminal simply shorted out the resistor so it would get full battery voltage during starting, which would help ignite weak mixtures with a full power spark even with a battery down to 6V.

author: ciccio
author_rank: Frequent Contributor
timestamp: July 25, 2015, 04:58:24 pm
content: When I designed industrial controls, we had many relays and a small transformer, so we usually powered them from a lower than nominal voltage (e.g. 18 V for a 24 V relay) and sometimes put an electrolytic cap (100 uF) in parallel with a resistor (about 1/3 the coil resistance) in series with the coil.At the moment the coil is energised by the driver transistor the cap is discharged, and full voltage is available to move the contact. If the coil remains energised, the cap will charge, and reduce the holding voltage (the coil will see 2/3 of the supply voltage).Our 24 V relays were thus powered continuously by 2/3 of 18 V = 12 V (still within specs)Power was 1/4, and it worked flawless.Best regards

author: aargee
author_rank: Frequent Contributor
timestamp: July 25, 2015, 10:43:10 pm
content: I built a small relay box to extend the Accessories voltage 'on' timeout in my car and built the relay into the small box. I wondered why the poor little PIC processor would shut down after a few hours, until I realised the relay coil was heating the interior up quite a bit. Replacing the relay with an external automotive type solved the issue.The one thing I would say about using a lower 'hold' voltages once the relay is activated is to watch for mechanical vibration/shock. The relay has a much lower hold force and if it lets go, may not be able to turn back on at the lower voltage and/or will be flickering on and off. But really you'd be having to get very close to the magnetic threshold for this to be a factor.

author: Circlotron
author_rank: Super Contributor
timestamp: July 26, 2015, 12:03:28 am
content: ^^ I found out recently that normally closed contacts can be much more sensitive to vibration than normally open ones. Only needed a modest tap on the relay to give a false trigger. Was in a high vibration race car application.

author: Psi
author_rank: Super Contributor
timestamp: July 26, 2015, 12:38:19 am
content: Unlike a AC transformer a DC solenoid/relay operates with a saturated core so it's exactly the same as a a length of wire shorted across the supply. So they will get warm.As TheMG said, a latching relay is the answer if you don't want the heat. They act like toggle switches. You pulse them to change their state and they use zero power most of the time.They come in two types;- One has the two coils wired across each other with diodes. They have 2 external connections and you pulse different polarity to switch them.- The other type has the wires for both coils available. So you pulse one coil for on and the other for off. This type is easier to work with.The disadvantage with latching relays is cost, they are more expensive because they are less common.

author: McBryce
author_rank: Super Contributor
timestamp: July 26, 2015, 07:54:16 pm
content: If heat's a problem I'd consider a solidstate relay or even just a MOSFET depending on how many amps the accessoiries are pulling.McBryce.