5 V Relay triggered by 12 V - Page 1
Forum section: electronics / beginners

author: kkessler
author_rank: Regular Contributor
timestamp: November 14, 2017, 03:13:03 pm
content: I'm trying to turn on and off a string of LEDs I use for backlighting my TV through the 12V trigger of my receiver. I have some 5V relays that will do the job, so I could simply slap a resistor in series to drop the extra 7 V in steady state, but, when the 12 V is first turned on, the full 12V will be across the relay coil until the electrons start to get moving. Is this going to be a problem? I could always slap in a 7805 to provide the 5V without the overshoot, or does the 5V rating really just mean to not exceed the current 5V pushes through the coil?An interesting side note, I used to use a power strip that monitored one outlet and turned on other outlets when the device on the monitored outlet turned on. Both my new TV and receiver draw so much current in standby, that this method is no longer reliable.

author: drussell
author_rank: Super Contributor
timestamp: November 14, 2017, 03:53:01 pm
content: Quote from: kkessler on November 14, 2017, 03:13:03 pmUhhh... ? ? ?Please explain.

author: Ice-Tea
author_rank: Super Contributor
timestamp: November 14, 2017, 03:56:35 pm
content: Quote from: drussell on November 14, 2017, 03:53:01 pmYeah, that's not how it works

author: langwadt
author_rank: Super Contributor
timestamp: November 14, 2017, 04:03:25 pm
content: Quote from: Ice-Tea on November 14, 2017, 03:56:35 pmin this case it sorta is since the relay coil is an inductorthat said, 12V will not be a problem as long as the steady state current is limited to what it would be with 5V

author: Ice-Tea
author_rank: Super Contributor
timestamp: November 14, 2017, 04:09:00 pm
content: Blimey.. You're right

author: Seekonk
author_rank: Super Contributor
timestamp: November 14, 2017, 04:15:31 pm
content: 5V relays can possibly take more current than the 12V source would like to see continuously. Once a relay closes, it takes far less current to hold it in. I would design your resistor to put 2.5V on the relay and bridge a 220uF capacitor across that resistor. This give a quick pull in and saves relay heating. Don't forget to put a diode across the relay coil.

author: kkessler
author_rank: Regular Contributor
timestamp: November 14, 2017, 05:29:03 pm
content: Quote from: Seekonk on November 14, 2017, 04:15:31 pmMy Denon AVR is rated at 150mA on the 12V trigger, so I'm probably good there, but I'll breadboard this up first to check it out. I like the idea of a cap across the resistor; that's something I'll definitely try out.Thanks

author: Peabody
author_rank: Super Contributor
timestamp: November 14, 2017, 08:26:26 pm
content: This is the opposite problem from yours, but interesting:http://www.ko4bb.com/ham_radio/Projects/24V_Relays/It appears to be a single stage charge pump to temporarily lift the voltage and get the relay moving.In any case, it appears the difference between the pull-in current and the hold current can be substantial. So if power consumption is an issue, and the relay is going to be ON most of the time, the R/C setup might be a good thing to do even if there is no voltage mismatch.

author: Zero999
author_rank: Super Contributor
timestamp: November 15, 2017, 09:32:28 am
content: Quote from: langwadt on November 14, 2017, 04:03:25 pmIt's true that relays are inductive and will switch faster, if given a higher voltage pulse, but that's not the reason for the hysteresis, i.e. on at a higher voltage than off.When the relay is unenergised, the moving iron is not magnetised. It's attracted to a magnet because it's made from a ferrous material, but not so much as a permanent magnet. When the coil is energised, to the point when it produces a strong enough field to attract the iron, it's pulled towards the coil and is magnetised. Now the coil current can be reduced significantly with the core and iron, in contact, before the contacts open. The spring mechanism which pulls the iron back into place also has some snap action, which contributes to the effect.

author: BrianHG
author_rank: Super Contributor
timestamp: November 15, 2017, 11:18:44 am
content: I would use a 6.8v 1watt zener diode in series. Your 12v - 6.8v (drop in zener diode) = approximately 5.2v. This will actually be around 5v to 5.4v, fine for the relay. Nothing else needed....

author: kkessler
author_rank: Regular Contributor
timestamp: November 15, 2017, 03:15:43 pm
content: The relay I was working with last night took about 70ma to trigger, but only about 15ma to stay on, so the effect is quite pronounced.

author: mariush
author_rank: Super Contributor
timestamp: November 15, 2017, 04:54:32 pm
content: A 100mA 7805 is less than 20 cents in quantity ... it will do 70mA and then 15mA should be no big deal [ ( 12v-5v) x 0.015 A = 0.1w, no big deal even for a to92 ]

author: Zero999
author_rank: Super Contributor
timestamp: November 15, 2017, 07:06:12 pm
content: Quote from: kkessler on November 15, 2017, 03:15:43 pmYou could add a resistor in series to limit the steady state current, to just above the hold current and put a capacitor in parallel with it, to provide the higher voltage required to energise the coil. If the relay's coil resistance is 60R, then the coil voltage would be 0.9V at 15mA, which is small enough to ignore, for the purposes of calculating the series resistor value. Suppose the current is limited to 20mA. R = 24/0.02 = 1200 = 1k2.Selecting the value capacitor to put in parallel with the resistor is a bit more tricky. It will depend on the inductance of the coil and the mechanical properties of the relay contacts. My gut feeling tells me it should be around 10µF, as it's a small relay, but that value might need increasing.

author: Seekonk
author_rank: Super Contributor
timestamp: November 15, 2017, 07:20:27 pm
content: Be aware that at extremely low currents the relay will be prone to opening with a mechanical shock. We had a go/no go current relay in production self powered from rectified CT . Pull in currents can vary quite a bit even with the same relay. Built a little tester for the production people as only 80% of relays would meet pull in spec. We used the same relay in other products so having rejects was not a problem.