Flyback/snubber/freewheeling/etc diode for 12V automotive relay - Page 1
Forum section: electronics / beginners

author: jsi
author_rank: Regular Contributor
timestamp: June 24, 2017, 10:18:45 pm
content: I've got an automotive application that requires a DPDT relay. Googling a DPDT 12V relay with diode turns up nothing. (except one relay with a built in LED - more on that below) This relay is part of a common modification that taps into my vehicle's wiring harness and improves the way an accessory works. I know enough to know that when power is removed from a relay there is a big voltage spike, unless there is a way to dissipate the energy safely.Given that 12V relays with built in freewheeling diodes are as a rare as hen's teeth, am I worrying about something that isn't a problem? It would be pretty simple to add a relay during manufacturing, but if it's not needed I could see why they don't.(I bought a relay with an LED pilot light thinking I could reverse the polarity and use the LED as my freewheeling diode. I ran some tests using my oscilloscope and if there were any improvements they were insignificant.)

author: keymaster
author_rank: Regular Contributor
timestamp: June 24, 2017, 10:49:54 pm
content: Quote from: jsi on June 24, 2017, 10:18:45 pmby put in a diode in the relay ,will constrain the coil to one way energizing . that is a "-" that cost . why would the manufacture do it?its easy to put a diode( the direction you need ) in parallel with the coil.

author: jsi
author_rank: Regular Contributor
timestamp: June 25, 2017, 04:54:57 am
content: Quote from: keymaster on June 24, 2017, 10:49:54 pmI understand that the relay would have to be installed correctly, but isn't that true of anything? I know internet instructions need to be taken with a grain of salt, but I seldom if ever see diodes included in automotive relay applications. Do automotive designs take into account there are spikes and voltage variations making extra diodes unnecessary?

author: Andreas
author_rank: Super Contributor
timestamp: June 25, 2017, 05:19:44 am
content: Hello,there are several considerations:- reverse battery protection. (there are "experts" which actually connect the battery in reverse not only on jump start)with a diode you may need a additional fuse to protect the wiring to the relay.- who guarantees that as replacement the correct relay with the correct polarity is used?so you have to do protection anyway in the control unit (or have to diagnose that the diode is available).- A diode makes the switch off time of a relay very long. The contacts may burn relatively fast due to this.Usually for on/off actors power stages are used with a higher clamping voltage (45-70V) to keep switching time of a relay low.- cost. (3 different relay types instead of one, additional tests for vibration etc).with best regardsAndreas

author: amb101
author_rank: Newbie
timestamp: June 25, 2017, 05:28:36 am
content: Single pole automotive relays with built in diode are very common. The built in diode is wired in parallel to the coil. It is your double pole automotive relay that is rare as hens' teeth.Why can't you just use 2 SPDT relays (with built in diode)?

author: ProBang2
author_rank: Frequent Contributor
timestamp: June 25, 2017, 05:52:56 am
content: Sorry, Andreas, but: Nope, nope, quite the opposite and nope...It is much more simple.Quote from: jsi on June 25, 2017, 04:54:57 amThey do. But in a working current relay mostly the flyback diode is replaced with a flyback resistor (in parallel with the magnetic coil).Look at thisexplanation of the working current relay, please.

author: X
author_rank: Regular Contributor
timestamp: June 25, 2017, 06:06:29 am
content: Quote from: Andreas on June 25, 2017, 05:19:44 amThis can be resolved with a zener diode:The zener voltage should be low enough to avoid damaging the driver, but high enough to prevent the relay holding on too long.What happens here is that when the power is released, the zener conducts until the voltage across the coil is low enough, then the zener stops conducting. This reduces the time as it allows the fyback voltage to be higher, but still shunts the excess EMF to prevent damage to the driver.

author: SeanB
author_rank: Super Contributor
timestamp: June 25, 2017, 07:02:16 am
content: If you are adding the relay in parallel with a resistive load ( a light) you do not actually need any diode, the parallel load will do the clamping for you. I see 3 types of automotive relay. Those with no diode, fine for use with a switch. Those with a resistror across the coil, self clamping and usable either way round but with a higher coil current. Finally those with a built in pair of 30V zener diodes or a transient suppressor across the coil, so they clamp the excursion of the coil voltage.Most are either SPST NO relays, though you do get SPST CO ones, but every time you want a DPDT you just use 2 SPST CO relays and parallel the coils.

author: floobydust
author_rank: Super Contributor
timestamp: June 25, 2017, 07:25:36 am
content: Some automotive relays use a 470-680 ohm 1/2W resistor instead of a diode across the coil to absorb back EMF.The diode causes smoke with the "reverse battery" requirement auto makers must meet.Automotive ISO pin-labelling does not seem to have coil polarity. 85/86 are the coil.

author: cs.dk
author_rank: Unknown
timestamp: June 25, 2017, 07:32:00 am
content: The numbers does met a standard;https://en.wikipedia.org/wiki/DIN_7255285 is ground, 86 is +12V/+24V supply.As mentioned, most relays are with a resistor build in.

author: jsi
author_rank: Regular Contributor
timestamp: June 25, 2017, 06:31:36 pm
content: Quote from: amb101 on June 25, 2017, 05:28:36 amI could, but it would fail KISS (keep it stupid simple). More to go wrong and more expense and more difficult to install. And I am trying to improve the mod for those that struggle with electronics. So the best answer is one DPDT relay.Quote from: SeanB on June 25, 2017, 07:02:16 amSean - thanks this taught me something and got me thinking too. So I connected my scope to the relay in question and the first image is what I saw when the relay was power cycled.The second image is what happened after I connected a 1N4007 diode in parallel with the coil. In this application the relay powers a linear motor. Nothing needs to happen quickly as the relay's only job is to signal the motor to run. It cuts its own power at the end of its stroke.Is that first image something to be concerned about in an automotive application?

author: Simon
author_rank: Unknown
timestamp: June 25, 2017, 07:10:41 pm
content: The standard auto relays typically made by Bosch do come in a version that has a diode, but i don't know any more than that as I have never used one.

author: LabSpokane
author_rank: Super Contributor
timestamp: June 25, 2017, 09:15:35 pm
content: Quote from: jsi on June 25, 2017, 06:31:36 pmIt should concern one in any application. You just need to decide if you want to use a diode or a resistor as a snubber.

author: floobydust
author_rank: Super Contributor
timestamp: June 26, 2017, 08:56:46 pm
content: Hella Automotive relays has resistor, one and this two-diode offering, which meets the reverse battery requirement. SSR's are new.Hella Electrics Catalog {slow load pdf}