Is there a danger with relays without diodes

mollydog · Apr 30, 2015

I came across this LINK on Wikipedia the other day, since then I’ve done some googling and found out this is a known fact that if relay hasn’t got a diode fitted, this might/will cause a spike and might damage something on the Elise S1 circuit.

I copy~pasted this form google

"Since an inductor (the relay coil) cannot change it's current instantly, the flyback diode provides a path for the current when the coil is switched off. Otherwise, a voltage spike will occur causing arcing on switch contacts or possibly destroying switching transistors”

My concern is that a year or two back I fitted a relay to the coolant temp circuitry to be able to also read the oil temp on the Stack, and another relay to be able to switch on the rad fan should the temp rise too much in traffic jam,

I am sure these relays didn’t have a diode fitted in them, is this something I should rectify or is there nothing on the Elise S1 circuitry that fragile?

Relay without diode:

Relay with diode that will prevent Flyback:

(I have the flyback showing going in the wrong direction)
I have come across these inline diode,

would these be an easy option instead of having to replace the old relays with new ones fitted with a diode?,

as the relays are in very tight spaces, it’ll be hard if not impossible to see the pin numbers to re fit the wires to the correct pins after un plugin them

many thanks for any advice that can be given

plugsandsparks · Apr 30, 2015

For the rad, if you have simply put a relay across the existing rad switch and you operate it with a switch on the dash, I cannot see how the relay coil can interfere with anything in the ecu. That's how my BMW is fired as I do the same thing. As for the stack I am not sure how you have wired it and what sensors have been used, i.e. does the ecu use the same sensors. If you can draw out your wiring we can see if the coil has any effect on the ccts you have cut into.
cheers
P&S

tazz · Apr 30, 2015

Reverse diode protection, is another phrase use, and the idea is to protect sensitive electronic components from high voltage spikes. As an inducter, it can produce quite high voltages for mS whilst energising. Most older vehicles do not contain such sensitive components, and if you are just switching straight off the battery circuit via a mechanical switch. Then a diode is not really required. Since relay coils are very low power, most modern ones have a 1A diode built in. This is easily installed, by placing the diode across the coil terminals, inside the relay enclosure. The 1N4001 is small enough to do just the job, being careful of polarity, and costing around 5-10p.
Hope this helps

mollydog · May 16, 2015

Thank you for getting back to me, been busy doing other wiring to the car and only now have got round to replying

I came on this site last year for advice and help, I had tremendous help from forum members on here that without it I’d have been struggling to get that project finished properly

that project was fitting an override timer to the fan of my car, so if I was in a traffic jamb, and the temp was going high but not high enough to get the fan to come on I’d press the timer and the fan would come on for the set duration.

and wit help from here I got the circuitry right.

reading about the high voltage spikes possibility with the relays and sensitive components (in cars) I thought I’d come back and ask for further advice

this is the timer I’m using

so I have been told by others that have done this to their cars (its a Rover 1.8 K series engine) , if the ECU senses the temp is high and the rad fan needs to come ON, the ECU has a wire going to a relay, and earths the fan wire at the relay to switch on the rad fan

what others have done though is use just an ON / OFF switch to earth the wire from the ECU at the relay, I have done the same but using the timer, with help from this forum this is how with the timer I made up the circuitry

I’ve already purchased a few 1N4002 diodes should I need to place them to my relays, I’ve seen it done like this:

one thing that I do see thats not consistent, I’ve seen most relay circuit wired like this:

On a few occasions I have seen the ground (85) reversed with IGN (86), now I have been told this will not be an issue (unless like me, maybe plan on fitting a diode), But I would just like to know if there is a set rule to follow regarding pin 85 and pin 86

one more question, if I may, my next project is to fit an on board camera (DVR) it’s like a black box for a car, it records your journey, so should you be involved in a road collision, you have recorded the incident, (crash for cash) so can prove if its not your doing. These DVR’s (all they are is a camera that is plugged in to the cigarette lights and when you start your car (switch on ignition) it starts to record, and when you switch off your car (ignition) it also switches off.

Thats fine, but what if you still need/want to record but piratical to keep the engine running. I always drive with my lights on, day or night (safety) so I was thinking of wiring my DVR to start recording only if sidelights are on, this way I can record with ignition off as long as I have the sidelights on, if for example an incident has happened and I still need/want to record it but hoeing the engine running while I’m out of the car is not practical/safe

I have come up with this wiring:

the car is a 15 year old Lotus, so apart from the ECU I shouldn’t think its got many sensitive circuitry

Any help is most welcome