Some basic questions on use of relays

[John Ibele]

I'm getting ready to use a few relays in my wiring setup, nothing fancy. They'll be on the passenger side of the firewall ... all lower current stuff so no issue there. I'll use one to avoid having the current energizing the starter solenoid go through the ignition. I'll have one for flash-to-pass using the button on the end of the RT turn signal. I'll have two for dimming my seat heat indicator lights when heat is set to low.

I realized there were a few gaps in my knowledge so I figured I'd just ask them here.

1.) I've seen more than a few examples where knowledgeable folks use unfused 12V power straight from the battery, but I realized I don't have a clear set of guidelines when and when not to do that. Here are the cases where it seems like use of 12V without current protection would be okay:



When the manufacturer tells you to. In my case, Holley telling me to hook Sniper and HyperSpark components directly to the battery.
When you know the device being hooked up is internally protected, and will isolate power from ground in the case of failure.
When the device isn't internally protected, but it's low cost, easily replaceable, and won't short to ground when it fails. Is the 12V side of a relay coil one such example, or shouldn't I take relay coil power directly off the bus bar?


What else am I missing, or am I on the wrong track?

2.) I've read up on the use of resistors and diodes incorporated in relays, and understand the principals of why they're used and when. In practical use, though, I have no idea how sensitive 'sensitive' is when it comes to the components we typically use in wiring up a roadster. I've got LED dash lights on my dash, for example. Any general guidelines for when and when not to use resistor or diode protected relays? Any downfall in using relays incorporating one or the other in general, other than a very small amount of additional expense?

Shoot, I can't count. Well, I can, and that makes 4 question marks. Anyway, thanks in advance for the advice.

[edwardb]

My thoughts FWIW:

1. You alluded to a distinction between the coil side of the relay and the circuit side which is powering whatever device is being used. That's important. The device powered side (typically pins 30 and 87 for automotive style relays) must always be protected. Could be any variety of ways as you described. The coil side could also be protected. If you search on the topic, you'll find many who say it's required. I will admit to (1) adding a number of relays in my builds, and (2) not always having protection on the coil side. My thinking? The draw on the coil is very low. Typically .1 amp or so. Even a very small fuse (2 - 3 am) may not blow if the relay fails in some way. Additionally, what are the failure modes? Even though typically they're very reliable. It could fail open or fail closed. Depending on which mode you're using the relay (NC = 87A, NO = 87) the attached device isn't going to work. But I don't see the risk of either a short or open causing a wire meltdown or worse. I'm positive not everyone will agree so please no flaming. Obviously nothing wrong with fusing that side of the relay if that's what you want.

2. I'm not familiar with having to add diodes or whatever for a relay to function. Like I said, I've added dozens and typically they're plain vanilla automotive style relays.

One other aside comment. There was a lot of forum chatter several years ago about whether a relay was required for the start wire in order to protect the ignition switch. Enough that on my #7750 build I did add the relay as you described. But by all accounts the typical automotive starter relay only draws 8-10 amps, after a very brief higher inrush when first powered. By all accounts the ignition switch can handle this current and thousands of builds run that way. Additionally, if you're using a clutch safety switch, likely it has even less current rating than the typical ignition switch. Again, FWIW.

[John Ibele]

My thoughts FWIW:

1. You alluded to a distinction between the coil side of the relay and the circuit side which is powering whatever device is being used. That's important. The device powered side (typically pins 30 and 87 for automotive style relays) must always be protected. Could be any variety of ways as you described. The coil side could also be protected. If you search on the topic, you'll find many who say it's required. I will admit to (1) adding a number of relays in my builds, and (2) not always having protection on the coil side. My thinking? The draw on the coil is very low. Typically .1 amp or so. Even a very small fuse (2 - 3 am) may not blow if the relay fails in some way. Additionally, what are the failure modes? Even though typically they're very reliable. It could fail open or fail closed. Depending on which mode you're using the relay (NC = 87A, NO = 87) the attached device isn't going to work. But I don't see the risk of either a short or open causing a wire meltdown or worse. I'm positive not everyone will agree so please no flaming. Obviously nothing wrong with fusing that side of the relay if that's what you want.

2. I'm not familiar with having to add diodes or whatever for a relay to function. Like I said, I've added dozens and typically they're plain vanilla automotive style relays.

One other aside comment. There was a lot of forum chatter several years ago about whether a relay was required for the start wire in order to protect the ignition switch. Enough that on my #7750 build I did add the relay as you described. But by all accounts the typical automotive starter relay only draws 8-10 amps, after a very brief higher inrush when first powered. By all accounts the ignition switch can handle this current and thousands of builds run that way. Additionally, if you're using a clutch safety switch, likely it has even less current rating than the typical ignition switch. Again, FWIW.


Hey Paul - I'm aligned with your thinking on question 1.

Question #2 ... as I understand it they aren't used to help the relay function per se, but are included as part of the coil circuit to reduce the introduction of transient noise into the system when the coil is activated. Having said that, you gave me precisely the answer I was looking for: use plain vanilla relays and you're unlikely to get into trouble based on a wide variety of auto applications.

Appreciate the added note. Makes sense. I think I'll do whatever suits my fancy when I get to that point :rolleyes:

Thanks for the advice.

[rich grsc]

Hey Paul - I'm aligned with your thinking on question 1.

Question #2 ... as I understand it they aren't used to help the relay function per se, but are included as part of the coil circuit to reduce the introduction of transient noise into the system when the coil is activated. Having said that, you gave me precisely the answer I was looking for: use plain vanilla relays and you're unlikely to get into trouble based on a wide variety of auto applications.

Appreciate the added note. Makes sense. I think I'll do whatever suits my fancy when I get to that point :rolleyes:

Thanks for the advice.
No, the relays don't produce interference, and there is no need to use a relay to active a relay(starter solenoid). As Paul say's it's a low amp load, easily handled by the switch

[Blue Viking]

I just recently learned something new about relays. The relay is spring-loaded to the 87a position. When it is energized, the magnet pulls it off the 87a and onto the 87, where it stays as long as the relay is energized. When the relay is de- energized, the spring pulls it back to the 87a position. That quick spring-activated motion apparently can induze a voltage spike of quite high voltage. That's where the diode comes in to play. It is positioned "backwards" in relation to the energizing current, and as such does nothing when energized, but it kills the potential damaging voltage spike when de-energizing.

[John Ibele]

I just recently learned something new about relays. The relay is spring-loaded to the 87a position. When it is energized, the magnet pulls it off the 87a and onto the 87, where it stays as long as the relay is energized. When the relay is de- energized, the spring pulls it back to the 87a position. That quick spring-activated motion apparently can induze a voltage spike of quite high voltage. That's where the diode comes in to play. It is positioned "backwards" in relation to the energizing current, and as such does nothing when energized, but it kills the potential damaging voltage spike when de-energizing.

Exactly. What I'm taking from this conversation is that when you're designing PCB's with sensitive components and worrying about things like ESD, it's a consideration. When you're building a car, it's not.

[sread]

Actually there is at least one application on your car where it would be recommended to use a diode protected relay. If you are using the ecu to trigger something, typically a cooling fan, it can protect the ecu from flyback voltage or some other issue with the ground circuit. The guys on the Holley efi support forum mention it repeatedly although I don't recall seeing it in the Holley install manual. There's really no reason not to use one - they don't cost much more and often good quality relays include them already.

[Nigel Allen]

Sread is spot on. 'Freewheel' diode is the term used. It is there to short out the high reverse voltage (back EMF) that comes from the relay coil as the magnetic field collapses in the coil when the power is removed. For any applications where a relay is controlled by an electronic device, a diode protected relay is a must. This includes indicating LED's wired in parallel with the relay coil. I have made these fail by neglecting to use relays with freewheel diodes incorporated into them.

The only issue with using diode protected relays is that the correct polarity must be observed for the coil. Terminal 85 must be negative, terminal 86 must be positive. If reversed, the diode will act as a short circuit and will blow the control fuse, or the diode will turn into smoke.

Oystein, close :) . In an automotive relay, the movement of the spring-loaded contact arm does not cause the reverse voltage (back EMF). Instead, reverse voltage is caused by the collapsing field of the winding in the electro-magnet (coil). Its the same principle of an ignition coil, except that the collapsing magnetic field is directed into an adjacent coil to produce a much higher voltage, which then goes to the distributor.

On the subject of fuse protection. In order to reduce electrical fire risk, every circuit in a car should have a fuse in the positive supply, only exception being the heavy conductor from the battery to the starter. So, what size fuse to use? The fuse should have slightly higher current rating than the device to be protected, but also lower than the cable current rating between the fuse and the device. In relation to protecting the coil (control) of a relay you only really need to protect the wiring to the coil of the relay, as the coil current draw is very low. Fortunately, when coils fail, they usually go open circuit and dont cause a short circuit.

Hope this helps a bit.

Let me know if you need more info.

Cheers,

Nigel

[Blue Viking]

Sread is spot on. 'Freewheel' diode is the term used. It is there to short out the high reverse voltage (back EMF) that comes from the relay coil as the magnetic field collapses in the coil when the power is removed. For any applications where a relay is controlled by an electronic device, a diode protected relay is a must. This includes indicating LED's wired in parallel with the relay coil. I have made these fail by neglecting to use relays with freewheel diodes incorporated into them.

The only issue with using diode protected relays is that the correct polarity must be observed for the coil. Terminal 85 must be negative, terminal 86 must be positive. If reversed, the diode will act as a short circuit and will blow the control fuse, or the diode will turn into smoke.

Oystein, close :) . In an automotive relay, the movement of the spring-loaded contact arm does not cause the reverse voltage (back EMF). Instead, reverse voltage is caused by the collapsing field of the winding in the electro-magnet (coil). Its the same principle of an ignition coil, except that the collapsing magnetic field is directed into an adjacent coil to produce a much higher voltage, which then goes to the distributor.

On the subject of fuse protection. In order to reduce electrical fire risk, every circuit in a car should have a fuse in the positive supply, only exception being the heavy conductor from the battery to the starter. So, what size fuse to use? The fuse should have slightly higher current rating than the device to be protected, but also lower than the cable current rating between the fuse and the device. In relation to protecting the coil (control) of a relay you only really need to protect the wiring to the coil of the relay, as the coil current draw is very low. Fortunately, when coils fail, they usually go open circuit and dont cause a short circuit.

Hope this helps a bit.

Let me know if you need more info.

Cheers,

Nigel

Thank you for the clarification Nigel. I didn't quite understand the exact cause of the spike when it was explained to me the other day, your explanation makes perfect sense to me. I have installed a relay in the brake light cirquit to control the release of the parking brake. I now have a flickering of some of the lights on the instrument panel, and I suspect said spike to be the culprit. The relay seems to flutter a bit when activated, I think it is a cheap poorly constructed relay. I have now ordered some Bosch relays with diodes built in, hopefully they will solve the problem.

[CraigS]

One easy way to protect the coil power is run your main power lead to the 30 using a 'doubler' wire end and run a short jumper from 30 to 86
161685
https://www.westmarine.com/buy/ancor--nylon-multi-stack-disconnects--P009_275_004_500?recordNum=28
A way to provide protection for your relays would be to use one of these.
https://ae01.alicdn.com/kf/HTB123OiSXXXXXXsaXXXq6xXFXXX6/4-Way-Fuse-Box-DC-12V-24V-Max-DC-30V-Car-Truck-Auto-Blade-Fuse-Box.jpg
or
https://i.ebayimg.com/images/g/FFsAAOSwqu9VUog5/s-l300.jpg
You mention passenger side firewall location. Depending how you wire your battery to the starter you could mount one of these in that area.
https://i.ebayimg.com/images/g/MAQAAOSwF6dZc1ti/s-l300.png
Put the battery cable, and the starter power cable, and maybe the alternator on one stud and use the other stud to supply your fuse block or relay block. BTW those three heavy cables can be protected by a double layer of split loom.
https://i.ebayimg.com/images/i/141775695195-0-1/s-l1000.jpg
There are other types of wire protection. I like the looks of the braided stuff but the split loom has the huge advantage that it can be installed after the fact w/o needing to disconnect a wire.

[John Ibele]

CraigS, I'm well down a path generally in line with your suggestions, with my beefy wires protected and brought together on a bus bar behind the firewall, and a few auto-resetting breakers used in the right spots. But your links will be of value to others, I know. The use of unfused power to the 12V side of the relay coil in certain cases is the main thing that caused me to ask my initial question, and Paul's answer sorted me out there. I'll take your suggestion of jumpering from 30 to 86, since the current requirement for the relay coil is miniscule.

Nigel, thanks a bunch, that confirms what I had read elsewhere, but just couldn't tie to the sensitivity of devices we typically have in our cars. It's coming together now.

And like Oystein, I too have some flickering LEDs on my dash when I have a dimmer activated, which suggests some abuse of the LEDs over time. That's something I'll need to rectify (sorry, couldn't resist ... oops, did it again ...) with an added diode or two.

Thanks all for the contribution of knowledge!

[Blue Viking]

I ordered a quality relay from Bosch with a diode, and replaced the cheapo non-diode relay, and that cured the flickering. I tried to upload a couple of video clips, but couldn't figure out how, so heres a link, hope it works. : https://photos.app.goo.gl/U8QwWsXDzVCVYPfw5

The relay is driven by the brake light feed, and manages the"OFF" signal lead from the electronic parking brake switch to the computer. The lights flickering were the High beam and L+R turn signal indicators.