A word of warning !!

While I was working on my electrics today I noticed something with the relays I will be using that I thought I would share with you, have you ever noticed that some relays have a "Resistor" or a "Diode" in the primary side of the relay ? If so have you ever wondered why it is there ?
Lot's of you guys are fitting more and more electronic devices to your bikes (ignition modules, instruments, system controllers etc) so I thought it made sense to cover this topic - the old motto "better safe than sorry" comes to mind as diagnosing the potential issues that can be caused by using "un-protected" relays with electronics will catch out even the most experienced enthusiast 

These relays have the resistor or diode to block "reverse voltage spikes" when you turn them off - sounds strange I know but the winding in a relay generates a magnetic field (just like an ignition coil) which when turned off inducts a voltage (same as a coil secondary/HT)

I have included a screen shot from an oscilloscope which shows a regular 4 pin autoparts store relay inducing a momentary spike of nearly 400v 

 



This is the potential scenario if not using a protected relay with electronics

You fit your new gizmo with a regular relay - turn it on, works great - turn it off - turn it back on and it doesn't work (the 400v back EMF has fried it) so you do your checks, relay works, wiring is all good so "gizmo" must be faulty. Take it back to the store and get a new "gizmo" fit it, turn it on, happy days, turn it off and "poof" new gizmo is fried 
Do your checks, wiring is ok maybe a "bad ground" no that's ok - so back to the store. Store guy is not happy to replace another gizmo but does begrudgingly. You take it home fit it and guess what happens next !!!!!!!!!!!! This time store guy tells you to f#*k off 

A "flywheel diode", reverse polarised, across the coil's terminals will "snub" that spike, even if using a relay that does not have an internal resistor.
I routinely use 1N4001 or 1N4007 diodes.

 

H.

Thanks for that Wookie.
I have to throw a diode on my starter solenoid and horn relay soon.
I was just looking at a video last night concerning this.

Here's a video from EEVblog discussing the issue.

I thought the symbol between 85-86 was a coil. Are you saying it’s indicating a diode?

In Wookie's photo between 85 and 86 is a coil. You are correct.

The one below has a built in diode.

 

F64 wrote:

In Wookie's photo between 85 and 86 is a coil. You are correct.

The one below has a built in diode.

 

 

With the one you posted you need to be mindful of coil polarity?

Yes, with a diode inside the relay, you need to have the correct polarity on the relay's coil.
There is a "+" sign next to the 86 terminal.

Mikaw wrote:

I thought the symbol between 85-86 was a coil. Are you saying it’s indicating a diode?

Good spot Mikaw (I must have been having a senior moment with that diagram) this is what a relay with resistor looks like

 

slmjim here.  Twenty+ years career in photocopier & printer hardware before moving into print / scan / fax Network Analyst arena.

Low voltage DC solenoids are all over those devices, some of which are quite robust.  All have spike suppression diodes built in where they can't be seen except as a small lump under the coil wrap, or in the schematic.  Those diodes serve exactly the same purpose; to keep spikes caused by the field collapsing through the coil from spiking voltage & current back into the control PCB's.

I've never seen a resistor as spike suppressor across a coil in that type of equipment, only diodes.  What is the value of the resistor?  Seems it would add undesireable load.

Good Ridin'
slmjim

slmjim+Z1BEBE wrote:

slmjim here.  Twenty+ years career in photocopier & printer hardware before moving into print / scan / fax Network Analyst arena.

Low voltage DC solenoids are all over those devices, some of which are quite robust.  All have spike suppression diodes built in where they can't be seen except as a small lump under the coil wrap, or in the schematic.  Those diodes serve exactly the same purpose; to keep spikes caused by the field collapsing through the coil from spiking voltage & current back into the control PCB's.

I've never seen a resistor as spike suppressor across a coil in that type of equipment, only diodes.  What is the value of the resistor?  Seems it would add undesireable load.

Good Ridin'
slmjim

I don't know the value of the resistor unfortunately, only that it is greater than the resistance of the relay coil. Because it is in "parallel" with the relay coil and electricity is lazy, under normal operating conditions the current passes through the relay coil (the path of least resistance) so any load increase is negligible 

Wookie58 wrote:

slmjim+Z1BEBE wrote:

slmjim here.  Twenty+ years career in photocopier & printer hardware before moving into print / scan / fax Network Analyst arena.

Low voltage DC solenoids are all over those devices, some of which are quite robust.  All have spike suppression diodes built in where they can't be seen except as a small lump under the coil wrap, or in the schematic.  Those diodes serve exactly the same purpose; to keep spikes caused by the field collapsing through the coil from spiking voltage & current back into the control PCB's.

I've never seen a resistor as spike suppressor across a coil in that type of equipment, only diodes.  What is the value of the resistor?  Seems it would add undesireable load.

Good Ridin'
slmjim

I don't know the value of the resistor unfortunately, only that it is greater than the resistance of the relay coil. Because it is in "parallel" with the relay coil and electricity is lazy, under normal operating conditions the current passes through the relay coil (the path of least resistance) so any load increase is negligible 
 

Seems the spike would conduct through the coil instead of the resistor then?  Perhaps the device printed as a rectangle across the coil on the relay cover is in fact a a diode, but is represented as a generic symbol?  Although the relay posted doesn't indicate coil polarity as does the one F64 shows.  Feel like I'm missing something obvious...

Good Ridin'
slmjim

slmjim+Z1BEBE wrote:

Wookie58 wrote:

slmjim+Z1BEBE wrote:

slmjim here.  Twenty+ years career in photocopier & printer hardware before moving into print / scan / fax Network Analyst arena.

Low voltage DC solenoids are all over those devices, some of which are quite robust.  All have spike suppression diodes built in where they can't be seen except as a small lump under the coil wrap, or in the schematic.  Those diodes serve exactly the same purpose; to keep spikes caused by the field collapsing through the coil from spiking voltage & current back into the control PCB's.

I've never seen a resistor as spike suppressor across a coil in that type of equipment, only diodes.  What is the value of the resistor?  Seems it would add undesireable load.

Good Ridin'
slmjim

I don't know the value of the resistor unfortunately, only that it is greater than the resistance of the relay coil. Because it is in "parallel" with the relay coil and electricity is lazy, under normal operating conditions the current passes through the relay coil (the path of least resistance) so any load increase is negligible 


 

Seems the spike would conduct through the coil instead of the resistor then?  Perhaps the device printed as a rectangle across the coil on the relay cover is in fact a a diode, but is represented as a generic symbol?  Although the relay posted doesn't indicate coil polarity as does the one F64 shows.  Feel like I'm missing something obvious...

Good Ridin'
slmjim

Hi Jim, please see below