Air Horn Relay question

[hootmon]

OK.. This is a Guru Question..
I've installed a few air horns and have never done what is described.. BUT, is it a good idea??
(Maybe this is the reason for all the bad regulators on Magna's  )

In the video below (about installing a horn on a Victory Cross Country)..
The presenter spends a good part of the video explaining a relay - nothing I didn't know -
But then says you need to add a Diode to the activating voltage posts..
(You can jump to 3 minutes into the video)

Any one want to agree or disagree with this mans logic on this point???

Thanx in advance...

VIDEO

[TLRam1]

From my little bit of knowledge, diodes change AC to DC and allow current to go one way and not the other, so I have no idea.

[lragan]

I would be surprised to learn, in fact shocked (no pun intended) to learn that the voltage kick from a small relay, like my horn relay, would take out my regulator/rectifier.  I don't think that is why we lose them.  If we surveyed the folks who have lost R/R modules, we would find that most of them do not have air horn relays installed.  These little relays don't store enough energy to do much damage.

I could be wrong about this.  I have air horns on my bikes, and have lost one R/R. -- the one that I had been too lazy to relocate.  I posted elsewhere about the price collapse of replacements, and I have replaced the dead one, and have a second as a spare.  So far, no problems.  I did relocate the new unit into the air flow.  I have yet to learn of a relocated unit failing.

As for his explanation, he confuses power and current rather annoyingly.  But he is right, the coil will generate a voltage kick when you open the circuit that drives it.  This is how an ignition coil gets the high voltage to drive a distributor/spark plug.  Remember breaker points?  That was their function.  The diode will limit the voltage that is generated by providing a path for the current around the coil.  Instead of, as he says, tens of volts, the voltage will be limited to less than one volt.  This will keep his Victory status lights from going nuts.

In my opinion, this is not necessary on your Magna.  It won't hurt anything, and might even preserve your horn switch by preventing it from arcing as it opens.  Next time I am into mine, I may just add a diode to each horn relay.

[hootmon]

Thanx Lawrence..
I was TOTALLY joking about the Regulator thing...

This will be going on the wife's new Victory.. A voltage spike like that would much more likely take out a ECM than a R/R

Seems more like an insurance policy than a necessity, but Who knows when you will need your insurance... Not often, but when you do, you're glad it's there..

Other than the 12 volt requirement, is there anything else I should look for in the diode? I know resistors are rated in Watts, but can't remember about diodes...

[lragan]

Diodes come in a wide variety of sizes and materials.  The smallest ones, glass cases, are very cheap and probably adequate.  The primary specification you need to check is the reverse breakdown voltage.  I would not use one with less than 25 volts.  Our systems are only nominally 12v.  Peaks of 18V are not uncommon -- even though they don't last long enough to read on a voltmeter, they are long enough to break down a diode.  Once breakdown occurs, failure happens in microseconds.  Also look for forward current maximum of 50 mA or more. 

Finding a diode to meet these specs should not be difficult or expensive.  The diode shown in the video will, in all likelihood, exceed these specs. 

[Smoked U]

If the Horn/relay is on a separate circuit in the fuse box of the bike, how big of a spike could you possible get?

Also, if the circuit has already been converted at the regulator to 12v DC, how would the diode help anyway with a spike?

[lragan]

If the Horn/relay is on a separate circuit in the fuse box of the bike, how big of a spike could you possible get?

Also, if the circuit has already been converted at the regulator to 12v DC, how would the diode help anyway with a spike?

Good questions, Dave.  

1) As I mentioned, I don't think this is an issue for the little horn relays I use, on a Magna.  Apparently, the Victory has some indicator lights whose drive circuitry is a bit sensitive.  The separate fuzes are commonly connected across the power buss, so spikes can migrate right through the fuses to the other circuits.  It is true that separate circuits minimize the spikes on other circuits, but they are not eliminated.

2) "12 VDC" is a myth.  The only time your bike really has a constant voltage across the battery is when it is not running.  The very nature of our regulator/rectifiers creates large spikes on the "12V" line -- meaning to every electrical part on the bike.  I wish I had captured the oscillographs of my battery voltage during charging when I took them.  You would have to see it to believe it.  If I ever have cause to look at them again, I will capture and post them.  That all our electrical stuff works with these spikes on them is why I don't think the relay spike is significant.  It could not possibly approach the voltage of the spikes that the alternator/regulator/rectifier system puts out.

[Smoked U]

Thanks Lawrence. i figured you or Rick would clear this up.

Electricity is a strange and awesome universe, indeed.