Power to alternator excitation circuit (1989 R100RT)

A question to all electrical gurus out there. I recently purchased and installed an Omega 600W alternator. The kit comes with a 'bypass circuit' used to provide redundancy to the alternator excitation power in case the instrument panel GENERATOR bulb burns out. I have installed a Katdash on this bike and believe that each light is serviced by two LEDs. if I understand this correctly, then the excitation circuit should already have a redundancy built in via the Katdash, so the extra bypass provided in the kit is double redundant, if that makes any sense.
Am I correct in my assumption, or should I go ahead and install the bypass anyway?
Thanks,
Marc

I bet the Katdash folks would know! If you hear something from them let us know, too!

I find no mention on their website of any additional bypass circuitry however. Probably makes sense...their product just replicates what is inside the instrument pod.

Omega Kit Bypass Circuit?

I too, recently installed an Omega 600, but do not recall the excitation redundancy circuit you describe. I like the idea, but do not recall there being one. I do recall an extra/add on wire from the diode board to the starter, but ASSumed that was just over engineering to carry the extra current. So I used it, and also kept the original wire in place also. This was possible, since the diode board had extra plug in spades.

Not interested in a KATDASH though, as I have grown too used to the idiot lights as they originally came. It would take me 10 years to get used to any changes. LOL

On any Airhead, Snowbum describes how a modification can be done to ensure that the excitation voltage gets to the rotor even if the stock path/bulb burns out:

https://bmwmotorcycletech.info/genlampresistor.htm

Omega 600W installation

jhall: the Omega 600 is offered by several vendors, including Ted Porter and Motorrad Electrik. I got it from Motorrad Electrik, there is mention of this bypass circuit on his web site. If you got it from Ted Porter, that could explain the difference.

Kurt: The bypass is EXACTLY as described in Snowbum's method #2 of the article you pointed to. The resistor value in mine is 330 ohm, which is one of the resistance values recommended by Snowbum. In fact, Snowbum was one of the first to evaluate the Omega 600, so there's no doubt in my mind that some exchange of information has been happening between the Omega designer and Snowbum.

My question was not whether the bypass installation was technically valid (I know it is), but whether it would be unnecessary for those who have installed a Katdash. The point being that since the Katdash uses two LEDs per idiot light, there is built-in redundancy making the bypass unnecessary.

Incidentally, I tried to find a way to contact the Katdash manufacturer, but after a quick perusal of their site I came up blank. I probably missed something. I'll try again and let everyone know what I find out.

Thanks,
Marc

Marc -

On the About page, there is contact info at the bottom. There is an email for "Kat" and when I click on it, it get the email address kat@katdash.com . There's also a street address and a phone number.

jhall said:

Not interested in a KATDASH though, as I have grown too used to the idiot lights as they originally came. It would take me 10 years to get used to any changes. LOL

Click to expand...

The Katdash board doesn’t change any of the lights on the dash. Everything appears exactly the same, just bright. The board is solid state and LEDs instead of foil embedded in plastic with incandescent bulbs. You install it and you should never have to remove or open the headlight cluster again.

I'm not specifically familiar with what's inside the KatDash, but to stir the pot a bit:
The factory setup uses an incandescent bulb of a specific wattage (see Snowbum's page). This bulb has a known resistance which varies as it lights up (or dies down), and this is the supplier of the desired excitation current.
Incandescent bulbs are not polarity-sensitive. And, their resistance, whether on or off, will be the same in either direction, and will depend on how much current is going thorough its filament.
Now the question is what happens if the bulb is replaced by a LED or two. LEDs definitely are polarity-sensitive.
As a solid-state device, it takes a specific amount of voltage and current to initially "ignite" the LED; this may be anywhere from about 0.2 to 0.8 volts, depending on what semiconductor materials the LED is made of.
When the LED is "Off", its forward resistance (in the "conducting" direction) is low, on the order of 20 to 50 ohms (again depending on what the LED is made of). The reverse (non-conducting) resistance is high, on the order of 10K (10 thousand) to 50K ohms.
To further pollute this, once the LED is "On", its resistance will change depending on how much voltage and current it is passing. This is a typical characteristic of a semiconductor device.

I really don't know "fer sure", but I'd wager that the bypass wire should be used with a KatDash.
Maybe Kat will jump into this discussion...?

Kat Dash plus

I have two airheads I have put Kat dash units in. My 78RS has only popped two bulbs in the time I have owned it but then again, I only ride it 5% of the time. My 84 R80RT has blown enough bulbs and the instrument cluster has been apart enough in 36 years and 240K miles and the plastic screw bosses broke or pulled out over time leaving me with only one or two screws holding the cluster together. I purchased a new housing from Siebenrock, and a Kat dash unit with the thought, never again will I have to replace bulbs or mess with the foil contacts in the OM system. Once the 84 was done, the RS was next for the Kat dash. Best money I have spent. Only time will tell when I will have to take the cluster apart again, and I am banking it will be a long time. St.

Incandescent bulbs die of age or high voltage (actually the high current). LEDs pretty much don't 'age' but die with too high voltage as well.

So the LEDs could die, hence a good old bypass with a resistor functions the same as with an incandescent bulb.

I think the reason for TWO LEDs is the ~0.25A that flow through the LEDs when you switch ignition on. One LED might not have the capacity, so they used two in parallel. But there might be another reason - who knows?

/Guenther

Versus

One of the nice things about the Kat dash I hope will mean I won't have to take my instrument cluster apart is diodes don' have filaments which are subject more to vibration. Also, the Kat dash units don't have the damned "foil" "flexible" contacts for the lights as found in the BMW stock units. Between blown bulbs and corrosion on the contacts over the years, I got sick of taking things apart to fix. Oh yes, I forgot the diodes are firmly seated and not subject to shaking out of their sockets another cause of failure in the OM system.

Face it, if diodes such as Kat uses in her units were available at a reasonable cost/selection back when the bikes were being built, I bet BMW would have used them instead of the system they came up with. St.

Condensation Issues

Having now read more on the KatDash, only possible drawback I see is the LEDs do not get as hot as incandescents, so my never clear up condensation under the speedo and tach lenses. I know condensation is not much of an issue, especially on BMW with fairing, but still that could be a detriment. Reading on KatDash website, they mentioned this issue, but sorta glossed over it.

Also, I would prefer the idiot lights not be any brighter than they already are. The current glow is easily visible in daytime, but not excessive at night. The KatDash brighter lights may be too bright at night time for me.

And the answer is...

Based on an email from Katdash

There isn't enough current going through the LEDs in the Katdash to use them as a path for alternator excitation current. Therefore, there is a dedicated circuit within the Katdash to provide said path. This circuit is completely independent of the LEDs, and therefore not subject to the same failure mode as the legacy instrument lighting design.

Said in other words: the purpose of the bypass circuit is to provide a backup in case of the failure of the GEN light in the legacy instrument lighting system. If you have installed a Katdash, a failure of one or even both the LEDs that illuminate the GEN light, WILL NOT cause the alternator to malfunction as it relies on a dedicated circuit independent of lights or lightbulbs.

Bottom line. If you have a Katdash, installing the bypass circuit is not wrong, but unnecessary because it addresses a failure very unlikely to occur. So my initial conclusion was sort of correct even though my reasoning was completely wrong.

Hope this clears the air, although at this point, I doubt it.

I would hope that Katdash adds something to their website and FAQs. I just searched their site for the word "bypass" and there were no hits. Maybe they have it explained under other terms. But again, it should be stated more clearly somewhere.

jhall said:

I too, recently installed an Omega 600, but do not recall the excitation redundancy circuit you describe. I like the idea, but do not recall there being one. I do recall an extra/add on wire from the diode board to the starter, but ASSumed that was just over engineering to carry the extra current. So I used it, and also kept the original wire in place also. This was possible, since the diode board had extra plug in spades.

Not interested in a KATDASH though, as I have grown too used to the idiot lights as they originally came. It would take me 10 years to get used to any changes. LOL

Click to expand...

The Katdash is just like the original only no bulbs to burn out and noticeably brighter. They do include the exciter circuit.

Scarlet - Goodonya for following this up! I was really curious.

Already Stated By Kat Dash

Scarletswirl said:

Based on an email from Katdash

There isn't enough current going through the LEDs in the Katdash to use them as a path for alternator excitation current. Therefore, there is a dedicated circuit within the Katdash to provide said path. This circuit is completely independent of the LEDs, and therefore not subject to the same failure mode as the legacy instrument lighting design.

Said in other words: the purpose of the bypass circuit is to provide a backup in case of the failure of the GEN light in the legacy instrument lighting system. If you have installed a Katdash, a failure of one or even both the LEDs that illuminate the GEN light, WILL NOT cause the alternator to malfunction as it relies on a dedicated circuit independent of lights or lightbulbs.

Bottom line. If you have a Katdash, installing the bypass circuit is not wrong, but unnecessary because it addresses a failure very unlikely to occur. So my initial conclusion was sort of correct even though my reasoning was completely wrong.

Hope this clears the air, although at this point, I doubt it.

Click to expand...

Reading on the home page of their web site: "And KATDASH has additional resistors in the GEN light circuit for the initial "excitement" current to get your alternator spun up, and keep your charging system charging. Works with the OEM alternator, as well as modern upgraded alternator systems." Cannot get any plainer than that.

They also say they now have a new, darker, frame color, since the original white one tended to leak light over in some places. I plan to order one today.

My confusion is using the word "bypass" versus replicating the OEM path for rotor excitation. It sounds like the Katdash system replicates the original excitation because the LEDs can't do that job...that's not a bypass. The term "bypass" used elsewhere to me is more about another path for this excitation that is not like the OEM approach. Snowbum talks about such a bypass by adding a resistor elsewhere in the bike's wiring for this. I still think the Katdash verbage needs more specifics to explain clearly what it is they're doing.