Size resistor for R27 LED charging light?

[hutchinson5234]

I add resistors to the post '70 airheads all the time when change to LED and customer doesn't want to install a Katdash.

I'm trying to do the same with a '66 R27.

Ohms Law was 50 years ago for me so need some help.

Stock 6V system, probably .03 watts LED.

I tried a 1 watt, 130 ohm and then a .5 watt 130 ohm resistor..

Red light stays on when engine revs up.

Am I at least in the ballpark for a resistor?

[schrader7032]

I'm not good at this stuff either. But you probably already know what Snowbum recommends for the Airheads...looks like he suggests a 1 watt 470 ohm resistor. Since you're dealing with a 6v system, should the resistor be half of 470??

[wa1nca]

1/8 watt resistor is all you need as far as size
You need to know how many watts the led bulb is using and then can calculate the resistance and add a dropping resistor in series with the new led and if you ever need to replace the led bulb with a different led bulb then you may also need to change the value of the resistor to match the new led or remove the resistor to go back to a original bulb.

Keep it simple:
Just use a 6 volt led bulb with the same base socket
I have used this bulb for all my 6 volt bikes and never had a failure

Vintage BMW Motorcycle Parts
https://vintagebeemerparts.com/bmw-6-vo ... odels-new/

[Seek]

All LED's are a bit different, but say yours uses 20 mA at 2 Volt. Then you have to burn of 6-2= 4 Volt in the resistor at these 20 mA. Ohm Law says R = U/I = 4/0,02 = 200 Ohm. I would choose a bit more because the Voltage in the bike can easily go up to 7.2 Volt. You don't want to fry your nice new LED. So 5.2/0.02 = 260 Ohm.

Use a normal light bulb. There is not a really good reason to go to LED in this application.

Edit: Doesn't the light bulb also have a purpose in providing a little bit of current to the generator at startup? Or is that only in the post '70 bikes? I kind of forgot, maybe the residual magnetism is enough to fire up the generator.

[hutchinson5234]

Forgot to mention that the stock 6V bulb doesn't light up when the ignition is on..fully charged battery..6.7V
Insert the LED and it does light up..go figure.

Checked the power in and ground..both perfect. Tried 3 different new 6V bulbs. None light..the LED does.

[wa1nca]

Seek: the red charge light/led is just a indicator that the generator is charging unlike the post 70 bikes
One side of the bulb is connected to the battery

The other side of the bulb is connected to the output of the generator
When the ignition is turned on (bike not running) their is no output from the generator

Now the bulb has battery voltage on one side and ground on the other side so it is on full
It will not work if their no battery or be dim with very low battery
When the bike is running and charging the bulb go out because now the bulb has battery voltage on one side and generator output on the other side (6 volts=6 volts) so no current flows thru the bulb

Greg
Forget the led bulb for now, your generator is not charging
I didnt note that earlier because you said your battery was 6.7 volts
That is your battery voltage but doesnt mean that your generator is charging, the gererator bulb is on because the generator is not working so the the generator bulb has zero volts on one side and 6.7 battery volts on the other side

[wa1nca]

Greg
Sorry you stated that your 3 bulbs for the generator light did not light with the key in and you have good battery voltage

The generator and neutral light are connected together on one side and get power off the switch board terminal 15 when the key is in
The neutral lights when the shifter switch goes to ground when in neutral
The generator light gets it ground from the small blue wire that goes to #61 on the switch board connection with another small blue wire that goes to #61 on the generator housing (which is almost ground via the generator brushes and armature while the engine is not running)
Tommy

[wa1nca]

Greg
Did you solve your problem with the generator bulb

PS :

The link you provided for your website: http://www.airheadadventture.com/
Should have only one T in adventure

https://airheadadventure.com/



Tommy

[hutchinson5234]

Thanks Tommy,

I should learn not to post at 2 a.m. It's amazing what I order and forget about until UPS drops off lots of boxes... The same goes for posts.

I just got back to the R27 late yesterday

Everything on the bike works EXCEPT the strange alternator light.

With the ignition on, the LED charging light will light but not go out with the engine running, but a switch to the stock 6V, the bulb will not light with the engine off or on. Voltage measures 6.2 in and out of the bulb contacts but no lit bulb. Tried three different 6V bulbs and swapped in the working bulb from the neutral side, where they all light up. Still no charging light or battery charging.

Got to be a simple blue wire issue...I hope. Back to point-to-point checking later today.

Same with websites...

www.airheadadventure.com

[wa1nca]

Thanks for the updated info

Forget the LED bulb for now It cant work if the standard bulb also doesnt work
Once the standard bulb gets working then the LED bulb should work, I have them in all my bikes

You stated you have 6.2 volts on either side of the bulb: It should be off !!
The bulb needs power (6.2 volts) on one side and ground on the other side to light up

The blue wire connected to the bulb goes directly to # 61 on the generator and is grounded when the engine is not running (via the brushes and armature )
When the engine is running and charging the voltage to the blue wire is about 6-7 volts so the charging light goes out because it has battery voltage on one side and generator output voltage on the other side which is almost equal (no current flow)

To trouble shoot:

Key on, ground the blue wire at the bulb, It should light because it has 6.2 volts from battery on one side and ground on the other side

Next ground the blue wire on #61 on the generator it also should light or the blue wire going to the bulb is broken
If it does light when #61 is grounded then their is a problem with the generator brushes,brush springs or unlikely a wide open armature

Ps
The headlight shell is not ground
Best ground is large brown wire on ignition switch, engine, battery negative

Dont give up