Electrical Question for Honda CB Headlight Resistor

[caferacer]

Awhhhh... badgers nadgers...

Bike: Honda CB 125 RS
Age: 1984
Problem: Ceramic Resistor for Headlight Circuit Broken
Mission: Replace Ceramic Resistor

Background
The ceramic resistor on my 125 has given up the ghost. It looks like the ceramics cracked a long time ago and the resistor wire inside has rusted through and broken.

Info
The electrics on the bike are 12 volt and from what I can tell the resistors are included in the circuit between the headlight and earth to protect the headlight from surges in power as it's directly powered by the stator.

First Attempt
The wiring diagram says I need 6.7 Ohms so I bought two 3.3 Ohm 10 Watt resistors from Maplin and put them in series to produce 6.6 Ohms at 20 Watts.

https://www.maplin.co.uk/p/wirewound-10-watt-33-ohm-resistor-h3r3

When I turned the engine on they immediately got hot and started smoking.

Something more substantial required?
So I assume the wattage is a lot higher than 20 Watts even at idle speed and as maplin don't have 7no 1 Ohm 10 Watt resistors in stock I started looking at 100W ones from RS Components.

https://uk.rs-online.com/web/p/panel-mount-fixed-resistors/0188116/

Not sure how I'd draw the heat away from this though.

Honda CG Resistors
Having noticed that there are plenty of CG headlight resistors on ebay which could be quite similar to my CB headlight resistor I started looking at CG wiring diagrams. Looks like they are 10 Ohm 20 Watt resistors. So why am I having this issue with wattage?

https://www.cmsnl.com/honda-cg125-1993-singapore_model2607/resistor-comp_35400kcs651/#whereused

Please help
Has anyone successfully replaced these resistors?
Am I going about this in the right way?
Has anyone seen replacements for sale?

Thanks for reading, I would appreciate any comments - even if it's just moral support![/b][/u]

[caferacer]

Ok, so here's a scanned image of the wiring diagram for the bike.

https://farm9.staticflickr.com/8096/8441419104_e6fff249c7_o.jpg

The resistor is at the top in the centre and shows it needs 6.7 Ohms on the left hand side. The 10W 3.3 Ohm resistor from Maplin for the right hand side seems to be working fine so lets not complicate things by worrying about that.

The P wire runs to the left handlebar headlight switch. The G wire is earth. The headlight is powered directly from the alternator.

[caferacer]

Here are two photos of the faulty resistor. The first when I removed it and found the problem:
https://farm8.staticflickr.com/7412/14128446322_1b48fb9152.jpg

The second after I'd tried to solder it back together and it had caught fire under my fuel tank:
https://farm6.staticflickr.com/5467/14151494803_f12dd3c410.jpg

Look familiar to anyone?

[davebike]

I think 20W rating is pushing it !

Potentially 14 v and 6.6 ohms gives 25watts
It is custom to over rate such thing at least 20% normally 50%

Did you bolt the 20w ones to a heat sink ?? they need one !

I try with a good heat sink if you not destroyed the resistors

then a series parallel matrix to give you 6.7/6.6 ohms at 40W

Must be bolted to a good heat sink alley plate will do

Dave
____________________
Dave
NC750Xdct + others at work !
davebike1@gmail.com

[caferacer]

Thanks Dave,

You're right, I was pushing the power levels going through these resistors at 20W.

I didn't realise people usually over estimate the wattage by up to 50%. I'm really only just getting to grips with electrics.

The 20W ones were not attached to anything as I was only trying them out.

With the 100W one from RS Components I was thinking I could bolt it to the panel the original ceramic one was mounted to and I could stick some of that heat transfer paste behind it? That way my whole bike frame will work as a huge heat sink - do you think this will work?

[davebike]

It will. work if you get a good thermal path

Dave
____________________
Dave
NC750Xdct + others at work !
davebike1@gmail.com

[caferacer]

I'm still interested if anyone knows anything about the Honda CG 125 Ceramic Resistors?

That still looks like it could be an easy solution?

[caferacer]

[Tedwood]

I bought a CB125RS a couple of years ago and that resistor was missing.

After a bit of research I found that the part number was 35400-KJ6-0003. Couldn't find it anywhere, but I found part 35400-KB9-9200 which I purchased on ebay from Italy it cost nearly £20.

The ebay seller I bought mine from was "bbricambi" in Italy and still appears to be selling them although not on ebay UK. Do a google search for the part number.

The resistors are rated 30W 3 ohm and 30W 6.7 ohm and was the resistor fitted to the XL125R. Haven't put my bike back together yet and tried the resistor but my guess is that it should do the job.

Also could try David Silver as he's very good at locating parts (even if they are not listed on his website) just email him and he will let you know if he can source it and the price.

I didn't know about David Silver when I got my bike so didn't try him before buying the XL125R one.

Link: https://www.davidsilverspares.co.uk/contact/

[smegballs]

I get all my electronic components from www.spiratronics.com. They are really fast delivery, cheap enough and have really good customer service.

https://spiratronics.com/50w-aluminium-clad-wirewound-resistor-6r8.html

50W 6.8 ohm resistor here for 2 quid.

[caferacer]

I bought the resistor smegballs suggested and attached it to a heat sink.

Seemed to be working fine but then this afternoon there was a pop and coils of wire all over the place.

Looks like I'm going to have to buy an original part from somewhere.

[Tedwood]

Did you see my message above re the xl125r resistor?

[Robby]

Can you re-wire it to use a more stable switched live and get away from needing a ballast resistor altogether?

If you want to avoid overloading existing wiring, you could run a feed direct from the battery activated by a relay connected to a switched live power source (such as the ignition circuit).

[smegballs]

Sorry my suggestion didn't work out. It must have been pulling a lot of juice though to blow up a heatsinked 50W resistor! Might as well put another headlight bulb in and benefit from extra lighting if that is the case.

[caferacer]

[Robby]

Yes, power it from the battery, but run the power feed through a relay connected to one of the switched live circuits so the circuit will only be live when the ignition is on - otherwise your headlight will work with the ignition off, and you'll end up leaving the light on and discharging your battery.

[Tedwood]

Hi caferacer,

The Italian seller has the Honda XL125R resistors (with a good photo) for sale online here:

https://www.ebay.com.my/itm/RESISTOR-ASSY-RESISTENZA-ELETTRICA-PER-HONDA-XL-125-R-ORIGINALE-35400-KB9-9200-/321401601526?

Also he will post to the UK, total 26 Euros (12 + 14 postage). I think that was the same price I paid a few years ago.

[caferacer]

[smegballs]

Glad you got it sorted.

My reaction would be to cry "oh hell naw" and start butchering the wiring, fitting a normal modern regulator/rectifier unit and having that do all the voltage regulation.

Bloody ballast resistors, not on my watch!

[caferacer]

Just a quick update.

The replacement ballast resistor arrived and works perfectly. Identical to the original. Well worth the money to get it right.

Thanks for all your help!

Unsurprisingly I'm now having issues with my indicators not flashing. I've created a new thread to explain:

https://www.bikechatforums.com/viewtopic.php?p=3905598