Yamaha XT550 regulator

**Wired1** · 25th May 2008, 18:26

Hi experts, I'm on the final leg of rebuilding an XT550 that came to me in a lot of boxes. I have fitted a pretty nifty looking headlight and want to power it off the charging circuit. Measuring the output from the single phase alternator gives me about 30 volts ac so that is a bit big for my 12 volt lights. I could change the bulbs to 24 volt ones and leave it as ac or I could fit a regualtor/rectifier and hope that it will happily shunt the excess voltage away - is this right? I don;t want to fit a battery so is there any other options? Cheers...

**FilthyLuka** · 26th May 2008, 00:27

Your best bet is running lights on DC, so setting up a rectifier circuit with some caps to even shit out. Then using appropriate resistors to get the voltage to 24 for the headlight

**Motu** · 26th May 2008, 19:12

You could use a zener diode like the Lucas system on British bikes - it shunts voltage over 14.75 or there abouts to earth.But your system probably puts out too much,so you'd need 2 zener diodes,as some of the later Triumphs and Nortons did.Use a cheap rectifier from Jaycar or Smith the Dick.

These work as a full shunt to earth,so the alt is working full out all the time - a voltage regulator would be better....run with a drain all the time,lights and maybe a resistor or capacitor to take some of the output.

**Wired1** · 26th May 2008, 19:35

Yes I think I need a regulator/rectifier. I bought one last night on turdeme so hopefully it will handle the input voltage and output load OK. Anyone know where I can find a generic wiring diagram for this? It appears to be a single phase rectifier and I'm hoping it's got a regulator built in as I think most of them do these days. If it goes bang it was only $13 to find out!

**Motu** · 26th May 2008, 20:06

What is the wiring on the bike? Usually yellow is ac from the stator,black is earth and red 12v DC out.So the important thing is how many yellow wires? 2 yellows will be a 3 wire reg,3 yellows a 4 wire reg.I'd say the yellow and green will be AC...but how many wires on reg,how many on the bike?

**Wired1** · 26th May 2008, 20:25

The XT has two white wires that weren't attached to anything and measuring them running I get 30 volts ac so I'm guessing these are the alternator output from a single phase alternator. Actually I've just checked the manual and this is right, two white wires are the alternator. The original bike had a five-wire regulator/rectifier - two ac in (white), a ground out (black), a red to the battery, and a brown to the rest of the load.

I haven't received the rectifier yet but it looks like it has red, black, pink, white and green. I guess the red and black will be the same but the other three might be a case of trial and error. I'm not planning on running a battery so I think I have to add an electrolytic capacitor from red to earth to simulate it.

**FilthyLuka** · 28th May 2008, 14:06

Originally Posted by Wired1

The XT has two white wires that weren't attached to anything and measuring them running I get 30 volts ac so I'm guessing these are the alternator output from a single phase alternator. Actually I've just checked the manual and this is right, two white wires are the alternator. The original bike had a five-wire regulator/rectifier - two ac in (white), a ground out (black), a red to the battery, and a brown to the rest of the load.

I haven't received the rectifier yet but it looks like it has red, black, pink, white and green. I guess the red and black will be the same but the other three might be a case of trial and error. I'm not planning on running a battery so I think I have to add an electrolytic capacitor from red to earth to simulate it.

Find out what each wire is and go from there. Electrolytic cap would be the way to go. Double check your manual and make sure its not a three phase generator, cause if so you will need a different form of rectumfrier

**FilthyLuka** · 28th May 2008, 14:10

three phase rectumfrier

**Wired1** · 28th May 2008, 17:43

Thanks, it is definitely a single phase alternator. I had it running tonight on an old yamaha regulator/rectifier that I'm not sure the history of - and it produced 30-odd volts dc which tells me the rectifier bit works fine but the regulator bit doesn't bugger it. I still have to try the cheap chinese regulator/rectifier I bought the other day. It arrived and only has four wires (2 x ac input and 2 x regulated dc output?) so I have hooked it up ready to test tomorrow. The first regulator blew my low beam headlight so I'll use an old indicator as a test load before getting too carried away!

**FilthyLuka** · 28th May 2008, 20:26

you could try a potential divider setup for getting the voltage right?

http://www.iop.org/activity/educatio...g_mid_4087.gif

something like that?

**Wired1** · 28th May 2008, 20:39

Originally Posted by FilthyLuka

you could try a potential divider setup for getting the voltage right?

http://www.iop.org/activity/educatio...g_mid_4087.gif

something like that?

Yes that's a possibility. That way I could leave it ac and no need for a regulator or rectifier. The only catch is the lamp filament has a very low resistance (if it's 50 watts then it must be 2.88 ohms) so I'd need a 4.6 ohm resistor rated at 50 watts and that's a heater.
Another more barbaric approach would be to put the tail light and headlight in series, which would split the 32 volts to 16 volts across each more or less (depending on the wattage of each), but 16 volts would probably blow them anyway.

**Wired1** · 29th May 2008, 18:36

I have had success with the new chinese regulator. I tried an old Yamaha regulator I had first just to check it was really buggered and it is so I blew all the bulbs! The buggered regulator was rectifiying but not regulating so it was pumping 30 volts dc into everything.

Next I experimented with the voltage divider but the resistors I was using were only rated for 5 watts so they quickly got bloody hot. If this had worked it would have been a nice simple fix as I wouldn't need a regulator as I don't need to charge a battery etc. I added a diode to chop out every second half wave and effectively halve the voltage but the peak voltage was still too much for the bulbs.

Finally I fitted the new regulator with my best guess for wiring and thankfully it worked. I am not switching the lights as they may as well be on all the time, and I have read some regulators need a minimum load to make them work so this way there is always a load connected. It works a treat and the regulator only got comfortably warm after half an hour or so of me buggering around connecting it up properly.

**Wired1** · 2nd June 2008, 12:58

Bugger - blew the new regulator up.
It was working nice for about five minutes, measured around 14 volts at the output of the regulator and the lights were working. No increase in brilliance with revs so I figured it was sorted.

I didn't bolt the regulator to the frame, just left it tucked under the seat so maybe it needs some more heat-sink?

I took her for a blat up the road (it has shitloads of grunt, what a blast) but by the time I got back, maybe a minute of high revs, it had stopped. My first thought was that I had blown the bulbs again but they work with a seperate battery so it's the regulator bugger it.

I guess the regulator I bought was designed for a lower input voltage as two bulbs is hardly a heavy current load. The problem is whenever you buy these things off the net the sellers never know the ratings of them or can supply wiring diagrams. I guess I can get the same sort of thing from an autoelectrician but at what cost?

**FilthyLuka** · 2nd June 2008, 15:38

Doesn't cost anything to ask em eh? Under the seat aint really a good place, you need somewhere with lots of airflow. You can do the old bonneville trick and bolt the reg to the front of the lower triple clamp, lots of airflow there