Just double checking.  Never modified the wiring to fit a dedicated lighting circuit voltage regulator before on a bike not fitted with one from the factory.

I have no wish to blow the new sealed beam unit and will fit a voltage regulator to protect it.  Is it like the Zener diodes on the old BSAs where it is just tapped into the feed from the lighting coil at one end and to ground at the other?

So I would be tapping into the yellow/white from the lighting coil with the yellow wire on the unit below and ground with the black?  I seem to remember the 175 had that arrangement.

If it's your 1977 DT100, I would expect that model to be "new" enough to have one fitted as standard - Service Bulletin referred to 1976 and earlier bikes. I guess you could double check by attaching a volt meter to the battery with the engine running (light switch on, but headlight bulb disconnected) and see what voltage it's getting.

If you do need to fit a regulator, then splice it into the blue wire going into the speedo backlighting - I made up my own pigtails. 

Service Bulletin is here.

Vintage Yamaha Enduro Fans - Service Bulletin #M5-084 re Voltage Regulators (yamaha-enduros.com)

 

YES!!

Exactly as GR8UNCLEAR states. I used a slightly different unit off eBay, but this one should surely work fine. Does the one you show have an indicator light? Mine does.

After I plugged it in and started the bike up I did not initially see that indicator light come on, but it did when I switched the lights on. Revving the bike up with the lights on, I could definitely see the headlamp brightness increase up to a point, and then the brightness dropped back as the revs continued. I did not check the voltage at the battery as I did this (I intend to), but feel quite sure the voltage to the lights are now restrained to an acceptable level.