You don't need a voltage regulator with a conventional car dynamo. Just place a wire wound variable resistance in series with the field connection and adjust as necessary {as you would on a full size dynamo}.

Worth remembering that the output voltage (without regulator) will depend on engine rpm, if you are looking at a steady load then simply setting the regulator and keeping steam constant will probably suffice, if your not to worried about lamps dimming when steam and revs drop a bit, if the load is variable, then you'll be relying on engine governors, which in 4" are probably not too reliable, so could lead to blown bulbs etc.

Yes , but the car type regulator does that automatically .

The original regulator if it is available would be the easiest solution, a variable resistor the cheapest, but one of these controlling the Field would be a modern efficient solution.

Or do it electronically, depends how much the load current is but you can get some fairly cheap small electronic units to give you a constant 12v output regardless of engine speed once its generating.
If I owned a 4" Showmans I'd also look at including a small'ish battery to enable the lights to run for a short time without having to belt up, also would allow the lights to run while travelling. Once belted up this would then recharge the battery.
Once you have a stable power source it then allows all kinds of modern paraphernalia to be fitted, how about a showmans with USB phone charging ports, charge your phone while at the beer tent thumbsup

My link above is to an "electronic unit" LiveSteam you must have missed it.


http://uk.farnell.com/linear-technol...9&CMP=KNC-GUK-

Apologies, yep missed it totally :o

Although 5A isnt a huge amount, around 60w, not knowing what the lamps would be on a 4" showmans but lets say each one is 2w that is only 30 lamps, might be enough though as thats 10 per side and 5 front and back ? doesnt leave anything in reserve, I'd be happier with something around 10amps in a TO3 package, may be a LM396K ?
http://pdf.datasheetcatalog.com/data...tor/LM396K.pdf

LiveSteam and anyone else who is interested.

The voltage regulator would not be directly connected to the output it would be connected in series with and control the field coil only, that is how the original type regulators are connected, the full output of 20 - 40 Amps at 12 Volts would then be available .

I have guessed the current requirements of the field coil, the first thing I would do if I had a suitable dynamo is to check the resistance of the coil to enable me to select a suitable electronic regulator.

I would also set the dynamo up on a Lathe or Drilling machine to get an idea about the speed needed for 12 Volts.
A quick Google suggests a speed of 1500 - 2000 RPM is needed for maximum output.



Below is a diagram of the original setup.





http://www.tractiontalkforum.com/pic...ictureid=40355

So are you saying if you 'clamp' the field windings at 12v you will never get more that 12v output no matter what speed you run at? :confused:

No, while Steamy1 is kind of correct with their comment connecting the electronic voltage reg to the field, this wont regulate the output, my assumption was for not using a battery, and thus connecting everything up in series and hoping for enough residual magnetisium to start the generation process.
If you are adding a battery then stick with the standard mechanical regulator.