Tarpen Generating set page 3

The Evolution of A JAP Powered  Tarpen Generating Set (continued)

Part 5 – Electrical matters

With a DC dynamo running off any engine other than its original ,the output voltage is uncertain ( the wires above are connected to a voltmeter measuring the output )There are two ways to control and adjust the voltage , adjusting the engine speed so the dynamo runs at its designed RPM and raising or lowering the voltage electrically .

To acheive a good range of control I chose to do both .

The original JAP governor acts upon a rod connected to the carb throttle but for this application is unsuitable on its own . A manual engine speed control was contructed using a Lawnmower throttle lever and cable along with whatever metalwork (brackets) were to hand in the workshop . The original JAP simple metal bar throttle lock ( acts on the brass governor arm ) was turned 90 degrees to the vertical position and a suitable spring attached between that and the carb throttle .


Manual Throttle arrangement

This allows the engine speed to be adjusted quickly and easily,the spring holds the throttle butterfly open ( fast ) and the speed is reduced ( slower) by operating the throttle lever and closing the butterfly in the carb .

This can be seen in this video ( you tube )

As mentioned earlier  the Original JAP petrol tank fixing bracket ( which is one half of the engine breather casting )was cut down and ground flat , one because it was no longer needed and was in the way ! and two it made a suitable place to mount a control box for the dynamo .


Ebay purchase – ex Woolworths Battery Charger
which was used to form the basis of a Control Box

The arrangements for controlling the output voltage follow .The charger seen above was in such good condition it was almost a shame to dismantle it but it was bought for a purpose and that was that !

The battery Charger seen in the previous part , was dismantled ,re painted green and re constructed as a control box for the generating set . The ampmeter was removed and replaced with a voltmeter ( for checking the output voltage) which was mounted behind the front panel for a neater appearance. The two swictches were re wired as an engine stop and a master on/off switch .

The original rear mounted fuse was kept and rewired directly into the output from the generator .

The wattage output of a DC device is obtained by multiplying the device voltage by its current -( volt x amps = watts ) so rearranging this to find the current  IE (amps = watts /volts )  for a 400w dynamo at 110v  that is  400/110 = 3.63 amps  ( max) so a 5 amp fuse is the minimum required , I am using a 10 amp fuse however  allowing for higher engine speeds ( and hence a higher voltage) during setup and adjustment .


Control box as constructed, the large Knob on top is described below

The output voltage can be controlled (changed) by a simple Potential divider (potential is the same as  voltage) If a varible resistance ( known as a potentiometer)is used ,the rating of the potentiometer must be higher than the maximum current and wattage of the output from the dynamo .

Basic wiring diagram of a variable potential divider

The potentiometer used was yet another Ebay purchase !

By careful use of the throttle and the electrical control ,quite an accurate and easy adjustment of the output voltage can be obtained -however this must be done with the load on and in circuit , IE starting at a lower voltage than is required and gradually increasing the voltage to its required level ( 110v) .

The completed set with  engine running and generating can be seen here
( you tube video)


Completed set


and its load – 6x60w bulbs – 240w total  ( 400w Dynamo )