Smart Diode Cut Out

Andy Marks
Dec 23, 2025
2 min read

Learn how to install and use a smart diode cut-out on vintage and classic motorcycles. This technical guide includes fitting instructions and specifications to ensure correct electrical operation during restoration and routine maintenance.

For use with three brush dynamos, some Miller two brush dynamos which use separate regulator and cut out units and all dynamo systems which use permanent magnets for the field including the ML/Lucas Maglita units.

Smart Diode Cut Out

£15.00

Buy Now

Included with every cut out we sell is a comprehensive instruction sheet. If you have misplaced yours or would like to take a look at how to fit one before buying, please read on. If you would like to download a copy to print out and use in the workshop, you can do so by clicking on the link at the bottom of this page.

Why fit a Smart Diode

When a dynamo is spinning it generates a voltage which will increase with revs until it is high enough to exceed the battery voltage at which time current will flow from the dynamo into the battery thereby charging the battery. When the dynamo stops spinning, it is necessary to have a device in the circuit which will prevent current flowing from the battery into the dynamo. If current did flow back into the dynamo, the battery would very quickly go flat. In a conventional two-brush dynamo, this device, known as a cut out, is normally combined with the voltage regulator. Earlier three brush dynamos do not have regulators and so the cut out used is a stand-alone device. These cut outs have taken several forms over the years including manual operation by the rider/driver, centrifugally operated mechanical switches and electro-mechanical switches amongst others. All these systems can now be replaced by this new Smart Diode Cut Out made here in the UK by The Magneto Guys.

Specifications

These Smart Diode Cut Outs uses a surface mount semiconductor mounted on a small circuit board measuring 27 x 25mm with two flying leads approximately 150mm long. The semiconductor device used has the following features:

Maximum reverse voltage: 30v - ideal for 6v and 12v dynamos
Maximum forward current: 15A - well above the output from these early dynamo designs
Low average forward voltage: 26mV at 8A – that’s a very low loss of dynamo output voltage
Reverse leakage current: less than 1µA – that’s 0.000001 of an amp so, pretty negligible
Maximum operating temperature: 125° C - much higher than anything likely to be seen in use

Wiring Instructions

This Smart Diode Cut Out is wired between the dynamo and battery. It can be used with either negative or positive earth systems as shown. All other electrical circuits (lights, horn etc.) are connected on the battery side of the Smart Diode Cut Out as on the original vehicle.