Value of Resistor in 6 Volt Cut Out

[Jamie]

The gubbins inside my cut out for my Seven were missing when I bought it, so I intend to replace the coils with a Schottky diode. However, I am not sure what the resistance of the original resistance wire was in a 6 volt car. I can find plenty of information on-line about the value to use if converting to 12 volt, but I am not sure whether it would be same for 6 volt.

I am basing my conversion on this from the Cornwall Austin Seven Club website.

 

Jamie.

[Colin Morgan]

Hi,

There is a copy of John Cornforth's Ruby wiring diagram on this thread, giving resistance values for the CFR (1.5 ohms)  and CFR2 (2.5 ohms and 0.5 ohms), if that helps? 

https://www.austinsevenfriends.co.uk/for...p?tid=7891

Colin

[Jamie]

Thank you. I am also mulling incorporating a zener diode to help with preventing over charging the battery, but need to do more research first.

 

[John Cornforth]

Hi Jamie

The resistance figures mentioned above are still the best information I have for the "Half/Summer Charge" resistors. The 6 volt third brush system relies on a combination of the lighting switch position and driver choice to set either full or half charging current, roughly 8 or 4 amps. The third brush setup stabilises (to some extent) the charging current with changing engine/dynamo RPM. The main aim of Half/Summer charge is to avoid overcharging the battery during long runs with no lighting load. The battery acts as the system voltage stabiliser, as its terminal voltage does not vary a huge amount with different charge currents.

I assume you are going for a 12 volt conversion. To allow for the higher system voltage the ideal would be to re-wind the field coils to double their resistance. Historically this was considered too expensive, so folks just put a resistor in series with the standard coils. A bit wasteful of power, but it did the job. If you want to retain the ability to rein back the charge rate by reducing the field current you will need two selectable values of resistor. This is the best way to avoid overcharging.

I have seen high power shunt zener diodes used in motorcycles to limit the system voltage, these being bolted to the frame as a heatsink. All that happens of course is that the zener diverts some of the current that would otherwise flow into the battery. The dynamo will still be producing full whack, which it may not enjoy especially on a long run in hot weather. I have not seen a zener proposed or used in a 12 volt Seven.

Although 12 volt conversions were all the rage at one time, the advent of LEDs for lighting has removed one of the reasons for their adoption.

[Jamie]

Thank you for your advice, John.

I do actually intend to retain 6 volt electrics, rather than the 12 volt coversion that I had planned originally. I have been given a sound dynamo and I do not particularly wish to mess with it.

My aim is just to use a diode and resistor to replace the cout out. I have just an empty case and none of the switches. A way to simplify things is my aim if possible.

Looking further into their use, I have also concluded that  there is no point in adding a zener diode.

Jamie

---

Thank you for your advice, John.

I do actually intend to retain 6 volt electrics, rather than do a 12 volt coversion that I had planned originally. I have been given a sound dynamo and I do not particularly wish to mess with it.

My aim is just to use a diode and resistor to replace the cout out. I have just an empty case and none of the switches. A way to simplify things is my aim if possible.

Looking further into their use, I have also concluded that  there is no point in adding a zener diode.

Jamie

[nrzam]

I used this from The Magneto Guys. £18

https://www.themagnetoguys.co.uk/smart-diode-cut-out

[Howard Wright]

+1 for the Magneto Guys smart diode.

Cheers

Howard

[Jamie]

Thank you for the suggestion of the Smart Diode.

May I just check: am I right in thinking that this only replaces the cut out coils and contacts, but not the resistors?

Jamie

[Howard Wright]

Hi Jamie

Yes I think the smart diode just acts as a cut out. All three of my cars have DEL dynamos with the summer/winter resistor incorporated in the dynamo itself so I can’t help with the value of the resistor.

Cheers

Howard

[Jamie]

Thank you, Howard.

I have just ordered one of the smart diode chips; it is not on a board, but was only about £7.00 including delivery.

I must confess that I need to check which dynamo I have been given. I was given it as a replacement for my original device which had failed. That had separate resistors and I am assuming that this one does, too. I shall look later to see what model dynamo it is. 

Jamie.