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Using a split-charge diode with a Lucas 11AC alternator


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I wonder if anyone has experience of fitting a standard (i.e. NOT a zero-drop) diode block to a Lucas 11AC alternator (this has a separate external voltage regulator and field relay). Will the volt-drop of the diodes cause problems with the relay pulling in?

Looking at the standard Lucas control circuit it would appear that the external regulator could be connected to the battery side of the diode block, thereby sensing the battery voltage and compensating for the diode drop. Does anyone know if this would cause any problems?

I Know that a simple split-charge block is not necessarily the optimum solution to the problem and that there are some quite sophisticated systems for separately controlling the charge to engine and domestic batteries but these are well over my budget.

Also, whilst a second alternator would be a nice alternative I don't have the facilities for making up a mount and can't justify the cost of getting it done professionally.

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  • 1 month later...


It's very dated now, but the standard setup for a Lucas 11AC alternator on many boats was just as you describe.

The 4TR controller took it's power (and reference voltage) from the battery via the relay contacts (with the relay coil being energised by turning on the ignition switch). Therefore the standard setup was "battery sensed" (as opposed to machine sensed) and would compensate for volt drop across the split-charge diodes.

I can't remember which battery feeds the regulator on Ella (I would guess that it's the starter battery).

The biggest problem with this early setup is that if you leave the ignition switch on without the engine running the regulator will be calling for full charge from the alternator by delivering full voltage to the field coil (not something that you want to do too long to a stationary alternator).

Similarly, the same would happen if your fan belt parted, but as this probably also drives the closed system water pump you will have other things on your mind then.

All the best,


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Thanks for the response Steve. I'll probably give the split-charge diode block a try and see how it works out. If I insert the block directly into the alternator output it will continue to sense the engine battery which seems the logical arrangement. My main concern is that even with the existing set-up I find that I have to increase the revs to over 1500 to get the 16RA field relay to pull in and suspect the additional volt-drop might make it even more reluctant.

I don't think you are correct in your statement regarding leaving the 'ignition' switch on with the engine stationary or the belt failed. In this condition the field relay should be de-energised and the power to the field winding will then be supplied via, and limited by, the charge warning lamp. Incidentally our boat has a direct raw water system so could continue to run safely without the belt just by turning the switch to 'off'.

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It seems that you don't have the basic setup that I was describing (which used a 6RA Relay - only 4 connections comprising of the energising coil and a pair of contacts). That system did suffer from the problems as described, as it initially (you could say brutally) effectively connected 12 volts across the stationary field coil with the current mostly only limited by the coils DC resistance.

I haven't come across the 16RA which apparently has 5 connections and I would be interested to see a circuit diagram if you have one.

For interest only the basic setup is described here:-


All the best,


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