Those who
know my car (once seen never forgotten!) will be aware that it runs 12Volts. As
a special it was cheap and easy to start from the beginning with 12Volts, both
for electrical reasons and cheapness of parts!
The 12Volt
conversion used a popular “solid state” regulator which may be familiar to some
of you. Basically it monitored and regulated the charge voltage and thus the
current by electronic means and therefore worked very well. In fact the ammeter
always showed a small (1A) charge no matter what the load on the system was.
However several conversations with Club members and some internet forum
discussions gave rise to some concerns about the durability of the device.
Being a solid state device it is susceptible to damage from “spikes”. Jump
starting or a flat battery are know to be fatal to the device!
The fear
was that a breakdown miles from home would be “unfixable”. The solid state
device was “potted” and thus fault finding is impossible and the dynamo had been
converted to 2 brush operation. Both of these facts could cause a headache for
roadside repair jobs!
I wanted to
go back to a 3 brush dynamo with some form of adjustable charge control to allow
a bit more selection than the standard half or full charge. Internet
discussions came up with several resistors rigged up to give a “switchable range
to replace the standard half charge resistance. This led to the logical
conclusion of a rheostat! Given the standard half charge resistance value and
the 3 ohms or so for the field coils a value of 0 -5 ohms was chosen for the
rheostat, which was thought to be good for a 12 volt system. The worse case
scenario for the rheostat would probably be 12Volts flowing through the field,
giving something like 4 amps and 48 watts flowing in the field at the “full
charge” setting (in reality this probably wont arise). Because of this a
suitable size rheostat needs to be used. I managed to buy a 150watt item off ebay from Canada as ex military old stock for about £23 air freighted to the UK
in about 3 days! Be cautious though as a similar spec bought from an electrical
supplier in the UK can cost a lot more than that!
All that is
required is to fit this rheostat in place of the standard half charge resistance
i.e. across the D and F terminals. You can now manually control the current
flowing in the field coils and therefore alter the charge rate. I mounted the
rheostat on the bulkhead and use a shaft through to the dash to allow adjustment
from the drivers seat, although on a special this is easier than it may be on an
original car. You can and should still use the 3rd
brush to limit charging and remember that between 8-10 amps seems to be the
sensible draw from the C35 dynamo, which is no problem at 12V with 36 Watt
headlight bulbs.
Of course
this leaves the cut out! An essential device to prevent the battery trying to
turn the engine over via the dynamo! 12v Cut outs seemed to be difficult
and expensive, so I decided after being offered much advice that a Schottky
diode would be the cheap and much simpler alternative to the electro mechanical
cut out. They are available in all manner of specs, but the one I settled on
has a 30A rating with a reverse breakdown voltage of 45 Volts.
Basically
the Schottky diode allows current to pass to the battery via the ammeter , but
does not allow current to flow back from the battery (technically a small amount
does flow backwards). Thus it does an excellent job of replacing the cut out
and has the advantage of only costing £1.71 from Maplins, so you can carry a
spare or 2! You should ensure that it is purchased with the correct silicone
mounting pad to allow it to be heat sunk to a piece of aluminium or a bulkhead
to dissipate the heat and prevent premature failure. A battery master switch is
a wise fitment too as the small leakage current can cause problems after long
periods of time, and I am informed that the failure mode of the Schottky diode
is short circuit so it no longer acts as a diode and an isolator switch would
prevent any damage!
And that’s
it! All you actually need for the 12Volt conversion is the schottky diode to
act as cut out, total cost £1.71! You could just put an extra 1 ohm series
resistance in the field to limit the current , but the rheostat conversion gives
you that bit extra fine adjustment! Of course bulbs will need changing and a
12Volt battery purchased, and a resistor is needed to limit voltage to the
petrol gauge if you have one. The conversion is as easy to fault find or repair
as the standard system with no complicated circuitry at all.
Parts: 1 x Schottky barrier rectifier part no:
MBR3045PT; 1 x 0-5 ohm 150W rheostat;
Assorted lengths of cabling; Adjuster knob and shaft for
remote adjustment
This article, written by Steve Martin, originally appeared in CA7C Seven Focus
in Mar 2007 p16-17.
