Dipping Solenoid headlamps.
This article is about the Lucas system of dipping headlamps that was fitted
to many pre-war British cars and continued even after WW2. Originally
the near-side lamp dipped and the off-side headlamp was extinguished via
an extra contact in the dipping solenoid unit. I understand that this
practice became illegal in 1937 and changes must have been made to existing
The solenoid appears to have been available in 6 and 12 volt versions.
I don't know how many owners of classic British cars still persevere with
the system. Many have found more modern units to fit their headlamp shells;
some from motorcycles and some the Lucas 700 headlamp. The latter probably
fitted 7” shells with no problem and step-up rims were available
at one time for fitting them to 6 ½” shells.
Enough of the history ! I have only had hands on experience of three 6
volt solenoid units having bought a pair of L133 headlamps from eBay which
included the original system with just one solenoid. A CA7C member gave
me a second solenoid and I set out to make a pair of dipping headlamps
for my ’38 Austin 7. Therefore, what you read here is more of a
blog than a technical article as I may have come to incorrect conclusions.
Please feel free to email me with comments or additional information that
I can add it to this page. I have not yet decided if I will actually use
them but am restoring them just the same.
Anyone reading this will no doubt of come across this webpage after a
net search. There is not a lot of information available on the internet
but Geoff Hardman’s excellent article on the Cornwall Austin 7 Club’s
website is excellent. Here it is : http://www.austin7.org/Technical
This diagram by courtesy of the CA7C website (above)
Firstly, here’s how to test a solenoid for electrical integrity,
preferably using a digital multimeter. Measure between the “Earth”
terminal and the “to Dipswitch” terminal with the solenoid
plunger out (ie, not operating the contacts ) The resistance indicated
should be about one ohm in a 6 volt device. I’ve not seen a 12 volt
solenoid so can only guess that it may read two ohms. Next, push the plunger
to the operate position and the new reading should be about twenty-one
ohms – possibly forty-two for a twelve volt unit.
If these readings are obtained then the unit may be operating properly
– and indeed might actually work. One of my two solenoids did work
and was more or less was satisfactory. The other showed a resistance of
only seven ohms when pushed to the operate position and it was concluded
that the built-in resistance element was faulty. So, no surprise when
It did not work properly and behaved like a pneumatic hammer when powered.
I secured a replacement NOS solenoid from eBay and carried out the basic
test described above. It showed that the in-built resistance was open
circuit. The seller refunded the purchase price and told me to keep it.
This then became the focus of my experiments.
The built in resistance is put into circuit somewhere near the end of
the plunger travel and reduces the current through the solenoid to a value
sufficient to hold it in the dipped position. I suspect that the resistance
wire in wound in the same direction as the main solenoid coil and therefore
adds to the magnetic holding field as well as setting the reduced current.
The first thing to try was to make it work with a substitute external
resistance. It worked after a fashion but the plunger action terminated
with short period of vibration. ( BBBrrrggghh! ) when fitted to a lamp
frame. Remember that this unit was supposed to be NOS and the contacts
seemed to be reasonably well adjusted. But I suspected that these contacts
impose a significant additional load for the plunger to push.
Examination of the faulty unit with the low value resistance showed showed
that after years of wear the underside of the sliding surface at the end
of the contacts loses it original smooth face exposing the fibrous nature
of the paxolin. This tended to add to the tendency to stick in the operated
position after the “dip” voltage is removed.
With the exception of the NOS solenoid both of the others also had roughness
on the outside of the plunger which also encouraged sticking. I cleaned
and lightly oiled the plungers.
So I came to the conclusion that the Achilles heel of the system is the
contact set. Correct operation seems to hinge around the contacts opening
at the optimum place in the plunger travel. My feeling is that it’s
about ¾ to 7/8 of the travel. I also suspect that if the settings
are not quite right the battery voltage might also be a factor in unreliable
operation. A six volt battery can range between six and seven volts in
normal running and a twelve volt battery can range between twelve and
After seventy plus years it’s anyone’s guess how many attempts
have been made to adjust and file those contacts. Note that as the top
contact starts to lift, the bottom contact rises with it until it reaches
the limit of its movement and the top contact comes clear, breaking the
circuit. The other contact which was intended to switch the off-side lamp
is now redundant.
I therefore decided to remove the contacts completely and fit a microswitch.
I sourced a couple of 16 amp, long lever switches from eBay. The 16 amp
rating is at 125/250 volts AC and the DC rating at is given as 0.6 amps
which is a small concern. Below is a photograph of the modified solenoid.
An experimental mind will see that a micro switch can be
mounted in any number of ways. In this case the long lever has been bent
as illustrated. The next modification was to add a 1N5401 silicon diode
(D2 in the final schematic) across the main coil (L1) to quench the spark
which must occur when the contacts break. This is to protect the micro
switch contacts. The later addition of a capacitor probably renedered
this redundant but it does no harm if left in. The quenching diode might
make a useful mod to an otherwise unmodified unit.
Note that the diodes are drawn assuming a negative earth
system. They would need to be reversed for a positive earth car.
The micro switch was set to open at about ¾ to 7/8 travel but there
was still a tendency to “hammer”. So the next thing to add
was an electrolytic capacitor across the contacts in order that instead
of an abrupt change of current to the holding value the current through
the main solenoid continued while the capacitor charged to create smooth
change between the initial (5ish amps at 6 volts) current and the holding
current (approx. 0.25 amps) This improved matters hugely but the capacitor
would be discharged through the short circuit of the micro switch contacts
upon release. To eliminate this and so preserve the contacts D1 is added
in such a way that the capacitor charges as before (smoothing the current
transition) but upon release it discharges through the holding resistor.
I haven’t yet mentioned that at the same time as repairing/modifying
these 6 volt units I was also converting them to 12 volts. Below is the
final schematic that achieved this.
R1 is included because the 6 volt solenoid responds too
violent ly to 12 volts and tends to bounce back causing the travel to
be terminated with the BBBbbbrrrgh vibration mentioned earlier. I found
1 ohm was fine.
R2 is of course the resistor that determines the holding current. With
a 6 volt unit modified to 12 volts, 47 ohms gives a holding current ofabout
0.25 amps. It is also the discharge path for the capacitor C2 upon release.
C2 is the capacitor that smoothes the transition and helps holds the plunger
in its activated position at the instant of “landing”. I have
tried values as high as 10,000uf with no discernible improvement.
Another point to watch is the arm with the rubber buffer,
attached to the device frame that limits the travel of the reflector.
Make sure that the reflector arm only just touches the rubber –
just enough to prevent the reflector being loose and capable of free vibration
when dipped. If the solenoid action hits the rubber stop that’s
another way that bounce back might occur.
With these additions the solenoid now operates perfectly and should continue
to do so subject to the micro switch contacts holding up. If it does fail
a more substantial switch could be tried. At the risk of further complication
a miniature micro-switch could be used in conjunction with a relay.
A fault free 6 volt solenoid could be used used as it is
at in a 6 volts car with the additional components. The junction between
the main solenoid coil and the internal resistance is to the upper contact.
It's possible to solder to the rivet at the underneath the fuse position.
D2 could be inserted in place of the fuse. R1 would be omitted. A capacitor
wired across the Lucas switch contacts.
Of course all this is academic if a suitable bulb can’t be obtained.
It’s easy to run 2 x 12v, 36watt headlamps from an Austin 7, 6 volt
dynamo (some folk run more) but the BA15S base limits the choice of bulb.
I would like to use halogen. More on that might follow if I learn of,
or can rig something suitable.
I have dispensed with the fuse inside the headlamp and, if the lamps are
ever fitted to the car, I will fuse it elsewhere. One final comment is
that this circuit – or indeed the original – do not like a
slow make of the dip switch. Ideally a faster acting toggle switch would
seem to me be to be a good choice.
Please email me with any comments on this blog - but not
just to tell me I'm mad for bothering :-)
email address is on my homepage.