Electro/Mechanical 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 cars.

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 Articles/Dipping Solenoids/

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 fourteen.

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.