Dipping Solenoids

Every now and then you come across something that makes you stop, look and say “That’s ruddy clever”. Such an occasion happened recently when a member asked me to check out some dipping headlight solenoids.  

Many folk regard the original dipping mechanism as unacceptable for modern traffic, and many are discarded during lighting upgrades or 12V conversions.  However, when considered in the context of the time, they were a very clever idea.  Firstly, as the light pollution was so much lower, people were quite happy with being able to see far enough when travelling at 30 mph.  After all, nobody else was going any faster anyway.  Secondly, bulb technology was such that twin filament bulbs were far too difficult to make. Thirdly, the power consumed from the battery was reduced, as the offside light was deliberately extinguished on dip beam.  Provided your car was first registered before 1936, this is still legal.  For cars after 1936 a conversion is required so that both headlights remain on when switched to dipped beam.

 solenoid top

On the top of the solenoid assembly are four terminals and two moving contacts.  These moving contacts are operated so as to make an electrical connection  to two fixed contacts on the bottom side of the insulating plate.  The left hand contact switches the supply to the offside headlight bulb, whilst the right hand one has a special purpose which will be explained later. 




solenoid circuit 2




In the “rest” position, which equates to Main Beam, the dipswitch is open so that no current flows through the solenoid.  A spring on the headlamp reflector pulls the plunger outwards. The contact arms are lowered under their own spring pressure and the contacts are closed underneath.  Note that should the fuse blow, the circuit through the dipswitch becomes open circuit and the system defaults to main beam. The fuse does NOT interrupt current to the offside lamp. 




solenoid 1



When the dipswitch is closed, current is allowed to flow through the solenoid. The windings of the solenoid are special, in that they consist of a normal winding to provide the magnetic field to pull on the plunger, which is IN SERIES with a resistance winding.



As the solenoid begins to operate, the plunger is pulled inward, pivoting the reflector. At the beginning of the travel, the right hand circuit is still made.  This contact short circuits the series resistance, ensuring that the maximum current flows through the windings for maximum pull.  The windings are only about 1.4 Ohms, so the current at 6V is pretty high - around 4 Amps. The windings won’t take that for very long without overheating.

solenoid 2As the plunger approaches the end of its travel, a small peg pushes the insulating plate between the contact arms upwards, opening the electrical connections.  The left hand contact opens, disconnecting the feed to the offside headlight.  The right hand contact opens which puts the resistance back in circuit.  This resistance is around 20 Ohms, so that the current is very much reduced, to less than a third of an amp.  Because the plunger is now almost fully home, very little magnetic force is required to keep it there and the power dissipated in the winding reduces to a fraction of what it had been during the travel.

So what can go wrong with these clever bits of mechanical logic? 
Firstly, the fuse holders tend to get grubby, which often means that the headlights are reluctant to dip.  Similarly, the contacts on the underside of the moving arm need to be clean: if the contact is dirty, the resistance will be in circuit and the solenoid will never pull hard enough to begin its travel. 

More commonly, as the picture shows, the top insulator can be very buckled and distorted.  It is made of a material correctly called Synthetic Resin Bonded Paper (SRBP), but often known by the trade names Paxolin or Tuffnol.  The snag with this can be seen from the photo.  It is hygroscopic, absorbing lots of water when stored in a damp environment, which makes it swell and buckle.  The arching shape means that this example doesn’t short out the series resistance, and the pull is feeble.  Do not be tempted to try to force it back: it is a one way trip.  In the case of the solenoid shown, the best remedy is to modify the contacts so that they still close despite the curve.

Converting 6V solenoids to 12V
These little pieces of period charm can be converted to 12V, but only if the disconnection of the offside headlight is abandoned.  With a pair of single filament bulbs, the dipping can be effected mechanically. 

It’s not quite as straightforward as putting a resistor in series, (such as for trafficators).  The reason is that with a resistor permanently in series, not enough current will flow to get the reflector moving; not fitting a resistor would cause the current limiting resistance to overheat and fail prematurely. 

solenoid circuit



There is a simple solution, and that is to use the offside bulb contacts to switch in an additional resistance only after the movement has been completed.  In the circuit shown, an additional 18 Ohm 5 watt wire wound resistor is placed across the bulb contacts on the left hand side.  One contact is wired to the terminal usually used for the dipswitch: the other is the new feed from the dipswitch.  In the main beam position, the resistor is short circuited and full current flows into the solenoid.  Once the dip position is reached, the contacts open, the additional resistor is put in series and the holding current is reduced to approximately what it would have been for 6V.






This article, written by Geoff Hardman, originally appeared in CA7C Seven Focus in July 2010 pp 15-17.