Solenoid Stuff
Some information about solenoids and how to make them work better.

In working on The Non-Rotary Valve Engine, I needed to boost the pull-in power of an existing solenoid without the extra winding in order to pull the latch pin that had a load on it.  Some solenoids, notably Warner Overdrive and some Diesel engine fuel cutoff types, there is an added heavy intermittent duty winding that is used to boost the mechanical power of the solenoid to get it moving against a load.  After the solenoid has bottomed-out, this winding is no longer needed and is switched out.

Two types of extended power solenoid drives.
On the left, is a solenoid with the extra winding, shown with a tap.  This could also be a totally separate winding.  The contacts at the bottom are wired in the normally closed position and are used to short out the tapped turns.  This increases the coil current and the force the solenoid can exert and does this until the solenoid pulls-in, operates the contacts and removes the short.  Since once the solenoid is pulled-in, it takes a lot less current to keep it there so current saving and heat build-up is avoided for long pull-in times.  In most types of solenoids like this, the high urrent "starting" position is only for short term intermittent use due to the heat buildup in the coil with turns shorted out.

If you happen to need a continuous duty solenoid that has a lot of starting force, the shunt coil type is good.  If all you have is a simple solenoid, you can effectively double the initial inrush current by using the circuit on the right.  It is necessarily more complicated but is easy to do and works well in most applications.

On the right is a schematic of what I came up with.  There are other ways to do the same thing but this seems to be the simplest.  Fundamentally, what this does is to dump a charged capacitor in series with the battery voltage across the solenoid coil.  This only gives a short boost but is enough in most instances to make it do what you want it to do.

What you have is a continuous duty solenoid with a voltage rating the same as that of the battery.  I show a pushbutton that energizes the relay coil.  This could be any kind of switch.  The circuit is shown at rest with the K1a switching the solenoid.  

K1b and K1c are used to both charge the capacitor and then connect it in series with the bottom of the solenoid coil.  As you can see, the capacitor charges to battery voltage while the circuit is deactivated.  When the relay pulls-in, K1b and K1c switch the capacitor from charging to discharging to the bottom of the coil, adding to the effective voltage across the coil.  When the capacitor has discharged, the diode connects the bottom of the coil to ground.

There are a few things to keep in mind here:
-  One of them is that a three pole double throw switch can be used in place of the relay if it works better in the application.

-  Two is that the value of the capacitor affects the initial pull-in surge and the amount of time the surge lasts.  I figured this out by trial and error and it will vary with the specifications of the solenoid.  Generally, the more capacitance, the more the surge time and the greater the surge force.

-  Three is that the voltage rating of the capacitor should be at least the same as the battery voltage.

Works for me!
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If you have any comments, I'd be happy to hear from you.  I can be reached via email at this address:
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