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. 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.
Two types of extended power solenoid drives.
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.
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:
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.
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!