Circuit operation explanation help please

[orion1210]

Hi,

I'd be very grateful if somebody much cleverer than me could explain how the attached circuit works. 28VDC at G and the output at D is a solenoid coil.

It is described as; when 28VDC is applied at G, 28V is initially present at D (to drive the solenoid) and then tails off to 6V (to hold the solenoid and prevent excessive heat build up).

I'm guessing the schematic may have been over simplified or I'm just too simple to get it ;-)

Many thanks in advance.

 

[MikeMl]

This tells the story. Pay particular attention to the power in the simulated solenoid coil (green trace).

 

[Mike odom]

When 28V is applied to G, the Base of the transistor gets the full 28V, and the solenoid is powered through Q1. As Cap C25 charges, it turns off Q1, and the hold current comes through R77. When G is opened, diode D229 allows C25 to discharge through R77.