The current gradually dies to 15mA.
We are not switching off the inducutor. The current drops to 15mA.
And secondly the energy produced (if the inductor is switched off) is determined by the voltage and current flowing though the inductor. These values are very small and so any back energy will be very small.
It's not even a value worth considering.
On top of this you have two diodes that will clip any back voltage so I don't know why you even brought up the point.
Hi Colin,
You're saying that there is no back emf, then you're saying that you have two diodes that will clip any back voltage. So which is it then?
The fact is that depending on the capacitance in the circuit and the inductance in the circuit we may see the back emf develop a current in the opposite direction. Some or all of that energy may be absorbed by the capacitors safely if they still contain some energy.
The other thing is near the input of the schematic we see "15v AC or DC". With 15v DC we'll see a one time only response, but with 15v DC we may see repeated responses similar to if we connected a 15v source, disconnected it, then connected it back up again. This means the load (solenoid) would be pulsed rather than turned on once and then turned off slowly. If the 'switch' you talked about is on the output, that means the switch may open during a time of relatively high current anyway.
In an more obscure design like this it is often harder to critique the design itself rather than the designer mostly because there are so many undocumented variables that only the original designer might know. This means we can question the designer and see if he is assuming something unreasonable. If we find something it doesnt mean that the design is flawed, but it does mean there's a good chance it is if he assumed something unreasonable that was more important. What would be done then is he could be asked to explain the reason behind the choice or spec of a component, or a circuit connection. If he cant explain it, then the design falls into a questionable area, but if he can explain it then perhaps it works to some degree or else he finds out that he has to improve the design.
Let me give a simple little illustration here...
In a certain design we find a resistor with a power rating of 10 watts, but in simulating the circuit as well as we can we find that the most likely power dissipation is 12 watts in that one resistor. We are also told that there are several units in operation in the field. What to do?
Well, we think there's a design flaw, so we look at the schematic and in the lower (or upper) right hand corner we see a block of text with the date and designers name. So we go back to the designer and ask him/her if something could be wrong.
He/she said they dont think so, but they will go over that part of the design again and see if they can find anything. We also ask him/her to back up the calculated data with some real life measurements and if necessary some scope pictures.
So it is a little harder sometimes to get to the bottom of it all, but one thing that helps a lot is some real life test data which in the case of a transient operating circuit like the one we are talking about in this thread some scope pictures could be the ultimate turning point, where we either verify the design or improve it.