Protecting an H-bridge, from the unusual

[mutas]

Hi everybody,

I'm a beginner, and am trying to solve somewhat of an unusual problem.  The project involves protecting an H-bridge circuit that is used to drive a telephone bell ringing solenoid.  The circuit that I am working with is essentially the one seen in the Sparkfun Port-o-rotary phone project, seen here;  the "High Voltage Ringer" page:

https://www.sparkfun.com/datasheets/Port-O-Rotary/Port-O-Rotary-v29-2.pdf

For my application, the solenoid also happens to be connected to a 5-bar magneto, that can produce 80+V AC.  My goal is to use the Port-O-Rotary ringer circuit to drive the ringer solenoid of an old-time hand crank telephone.  Somehow, I also want to maintain the original behavior of the telephone by allowing the bells to be rung by turning the hand crank magneto.  Since the ringer solenoid must be driven by both the H-bridge, AND the hand crank, I am really stuck on what (if anything) must be done to protect the H-bridge (and by extension, the microprocessor which drives the H-bridge) from any voltage emitting from the hand-crank magneto.

I would really appreciate any guidance or hints you folks might have.

Thanks,
_matt

[jeroen79]

What current will the ringer draw?

The microcontroller can be protected by isolating it with optocouplers.

For the H bridge:
-Use components that can withstand 80VAC.
-Use a transformer to keep the voltage on the H bridge side low.
-Use a relay to switch the ringer between the H bridge and the crank.

[mutas]

Hi, thanks for the reply and suggestions!

The solenoid draws around 6mA at 30V.

If I were to go with something like a SPDT relay to switch between either the magneto or H bridge, as the signal voltage for the solenoid, would that pretty much get the job done?  Would I even need to worry about opto-isolation, or other forms of protection at that point?

Thanks again!