I'm looking for a component to protect against overcurrent if the output of my Teletype current loop driver is shorted. The output end of this system is a 1uF aluminum electrolytic capacitor charged up to 120VDC, with the output switched by an Omron G3VM401E optoisolator. Normally, this drives an electromagnet with 55 ohm resistance and 4 H inductance. With that load, the maximum current is about 60mA, which is what it's supposed to be.
But what if someone shorts the output? It could easily happen; the output is to an ancient long-frame phone jack, which is what old Teletypes use. If someone plugs a keyboard into the printer jack, the output will be shorted.
If shorted, the capacitor discharges into a dead short. The only resistance in the circuit is the equivalent series resistance of the capacitor. There will be a huge current for a few microseconds, and 0.005J of energy will be dissipated.
Is this likely to blow the capacitor or the optoisolator? It's a high-current transient that lasts a few microseconds, without much total energy. It could repeat at about 5Hz as the charging circuit recharges the capacitor.
Both regular fuses and polyfuses are too slow. There are current-limiting ICs, but I haven't found any rated for more than 40V. Any ideas? Is protection even necessary?
Here's the whole board design in KiCAD, if anyone needs the details.
John Nagle