If your load is resistive (no reactances), you can use a power transistor or a MOSFET as a switch.
For your 50 V and 700 mA I would use a PVG 612 power opto-coupler which I tried many times, for up to 1.2 A load under 60 V. The electronic driver for it only should generate 5V, 10-20 mA for optocoupler internal LED.
Check with Digi-Key, PVG 612 should be available.
If your load is resistive (no reactances), you can use a power transistor or a MOSFET as a switch.
For your 50 V and 700 mA I would use a PVG 612 power opto-coupler which I tried many times, for up to 1.2 A load under 60 V. The electronic driver for it only should generate 5V, 10-20 mA for optocoupler internal LED.
Check with Digi-Key, PVG 612 should be available.