Passively, an SCR behaves as virtually an open circuit to voltage pressure (attempted current flow) in either direction.
When a small current flows from the gate to the cathode, and there is positive voltage applied from the anode to the cathode, the SCR will latch on and be conductive in the anode to cathode direction like a diode, until a reverse bias current (attempted current flow from cathode to anode) switches the device off.
Whe have used these principals in testing the devices removed from the rest of the circuit to determine functionality and condition. Of course, a 'hockey-puck' SCR must be under the appropriate pressure for any test to be meaningful.
We fire the SCR on the gate circuit with 3 AA batteries in series, with a resistor to limit current.
To supply the positive voltage from anode to cathode and verify current flow when gated on, we use an analog meter on Rx10,000 setting, since it 'pushes' harder than a digital meter, but still is capable of only a limited current.
We then use the same analog meter to reverse bias the SCR to switch it off, and verify high resistance in the off direction.
We then **briefly** use a megger on the appropriate setting (500Vdc for a 480Vrms circuit, 250Vdc for a 240Vrms circuit) in the reverse bias direction first, then the forward direction to verify very little current leakage when the device is supposed to be non-conductive in the recifier circuit.
Although an SCR is a solid state device, none of these procedures apply a higher than rated voltage to the device, or inject more than a fraction of the rated current through the device when in actual operation. They are not fragile and secret 'black box' devices, but robust semi-conductors that obey Maxwell's and Hall's equations and are designed to operate in a harsh, industrial environment. The only diference in the bench-testing and the actual operating conditions is arguably the continuous amount of time a voltage is applied to the device.
When a small current flows from the gate to the cathode, and there is positive voltage applied from the anode to the cathode, the SCR will latch on and be conductive in the anode to cathode direction like a diode, until a reverse bias current (attempted current flow from cathode to anode) switches the device off.
Whe have used these principals in testing the devices removed from the rest of the circuit to determine functionality and condition. Of course, a 'hockey-puck' SCR must be under the appropriate pressure for any test to be meaningful.
We fire the SCR on the gate circuit with 3 AA batteries in series, with a resistor to limit current.
To supply the positive voltage from anode to cathode and verify current flow when gated on, we use an analog meter on Rx10,000 setting, since it 'pushes' harder than a digital meter, but still is capable of only a limited current.
We then use the same analog meter to reverse bias the SCR to switch it off, and verify high resistance in the off direction.
We then **briefly** use a megger on the appropriate setting (500Vdc for a 480Vrms circuit, 250Vdc for a 240Vrms circuit) in the reverse bias direction first, then the forward direction to verify very little current leakage when the device is supposed to be non-conductive in the recifier circuit.
Although an SCR is a solid state device, none of these procedures apply a higher than rated voltage to the device, or inject more than a fraction of the rated current through the device when in actual operation. They are not fragile and secret 'black box' devices, but robust semi-conductors that obey Maxwell's and Hall's equations and are designed to operate in a harsh, industrial environment. The only diference in the bench-testing and the actual operating conditions is arguably the continuous amount of time a voltage is applied to the device.
