The answer to this question depends on the type of AMI power supply system you have and the specifics of the protection circuit on your magnet. The following outline is intended to serve as a general guide, however, what happens to a particular system may not be captured here for every case.

AC line power is interrupted while the magnet is being ramped/controlled by the Model 420 Programmer  

I.                Model 420 Programmer + Unipolar Power Supply, Magnet with Persistent Switch

1. All electronics will go dead.

2. Current from the magnet will come back to the power supply and trip the protection diode on the rear of the unit. This diode will typically drop ~0.7V which will be in addition to the voltage drop across the power cables to the magnet. Thus the magnet will begin to ramp down slowly.

3. The persistent switch will normally not lock in, even when not being heated by an external supply, as long as there is voltage across the magnet. This is because the current induced in the resistance of the switch itself is enough to cause self-heating, preventing it from becoming fully superconducting.

4. Upon return of AC power the Model 420 will be in Pause Mode with the switch heater Off. The magnet will continue to decay through the external diode. The Model 420 will regain control of the magnet and “Catch” or match the existing current when the Persistent Switch Heater is energized, and remain in Pause Mode at this current.

5. Normal operation is resumed.

II.             Model 420 Programmer + Unipolar Power Supply + Model 601 Energy Absorber, Magnet has Persistent Switch

1.  All electronics will go dead.

2.  The Model 601 will continue to operate, using power from the magnet, and will drop a continuous 5V. Additionally, current from the magnet will also come back to the power supply and trip the protection diode on the rear of the unit. This diode will typically drop ~0.7V which will be in addition to the voltage drop across the Model 601and power cables to the magnet.  Thus the magnet will begin to ramp down rather quickly.

3.  Depending on the protection circuit of the magnet, the total voltage drop may be enough to trigger the quench protection circuit. If this occurs an increase in helium loss will be observed and the magnet will ramp down very quickly but should not quench unless the helium level drops too low and the magnet warms up.

4.  The persistent switch will normally not lock in, even when not being heated by an external supply, as long as there is voltage across the magnet. This is because the current induced in the resistance of the switch itself is enough to cause self-heating, preventing it from becoming fully superconducting.

5. Upon return of AC power the Model 420 will be in Pause Mode with the switch heater Off. The magnet will continue to decay through the external diode. The Model 420 will regain control of the magnet and “Catch” or match the existing current when the Persistent Switch Heater is energized, and remain in Pause Mode at this current.

6. Normal operation is resumed.

III.           Model 420 Programmer + AMI 4Q05100 Bipolar Power Supply, Magnet with Persistent Switch

1. All electronics will go dead.

2.  The 4Q05100 Power Supply will drop approximately 6V which will be in addition to the voltage drop across the power cables to the magnet. Thus the magnet will begin to ramp down rather quickly.

3.  Depending on the protection circuit of the magnet, the total voltage drop may be enough to trigger the quench protection circuit. If this occurs an increase in helium loss will be observed and the magnet will ramp down very quickly but should not quench quench unless the helium level drops too low and the magnet warms up.

4.  The persistent switch will normally not lock in, even when not being heated by an external supply, as long as there is voltage across the magnet. This is because the current induced in the resistance of the switch itself is enough to cause self-heating, preventing it from becoming fully superconducting.

5. Upon return of power the Model 420 will be in Pause Mode with the switch heater Off. The magnet will continue to decay through the external diode. The Model 420 will regain control of the magnet and “Catch” or match the existing current when the Persistent Switch Heater is energized, and remain in Pause Mode at this current.

6.  Normal operation is resumed.

IV.          Model 420 Programmer + AMI 4Q05100 Bipolar Power Supply + Un-Interruptable Power Supply Unit (UPS), Magnet with Persistent Switch.

Note: Only the 4Q05100 Power Supply need be connected to the UPS unit. AMI recommends the use of a suitably sized APC brand unit that will deliver roughly 650 VA. The 4Q05100PS accepts an AC loss input signal from the APC brand unit via the DB9 connection on the rear panel of the 4Q05100 Power Supply.

1. All electronics will go dead.

2.  The 4Q05100 Power Supply will drop a continuous 2.5V which will be in addition to the voltage drop across the power cables to the magnet. Thus the magnet will begin to ramp down in a controlled manner.

3.  The persistent switch will normally not lock in, even when not being heated by an external supply, as long as there is voltage across the magnet. This is because the current induced in the resistance of the switch itself is enough to cause self-heating, preventing it from becoming fully superconducting.

4. Upon return of AC power the Model 420 will be in Pause Mode with the switch heater Off. The magnet will continue to decay through the external diode. The Model 420 will regain control of the magnet and “Catch” or match the existing current when the Persistent Switch Heater is energized, and remain in Pause Mode at this current.

Note: If both the Model 420 Programmer and the 4Q05100 Power Supply unit are connected to a UPS unit then the Model 420 will usually indicate a ‘Quench Detect’ at some point. Control of the magnet is regained after depressing the Quench Detect Reset/Zero button and subsequently turning on the Persistent Switch Heater.

5.  Normal operation is resumed.

V.             All Cases Above, No Persistent Switch on Magnet

Each case described above is the same without a persistent switch present. However, if the Model 420 Programmer is correctly configured for NO Persistent Switch present operation then the response upon return of AC power is different. In this case the Model 420 Programmer will automatically “Catch” the magnet or match the existing current without having to energize the switch heater and hold it there in Pause Mode.

In some rare cases on systems with a Model 420 Programmer + Unipolar Power Supply + Model 601 Energy Absorber enough voltage may be developed to trip a magnet protection diode. When this happens the Model 420 will be unable to "Catch" the magnet and it will simply discharge rapidly to zero current. An increase in helium boiloff will also be observed during such an event.

VI.              Model 420 Programmer + 4Q1075 or 4Q10150 Bipolar Power Supply, Magnet without Persistent Switch

1. All electronics will go dead.

2.  The 4Q1075 or 4Q10150 Power Supply is not designed to clamp the output in the case of AC power failure. High voltages may develop across the power supply output as a result of energy feedback from the magnet. This power supply depends on the magnet protection circuit to limit this voltage. If the magnet protection circuit does not limit this voltage to         < 20V then damage to the power supply may result. This power supply system is therefore not recommended for use on magnets without voltage limiting diode protection circuits.

3.  The protection circuit on the magnet should be triggered and will drop some significant voltage which will be in addition to the voltage drop across the power cables to the magnet. Thus the magnet will begin to ramp down rather quickly. When this occurs an increase in helium loss will be observed and the magnet will ramp down very quickly but should not quench.

4.  The persistent switch will normally not lock in, even when not being heated by an external supply, as long as there is voltage across the magnet. This is because the current induced in the resistance of the switch itself is enough to cause self-heating, preventing it from becoming fully superconducting.

5. Upon return of power the Model 420 will be in Pause Mode with the switch heater Off. The magnet will continue to decay through the external diode. The Model 420 will regain control of the magnet and “Catch” or match the existing current when the Persistent Switch Heater is energized, and remain in Pause Mode at this current.

6. Normal operation is resumed.