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AC Ramp down circuit

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Rumble

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First time user here...

I have a project that I am assigned at work. We manufacture customer inductors and transformers and part of the testing involves applying power, either AC or DC, at elevated temperatures. Some of the tests are completed during the night or weekends and we use timers to shut off the temp chambers but we do not have anyway to remotely remove the applied power. If we simply turn off the AC power supply, the feedback from the inductors in the temp chamber blows the output stage. If we interrupt the signal path, using a relay for example, then the output stage of the AC power supply is severely stressed. What I need to do is to find a way to ramp down the applied voltage over a period of time, say 60-90 seconds.

I was considering using a triac in a manner similar to a light dimmer circuit but instead of a rheostat, using a digital potentiometer in the RC circuit. My concern here is that we use a frequency range of 400Hz to 100KHz in our testing and the RC circuit may not provide enough phase shift for the entire frequency range.

I was also considering using a VCO to the trigger the triac and slowly ramp down the frequency but since the triac will not trigger at the same point on the sine wave each time the resulting ramp down signal will be very erratic and noisy.

One option would be to build a ramp down circuit for a smaller frequency range, for example 400Hz-1KHz, but I would like to have one, fit all circuit.
 
but we do not have anyway to remotely remove the applied power.???

the feedback from the inductors in the temp chamber blows the output stage.
Use an MOV to dampen this transient?

a way to ramp down the applied voltage over a period of time, say 60-90 seconds.
See below

that we use a frequency range of 400Hz to 100KHz in our testing
so you need a resistive path

If you can ramp down in steps, you could have series resistors that are shorted out by relay contacts when full power is desired, and then one-by-one the relay contacts open to give a several-step reduction in applied voltage, with the last relay totally opening the circuit.
The relays would be driven by a digital circuit.
 
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So how is it turned off manually?
 
If the test is scheduled to be completed during the work day, for example at 9:00am, we simply turn down the output on the AC power supply and turn off the temp chambers.
Do you turn down the output by adjusting a knob on a pot? If you can get at the control, my thought is to connect a digital potentiometer, such as ones from Analog Devices, to the control pot output and reduce the signal by slowly changing the pot value.

Otherwise Willbe's relay approach may be the best.
 
Does the power supply have a GPIB interface/option? Otherwise modifying it the way Carl mentioned would be the easiest.
 
Does the power supply have a GPIB interface/option? Otherwise modifying it the way Carl mentioned would be the easiest.

No, unfortunately not; it just has knobs to adjust the frequency and output.

I am now considering the digital potentiometer and the relay idea.

The digital potentiometer will require a way to adjust it manually; probably using a monentary contact toggle switch. (up to increase output, down to decrease output.)
 
The digital potentiometer will require a way to adjust it manually; probably using a monentary contact toggle switch. (up to increase output, down to decrease output.)
Yes. Some of the digital pots can be controlled with pulses. The timer control would need to provide a series of pulses to ramp the potentiometer and shut off the AC. The timer could turn on a pulse generator to do this which could be a simple 555 IC type circuit. The pulse frequency would determine how fast the power is turned off.
 
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