What's the simplest ballast to maintain 25mA-rms @ 98Vrms?

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Discussion Overview

The discussion centers around finding a suitable ballast to maintain a 25mA-rms current at 98Vrms for an AC switch controlling another circuit. Participants explore various solutions, including resistive, reactive, and inductive methods, while considering power dissipation, size constraints, and the behavior of the AC switch.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant suggests using a ~4k resistor in parallel with Circuit B, noting it would dissipate ~3W of heat, which is not optimal.
  • Another participant proposes the possibility of using a voltage regulator with an internal thermal overload feature.
  • There is a suggestion to consider an inductive approach, with questions raised about the importance of minimizing power dissipation and its relation to environmental concerns or battery life.
  • A calculation is presented indicating that a 0.68 uF capacitor could provide the necessary reactance at 60 Hz, though there is uncertainty about how the AC switch would respond to a reactive load.
  • One participant shares an experience with a reactive load of 1.1uF, noting that while the AC switch did not fail, it caused the LEDs to flash intermittently.
  • A request for clarification is made regarding the operation of the AC switch and the behavior of the LEDs when a capacitor is used in parallel with Circuit B.

Areas of Agreement / Disagreement

Participants express differing views on the best approach to maintain the required current, with no consensus reached on a single solution. There are also varying opinions on the significance of power dissipation and the implications of using reactive loads.

Contextual Notes

Participants highlight limitations related to the heat sensitivity of the application and the need for a compact solution. There are unresolved questions about the AC switch's behavior with reactive loads and the specific requirements for maintaining current flow.

KingNothing
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I have a circuit containing ACST6-7S AC switch. That power switch is used to put out 98Vrms, which drives another circuit (Circuit B). The problem is, the power switch needs to have 25mA-rms throughput in order to stay 'turned on'.

The simplest solution is to just put a ~4k resistor in parallel with circuit B. However, this solution is not optimum because it will always be dissipating an extra ~3W of heat.

What I'd really like is something which only draws extra current as needed. Circuit B will at times draw well over an amp. Another limitation is size - the solution, when manufactured, would have to fit in the palm of one's hand.
 
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Would there be any way to use a voltage regulator that has a internal thermal overload?
 
You could do it inductively. Not sure what your power source is and why it is important to not dissipate the 2.5 watts. Are you just trying to keep power consumption down to be 'green'? Or is there battery life compromised here? Would like to know a bit more.
 
I calculated that a 0.68 uF capacitor would have about the right reactance at 60 Hz, but I don't know if the AC switch would appreciate a reactive load like that.

In a device that usually uses about 100 watts, 3 watts of standby power doesn't seem too bad.
 
Last edited:
I want to keep power dissipation to a minimum because the application is very heat-sensitive. I did try a reactive load of 1.1uF...the AC Switch didn't blow up, and didn't get too hot, but the LEDs would be on for about one second, then off for about one second, and repeat this cycle.
 
Not sure I understand.

The AC switch is controlling power to the circuit B, but when the AC switch is turned off you still want current to flow through it, but you don't want this current to flow normally?

And if you put a capacitor in parallel with Circuit B, some LEDs flash? Which LEDs would these be?

Maybe you could explain a bit more or put up a circuit.
 

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