Cheap visual indicator for a PA loudspeaker line

Question

I have a PA system which is outputting at most 200Vrms AC peak, around 100Vrms AC nominal, from the transformer. The current is around 4Arms. Since the system is comprised of 12 output transformers, and the test system puts them all in the same room, I'd need a visual indicator of which are actually playing and which are in fault. Which could be the cheapest/easiest solutions to do such a thing? I thought of putting a small light bulb on each AC high-voltage line, but I'm afraid it would not be a good solution.

Answer 1

Many possibilities.
- How much do you want to spend per location and

• Have you got AC or DC power at each point?

  • IF this was line powered,what maximum power can you afford or want maximum to be taken per transformer from the audio signal for driving the inicator (eh 100 mW, 10 mW, 1 mW ...?)

Neon: The classic easy and cheap approach would be to use a Neon bulb and a resistor Would take some fine tuning in the design but not hard, line powered and there will be many existing circuits available. Note that, as Olin says, and as per graph at ane, for Voltages below 100v it will not always "strike".

LED Something using an LED will also work and be easy to do. You can easily produce a low voltage signal that varied with speech etc and which can drive an LED 'in time' with the sound, Or you modify the same circuit to light an LED steadily as long as there more than a certain voltage present.

• Component count for a steady or flashing indicator can be
  Line1 - Diode-|>|- Resistor - LED -|>|- Line 2

  LED flashes with AC

• As above but add capacitor across LED
  LED steady

• Tap resistor add zener from centrepoint of resistors to line 2
  Adds protection against overvoltage.

A 100 VAC RMS line will lose 100 mW for every mA drawn. A 1 mA driven LED indicator can be easily "good enough" for the purpose.

• eg as an extreme example a Nichia "Raijin" white LED (about as efficient as any on sale on earth) operated at 1 mA will produce about 0.5 lumen of light - which will illuminate a one inch square to about 600 candella or about twice the brightness of a typical modern LCD screen at full brightness.

A VU/bar / dot meter type display also easy subject to powering answers. VU meter using analog meter is passive.

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Minimum "Neon" strike voltage

Paschen curves.
Discussion here

Answer 2

I'd probably start with a capacitor, diode, and LED. Or if you want, two LEDs back to back and then just a capacitor. You will get some indication even at lower voltages. You can size the capacitor to get just enough LED current at the voltage you care about. The LED will be brighter with frequency too, although I'm figuring that won't matter much.

If you are sure the voltage you care to detect always has peaks of at least 90V or so, then the neon bulb and resistor will work. The problem with neon bulbs is that they take a fairly high voltage to fire. If you sometimes only have 70V peaks and want to detect that, the neon bulb method won't work.