mains voltage sensing

[zappedagain]

I'm designing a piece of equipment that has two AC powered servo drives. I have a power switch that cycles the mains power (110-240 VAC), with an LED inside. The LED isn't wired directly to the switch because the switch is designed for 120VAC 60 Hz or 240VAC 50 Hz. I am attempting to avoid bringing the mains onto a PCB (for regulatory reasons) just so I can sense them and drive an LED.

Is there a simple way to monitor the mains voltage with galvanic isolation so I can illuminate my LED when the mains switch is closed?

I thought about a current monitor, but that will give a false reading if the fuse is blown.

I could add a transformer, but that seems like overkill.

No, there are no signals that I can tap from the servo drive to power my LED.

Are there any simple mains sensors available?

Thanks,

John D

 

[dpc]

Some type of opto-isolator?

David Castor

http://www.cvoes.com

 

[BobM3]

Fairchild MID400 AC Line Monitor Opto (Digikey sells them).

 

[davidbeach]

Two-pole switch?

 

[VEBill]

Your post title ("...voltage sensing") is generating more answers than what is probably your actual question: How to easily and cheaply drive an LED from an AC power line?

The usual solution is a combination of a capacitor, resistor and perhaps a diode. Problem is, these components might need to be installed on a PCB. So you're back to where you don't want to be.

It might be (far) simpler to use a stand-alone panel mounted indicator light and ignore the troublesome LED in the switch.

 

[IRstuff]

Why not a step down transformer?

TTFN

FAQ731-376

 

[OperaHouse]

"I thought about a current monitor, but that will give a false reading if the fuse is blown."

I don't understand that at all. A self powered GO-O GO current relay could be a viable option. This device has a current transformer with a one inch hole and a set of dry contacts. Usually rated at 10A pull in, looping the power wire to your device 10 times gets that down to 1A. Hold in is much lower so if the device has an inrush current required amps is lower. This would detect a blown fuse.

 

[benta]

Does it have to be an LED?
How about an old-fashioned Neon indicator? (I believe that's the right name, English is not my native language. But you know, the amber ones that you see on stoves etc.)

Benta.

 

[ScottyUK]

Relay with a line voltage coil?


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If we learn from our mistakes I'm getting a great education!

 

[VEBill]

The relay with the line voltage coil would provide the logic, but not the power for the LED. Anything that depends on current would provide a false indication if the circuit is live but failed (no current).

What I have done in my own house is wire up an outlet to the switched AC circuit, and then plugged in a surplus AC Adapter to provide a low voltage signal. The pump in my septic tank is wired up such that the switched hot is brought back into the basement. I've got a 12VDC electromagnetic counter that tracks the pump cycles. For a one-off project, this approach might be the 2nd simplest.

 

[fangas]

I've used these in similar applications

http://www.crmagnetics.com/2550.pdf

Ed

 

[zappedagain]

This is getting awfully complicated to turn on an LED...

I'm stuck with LED in the power switch for consistency with our other products (that all have internal power supplies so normally this is a no-brainer). After working all day through transformer, power supply, AC relay, and neon designs I realized each servo driver has an unused RS-232 port on it. I'll tap enough current out of that to drive a transistor and power my LED! Innovation at its slowest...

Thanks for all your help everyone!

John D

 

[itsmoked]

That's a good idea! See - it all paid off.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[zappedagain]

Doh.. almost! The firmware keeps the transmitter on the RS-232 driver into standby when it isn't talking, so I only have a high impedance output. It looks the same powered up or down. Back to the drawing board!

John D