Newsgroups: sci.electronics.misc
Subject: Re: No-Ground detection circuit
References: <[email protected]> <[email protected]>  <[email protected]> <[email protected]> <[email protected]>
[email protected] (Gerard Bok) writes:

> On 8 Feb 2006 05:38:39 -0800, "TBass" 
> wrote:
> 
> >Why a neon bulb as opposed to a incadescent or LED?
> 
> Because a neon bulb glows on a much lower current than a LED.
> (Well, at least that used to be true.)
> 
> >What I'd like to do is get a 24VDC digital output if the ground fails
> >so that my controller can alert the interface of a ground problem and
> >alert the operator. I suppose I could use a comparator, and the relay
> >the 24VDC from an external power supply.
> >
> >[snip]
> >This 'trick' requires a series resistor in the 500 koms - 1 Meg
> >range.
> >[/snip]
> >
> >How did you make that calculation?
> 
> I didn't. I just read the color bands :-)
> 
> Actually: if you buy a panel mount indicator over here, you can
> find a 220Kohms upwards series resistor mounted.
> 
> The screwdriver type indicators rely on passing current through
> your body and they typically use a much higher resistor.
> Somewhere in the 500 Kohms - 1 Meg range.
> At 230 volts, with a 65 volt drop over the neon, you end up with
> 0,3 mA or less.

That's true. You can convert this kind of sensitive neon bulb signal 
to digital signal with help of some sensitive optical sensor. 
Pack the well insulated neon bulb (like that panel mount indicator) 
and the sensor of that optical sensor inside some some suitable black 
plastic box or heat shrinkin black insulating tube.. Idea is 
that only the light form the bulb can get to the sensor. 
Select sensor a that can generate needed 24V signal. 

Other idea: Take a sensitive optoisolator (AC input type or 
normal LED input type with external protecting diode). Put 
the optoisolator input in series with the neon bulb + resistor 
circuit. You wil get that current that goes though that also 
through optoisolator sensor. Have suitable circuit on optoisolator 
output to amplify circuit. If suitable optosolator and 
amplifying circuit is selected, I think that you shoudl be able to 
detect fraction of mA current that passes through...
 
> For comparison: the now common two-lead mains testers use leds,
> but require the return wire to be connected to either neutral or
> earth. And a ground current protector typically trips in the
> latter case :-)

-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/