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Metal Oxide Varistor (MOV) in series

I want to sense when all three legs of 220V three-phase circuit are active. I've built a sensor for a single leg using a capacitor to limit the current flow to a LED to test the concept of using an optoisolator. There is a diode mounted opposite polarity across the LED to protect the LED from reverse voltage and a resistor across the capacitor to discharge it when the circuit is turned off.

+-- CAP --+ +--- DIODE---+ o--+ +-----+ +------------o +---RES---+ +------LED-----+

This works fine most of the time. I can leave it connected for hours at a time but ocassionally when I first connect it I get a spark jump across the connection and it blows the LED. The only thing I can figure is that the intermittent connection is causing a high voltage spike.

So I figured I would put a MOV across the LED/diode pair in case this is the issue.

The problem is that I can only get 130V MOVs locally (Radio Shack... I live in a small town).

So my question is can I put two 130V MOVs in series to get 260V protection?

Thanks,

Jay

Reply to
jlb
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Adding MOVs across the entire circuit will limit the magnitude of the voltage transient, but I believe your problem is more because the capacitor is acting like a short circuit during the initial conditions. The current through the capacitor (and the LED) is C dv/dt which can be very large. I think you should be looking more at ways to limit the current through the LED (series resistor), possibly with a shunt element (5V TVS / MOV).

--
Jim
Reply to
Jim

"jlb"

** Nope.

You have failed to include the essential current limiting resistor.

When a source of voltage is first applied to a cap, the peak charging current is VERY large - easily large enough to kill the poor LED.

Add a series resistor of about 1000 ohms to the circuit, so the peak surge current is limited to 320mA.

..... Phil

Reply to
Phil Allison

On a sunny day (Mon, 08 Dec 2008 17:20:07 -0600) it happened jlb wrote in :

I've built a sensor for a single leg

using an optoisolator. There is a diode

voltage and a resistor across the

You need a resistor in series to limit the peak current when switching on at the moment the sine wave is at maximum amplitude. The value is R = Ueff * sqrt(2) / Imax LED. You need no varistors. The resistor should also be able to handle the power.

Reply to
Jan Panteltje

On a sunny day (Tue, 9 Dec 2008 22:54:08 +1100) it happened "Phil Allison" wrote in :

What is 'ECT"?

Reply to
Jan Panteltje

"Jan Panteltje"

** What you need lots of

- f*ck head.

..... Phil

Reply to
Phil Allison

On a sunny day (Tue, 9 Dec 2008 23:54:18 +1100) it happened "Phil Allison" wrote in :

ET Extra Terrestrial ECT Extra Cute Terrestrial?

LOL You need to change the code in your reply generator, it seems to be in a loop.

Reply to
Jan Panteltje

I've built a sensor for a single leg

using an optoisolator. There is a diode

voltage and a resistor across the

but ocassionally when I first connect it

I can figure is that the intermittent

you need a resistor in series with the LED.

when the cap has 0v charge and the AC is near its peak and you hook this thing up a big slug of current flys though the capacitor and blows the LED.

try a 2.2K resistor: that should limit the the peak current to something sensible without wasting too much energy.

Reply to
Jasen Betts

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