# Electrostatic Current Monitor

Discussion in 'Electronic Design' started by Mike Monett, Sep 14, 2008.

1. ### Mike MonettGuest

Monitoring the current in an elecrostatic air filter can be a
problem.

If you use an external dvm in series with the high voltage lead, you
are exposed to a potential arcover that could give a very bad jolt,

If you put the dvm in the ground lead, accidentally removing one of
the leads can place dangerous voltages on the remaining connections.

US patent 7258729 has a neat solution. It uses a NE-2 bulb is in
parallel with a capacitor, which goes in series with the high
voltage. (It probably should go in the ground lead to help minimize
corona from sharp points and small lead diameter on the
connections.)

The capacitor can be adjusted to suit the expected current.

The patent uses a 0.047uF cap. Assuming the NE-2 fires at 90VDC, and
the current drain is 1uA, the bulb would fire every

dt = 0.047e-6 * 90 / 1e-6
= 4.23 seconds

which is fast enough to monitor visually.

This is a very simple solution to what can be a difficult problem.

Best Regards,

Mike Monett

2. ### Mike MonettGuest

dt = C * dv / I

3. ### Mike MonettGuest

Yes, I realized that just after posting. Searching google for the
extinguishing voltage gives values all over the map. But I think the
bulb will light at about 90V and go out around 60V. So the equation
should be

dt = C * dv / dt
= 0.047e-6 * (90 - 60) / 1e-6
= 1.41 seconds

or thereabouts. The small voltage drop should have little effect on
a 12KV electrostatic filter.
I mentioned it should go in the ground side to prevent corona. The
firing rate will vary with different bulbs, and probably other
factors such as temperature. The cap will also have a tolerance. And
you monitor the firing visually, so it's not going to be anywhere
near the accuracy of a dvm.

But it's just a monitor, basically to alert you to problems. The
current drain of an electrostatic filter can vary by many orders of
magnitude, depending on leakage on the insulators from dirt or
contamination, ambient humidity, how much corona there is from sharp
points, etc. So it's more than good enough for the application.

My biggest problem is mounting the filter in the window and trying
to keep the rain out. Raindrops hit the window ledge and make a fine
mist. This gets sucked into the filter and shorts the high voltage
to ground. I was trying to measure the voltage by holding a grounded
lead near the electrode and estimating how much gap it would jump.
But the loud bang made me nervous every time it arced over. The NE-2
seems like a much better idea. Simple, reliable, and much quieter
Mike Monett

4. ### Guest

just put the ne2 in series with the unit's psu and monitor with a
photodiode, we used that technique to monitor currents down to 10 nA
or lower in a electrostatics lab, when we needed a floating fiber
optic current sensor. Or put a 100K ohm 1/4th watt resistor in
series with the device and put a 10 nanofarad ceramic cap across it
to protect the meter from the turnon surge. We also made floating
meter assemblies in plastic boxes with the RC circuit.

Steve Roberts