Overvoltage protection for sensitive device

[Hans]

Hello,

We have got a device which we prepare for marketing but there was a
problem during testing so it did not pass necessary laboratory test.
Problem is voltage durability e.g. device has test inputs (it's purpose
is acquiring data based on measuring of bio-potencial) which were tested
on 4 kV in duration of 10 seconds. Device goes up in smoke so we search
for adequate overvoltage protection to protect it's input circuits.

Voltage should not exceed 20-30 Volts, so I am interested for a
schematic that possibly includes standard spark gaps or varistors (not
any exotic or expensive components) so we can achieve protection on 4-5
kV in duration over 15 seconds.

If anybody has a solution please send some links or ideas.

Thanks,

Hans

 

[Joerg]

Hello,

We have got a device which we prepare for marketing but there was a
problem during testing so it did not pass necessary laboratory test.
Problem is voltage durability e.g. device has test inputs (it's purpose
is acquiring data based on measuring of bio-potencial) which were tested
on 4 kV in duration of 10 seconds. Device goes up in smoke so we search
for adequate overvoltage protection to protect it's input circuits.

Voltage should not exceed 20-30 Volts, so I am interested for a
schematic that possibly includes standard spark gaps or varistors (not
any exotic or expensive components) so we can achieve protection on 4-5
kV in duration over 15 seconds.

If anybody has a solution please send some links or ideas.

10-15sec is a long time. First, most resistors aren't spec'd for more
than 150-200V although you can get higher voltage versions from
companies like Vishay. Then, take a look at the dissipation. Proper
protection requires clamping diodes after that resistor so lets say they
clamp at 20V, then the other 3980V are dropped across the input series
resistance. Let's say you have 10K in there: 3980V^2/10kohms = more than
1.5 kilowatts ... one second ... two seconds ... phssst ... phooof ...
*BANG*

Somehow this calls for a totally isolated design.

 

[Hans]

10-15sec is a long time. First, most resistors aren't spec'd for more
than 150-200V although you can get higher voltage versions from
companies like Vishay. Then, take a look at the dissipation. Proper
protection requires clamping diodes after that resistor so lets say they
clamp at 20V, then the other 3980V are dropped across the input series
resistance. Let's say you have 10K in there: 3980V^2/10kohms = more than
1.5 kilowatts ... one second ... two seconds ... phssst ... phooof ...
*BANG*

Somehow this calls for a totally isolated design.

Thanks, Joerg

what about spark gaps ? I suppose all nominal spark gap voltages are too
high ? Device which I am trying to protect is not my design so I
mentioned 20-30 Volts just out of precaution. I suppose I can get to
max. 200 V. Is there a solution with spark gap only ?

I must mention that tests are for electrostatic charge, so they cannot
provide such currents.

 

[John Devereux]

Thanks, Joerg

what about spark gaps ? I suppose all nominal spark gap voltages are
too high ? Device which I am trying to protect is not my design so I
mentioned 20-30 Volts just out of precaution. I suppose I can get to
max. 200 V. Is there a solution with spark gap only ?

I must mention that tests are for electrostatic charge, so they cannot
provide such currents.

Perhaps you could explain more about the test.

An electrostatic discharge event is over in nanoseconds, so how do you
get to 10 seconds? Or do you mean there are multiple discharges for 10
seconds? Is this one of the a standard "human body model" or "machine
model" tests?

 

[Joerg]

Thanks, Joerg

what about spark gaps ? I suppose all nominal spark gap voltages are too
high ? Device which I am trying to protect is not my design so I
mentioned 20-30 Volts just out of precaution. I suppose I can get to
max. 200 V. Is there a solution with spark gap only ?

Generally, no. Spark gap performance varies greatly with tolerances,
humidity and so on.

I must mention that tests are for electrostatic charge, so they cannot
provide such currents.

Well, that won't be 10-15sec. All you need is to survive a few dumps
from a pre-charged (small) capacitor. You'd have to provide more info in
order to help, maybe post a schematic.

The usual method for inputs is a series resistor and clamp diodes to the
rails, then another smaller resistor to the pin of the chip. Or a
transient voltage suppressor (TVS). Outputs can really only be handled
with TVS unless it's not a very low impedance output.

However, you might want to gently mention to your bosses that in order
to make an existing design not fail ESD a re-design is usually required.
You can get there with rework but it'll be ugly and production will most
definitely not like that.

 

[PeterD]

so we can achieve protection on 4-5
kV in duration over 15 seconds.

4-5 KV for 15 seconds? SECONDS? That would require one hell of a high
input inpedence to avoid letting a lot of magic smoke out.

 

[Spehro Pefhany]

Thanks, Joerg

what about spark gaps ? I suppose all nominal spark gap voltages are too
high ? Device which I am trying to protect is not my design so I
mentioned 20-30 Volts just out of precaution. I suppose I can get to
max. 200 V. Is there a solution with spark gap only ?

I must mention that tests are for electrostatic charge, so they cannot
provide such currents.

If you want a sensible answer you must provide a schematic of the test
circuit (eg. 100pF charged to 4kV with 1K5 in series) and explain more
about your input circuit and source characteristics so we can
understand how much series and shunt impedance and leakage current you
can tolerate. Otherwise we're just guessing.

 

[Hans]

That was sort of the standard test for isolation between mains primary
and secondary for transformers.
That has nothing to do with inputs.

If they tested the input that way versus mains or ground, possible,
but not the way the test should be done.
The OP will have to be a lot more specific before any suggestion can be made.

That looks also absurd to me, testing inputs with 4 kV. What is more
absurd, the device is robust in design and always worked flawlessly and
now is threatened to be discarded by security and quality standards.

I do not have any schematic here, I will contact engineers about that.

Thanks.