The biometric signals we are measuring are 5 to 25 microvolts. To recieve FDA approval our device has to withstand a pulse of 15kV. We tried a relay but it was only rated at 1.5 kV and it arced and burned out. Do you have any suggestions, with or without the relay approach.
What was the item number for the relay you had tried?
It would be useful to define the precise need in more detail; it’s not clear to me what relay was used, the manner of its employment, the nature of the observed failure, the test stresses to be applied or characteristics of the normal signal, etc.
Insofar as biopotentials are generally low-level, low-frequency signals, crowbar-type protection devices such as GDTs and TVS thyristors may offer useful characteristics compared to clamping type devices, e.g. MOVs and TVS diodes.
Also, since the offending signal and signals of interest are typically well-separated in frequency, there’s potential to use inductance as a means of separating the two.
We used a S1-05-bdm. The relay is normally closed. When we activate the relay with a 5 volt microswitch nothing is supposed to pass through to the amplifier. Unfortunately an 8kV was sufficient to arc and the relay never opened and the surge blew the 11,000 gain amplifier.
The device we are using is an EEG machine. Signals range from 5 to 15 microvolts. The FDA requires the device not be damaged by a 15kV pulse. This occurence will never happen in the real world. We only need a solution to satisfy the FDA shocking the electrodes.
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Does the test allow advance notice/preparation for an ESD event? Rise times of ESD event models are typically on the order of nanoseconds, so unless one is assured of seeing it coming a long ways off, the event will likely be over and done before the the coil current on a typical relay has a chance to change much at all.
TVS diodes might be a thing to consider, though it should be appreciated that they introduce a bit of capacitance, leak (noisily), and that neither may be particularly welcome. Minimizing bias across the diode can help.
Since the ESD waveforms rise so quickly, even parasitic inductance can be substantially useful for staving off voltage and smoothing things out. Putting the ESD protection near the box entry and arranging for some nanohenries of extra parasitic inductance between that point and the amplifier can do a lot to steer things in the ~15-300MHz territory where a lot of ESD lives, while having negligible effect on the desired signal and not costing extra for additional parts to be placed.
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In the single discharge mode one discarge is made each time the button is pushed. Contact discharge of 8kV is 0.8ns.
I like your thinking. Can you recommend specific part numbers for me to look at?
DESD1V0ZS1BLP3-7 is an example of a single-channel “bidirectional” device so named because of its symmetric V-F characteristic; “unidirectional” devices behave more as a zener, with a normal diode drop in one direction and a higher breakdown in another. A person might consider such a device connected between the inputs and the common-mode reference, to minimize applied bias and consequent noise.
DT1042-04SO-7 by comparison is a 4-channel “steering” type array; this sort of device functions primarily by shunting applied transients into the power supply network of the system, where there’s usually plenty of capacitance hanging around capable of absorbing it, though it also contains a clamping type diode that will dissipate incident energy if it causes the bus to exceed the indicated breakdown voltage. This is potentially a more robust approach and conveniently suited to single-supply environments, though will likely result in higher noise contribution due to the the internal diodes being under bias.
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