What Do I Do if My Logger Says "Self-Test Failed?"
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CLEVELAND - OhioPen -- We occasionally receive data loggers in for repair that fail their internal self-test. This usually indicates damage to either the internal power supply circuits or to the amplifier/A/D converter. Recently, we received multiple units back from one customer that shared a similar problem with the main power supply. Going back to the customer, we found out that these were all being used to monitor an open pit mine. Since the loggers were in the field, they were powered by solar panels. The customer said there had been a particularly intense lightning storm at the site which, in addition to the data loggers, had also damaged the solar charge controller. Upon disassembling the loggers we found several of the inductor filters in the power supply were vaporized!

It doesn't take a direct hit by lightning to cause this kind of damage resulting in a "self-test failed" message.  A cloud-to-ground lightning bolt is actually a multiple-step process, involving an initial leader from the cloud, a streamer from the ground upward towards the cloud, the connection of these two to form a low resistance ionized path cloud to ground, and then the main discharge from the cloud to ground which is the lightning bolt we see.

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It turns out that there is a whole class of protection devices that were designed exactly for this application. They are called transient voltage suppression or TVS diodes. A popular model is the Littlefuse 1.5KE Series.

To use them on an analog input signal, I usually recommend a "belt and suspenders" approach to provide both protection to ground and protection across the + and – signal lines as shown in figure 1.  Since the signal lines are often floating or isolated from ground, the 2 unidirectional TVS diodes to ground prevent either of the signal lines from exceeding the common mode voltage limit of the data logger. The bidirectional TVS between the signal lines prevents the differential voltage between the = and – from getting large enough to cause damage to either the input multiplexers or the signal conditioning and A/D circuits. For the clamping voltage for the diodes, select a value that is a couple of volts greater than the normal operating voltage of the circuit.

Give us a call at 800-956-4437 or visit us at https://www.dataloggerinc.com to speak with one of our application specialists today.

Contact
CAS DataLoggers
Elizabethe Zala
***@dataloggerinc.com


Source: CAS Dataloggers

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