Self-Resetting Fuse Additional Information

Hardware: Multifunction DAQ (MIO)>>M Series

Problem:
National Instruments has learned of customers experiencing damaged modules and damage to chassis or adjacent modules due to improper connections.This document provides information on who may be affected, how to check for a problem, and what to do if you are affected.

Solution:

Background

A UL-Recognized self-resetting fuse (also called a resettable fuse) is used on many NI products to provide protection in the case of an inadvertent short circuit on the +5 V auxiliary power lines, but it is not intended to protect against prolonged short circuits (>1,000 hours). The fuses are typically used on the +5 V auxiliary power lines on PCI and PXI boards. For a list of NI products that use this self-resetting fuse see attached List of All Affected Products.xls.

Product damage has resulted from shorting the +5V to ground for an extended period of time. The fuse manufacturer has advised that the self-resetting fuses may not withstand an extended short circuit condition. The condition may damage the fuse and could cause arcing or flame on the device.

Example Diagram: NI USB-621x Self-Resetting Fuse Location.

Who is affected

Unless you rely on the +5V from the NI board for your application, unintentional short circuits of the +5V line may not be obvious. If the self-resetting fuse is tripped, there may be no effect on the performance of the remainder of the board. The most common causes of shorts are:

Inadvertent shorting of unused +5V pins to ground on custom test fixtures
Incorrect wiring connections on terminal block or accessory
Bent +5V or ground (GND) pin on a cable connector

How to check for a short

Please review your system, both by visual inspection and by checking any wiring diagrams, to determine whether the +5V from the NI board is being used. If your test system uses the +5V provided by the NI board, and the parts of the test system using the +5V are operating correctly, then you do not need to check for a short circuit.
If the +5V from the NI board is not being used, you should determine whether the +5V line is being shorted. Notice that most boards provide +5V on multiple pins, so be sure to check all applicable pins. If no short circuits are apparent, you can confirm there are no shorts by taking a measurement. This is particularly important if you have designed a custom test fixture that may have a short that is not visible.
You may use the following procedures to check simple test fixtures. If you have designed a custom test fixture, you may need to review the design of your custom test system in greater detail.

Measuring +5V node—If there is a short in your test system, the +5V node will not be at +5V. If it is shorted to ground, the self-resetting fuse will be “open” and the node will be at ground. This method requires access to the +5V line from the test fixture. When the board is plugged into a chassis, powered on, and connected to the test system as usual, measure the +5V node with a multimeter to confirm that it is at +5V. If it is not, see below: “What to do if you find a short.”

NOTE—The NI board MUST still be connected to the test system or accessories as usual. If you disconnect the cable to test the +5V pin, you will also be disconnecting any existing short circuit and will therefore be unable to identify a short.

Measuring resistance—If you do not have access to the +5V node on your test system, you can remove the cable from the NI board and measure the resistance between the power and ground pins on the cable. If you are not using the +5V, you should read an “open” (> 1 MΩ). Before measuring the resistance, you should review your test system and confirm that the +5V line is not being used. If it is being used, then you may get a reading other than open or short.

NOTES:

The cable MUST still be connected to the test system or accessories as usual. If you disconnect the cable to measure the resistance, you will also be disconnecting any existing short circuit and will therefore be unable to identify a short.
You must check for a short on every +5V pin

What to do if you find a short

If you find a short that has been in place for an extended period (>1,000 hours), your board may have been damaged and may be considered a safety risk. You should stop using any such boards immediately. Boards in this condition may be repairable if the short has not caused other damage. For additional information on repairing or replacing any affected products, contact your NI representative. You may also contact National Instruments support at ni.com/ask for additional information.

FAQS

Q1) How long can the self-resetting fuse be shorted and still be safe?

A1) The UL certification testing period for these fuses is 1000 hours. Beyond this, the manufacturer states that the fuse will eventually fail, and may cause arcing or flame.

Q2) I found a short, though I am not using the +5V line. Can I just remove the short and continue using my device?

A2) If the short has been present for an extended period (>1,000 hours), you should contact NI to have the board repaired.

Q3) Can I replace the fuse myself?

A3) Replacing the fuse, or making any other changes to an NI product, would void any warranty. Additionally, NI recommends that boards be returned to NI so that we may inspect for any related damage to the product.

Related Links:

Attachments:

<p>Note: CCA number refers to the part number listed on the product that is affected. This number will be printed on a label or on the PCB. All CCAs have the format 1XXXXXR-NN(L) where X's are unique numbers, R is the revision and is a letter, NN are variations on the same product and the L indicates Lead free. All revisions and variations listed in the list of affected products are affected.</p>

Report Date: 08/10/2007
Last Updated: 05/12/2009
Document ID: 4C9AO4NO

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