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# LM335: Diode test for LM335H

Part Number: LM335

I am using a LM335H sensor in a photomultiplier tube assembly. I was testing the temp sensor with the diode test on a digital voltmeter with a range of 3.1V. The DVM read 2.95V at room temp, which I expected. However, when I sent it to the customer they claimed that the sensor was broken and reading 2.5V. I tried again with another one and had the same issue. I tested with both a Fluke 83 III and 189 multimeter.

My question is: Is using the diode test an inappropriate way to check the sensor? Will it damage the sensor if testing without a resistor or does it need a resistor in circuit? Would this cause an improper voltage across the pn junction? I am trying to determine if the customer's test setup is improper.

I also applied a heat gun to the sensor to verify that the voltage jumped up in response, which it did. So would it make a difference whether I applied heat which would be within the operating conditions specified on the datasheet?

• Dear Derek -

Applying a voltage (which is what the multi-meter is doing) to a zener diode without a current limiting resistor is not recommended.

If you have followed the datasheet recommendations for ~1mA limit of current (as shown in section 8.3 of the datasheet) through the use of a selected value of resistance based on your input voltage, you will then be able to just use voltmeter to read the output of the device and not risk breaking the parts.

• Hi Josh,

I see where a 1mA current is recommended for testing, but Section 9 indicates that a range of 5mA is okay, no? Or does this not factor in the impedance of the diode?

What is the best practice for testing the sensor? Should I just place it in series with a limiting resistor such that the resistance value equals the difference in the source voltage (say 15V) - the voltage drop over the diode, divided by the 1mA current? What is the risk of the sensor breaking without the limiting resistor?

Thanks,

Derek

• Derek -

without getting into a big discussion about different diode types constructions -  I am sure you understand well the concept of current limiting and the fact that in all cases where diode is not used as a switch (i.e. Zeners and LEDs are common examples) a current limiting resistor must be present. Zener diodes are low impedance and therefore current limiting resistor not only must be present, but it must be small enough to keep the device in reverse breakdown, yet large enough to effectively limit the current so it won't break the part.

So - please use a current limiting resistor, power the part and check the output matches the temp you are operating at. This is the way to test the device.

• Hi Derek,

If I may add to the discussion, I think the best practice is to use an appropriate current limiting resistor, a voltage source, and a voltmeter. A multimeter set to diode test mode may have a test current which is not appropriate for LM335. One multimeter I tested produced a quite good 0.998mA test current. while another model produced 0.420mA. 400uA is the lower limit for this device. The multimeter also may not have enough working voltage range to properly measure 3V or to source current at 3V in this mode, as it is most likely designed to operate near 0.7V.

With LM335's Absolute Max current limits of 15/10mA, it seems unlikely that it would be damaged by a multimeter's diode test mode. It's more likely that it is incompatible with your multimeter, as discussed above.

thanks,

ren