Hello. I recently acquired the part, “P6SMBJ18AS_R1_00001”, from digikey. I don’t have much experience working with TVS diodes. The datasheet shows it’s rated for standoff voltage of 18V and a breakdown voltage of 20V. It also shows it is “unipolar” and “bipolar” and no other references on how to connect it. I have connected it one way, and under no input, its resistance is in the MOhms, and when applying the 18V input, the resistance drops down to 0 and it conducts really hard, protecting the rest of the circuit. My concern is that it should not conduct at 18V.
Did I connect it the wrong way? Does this part arrive as a bipolar or unipolar? There was a very thin black line marking, but I wasn’t sure if that was an indication of the cathode or if it’s just part of the product.
I would also like to ask, that if it is a bipolar diode, then the polarity should not matter, right? And it is rated for 18V standoff, so it is protected 18V above and below its two nodes.
I appreciate the help, thank you!

Hello Oaldaw!

Welcome to the techforum!

I can understand your frustration, and I’d be happy to help here.

This part is what is called a Zener diode, here is a bit of an explanation on how those work:

A Zener diode is a diode which acts like a regular diode in the forward direction, but will also permit flow in the reverse direction when the reverse voltage is above a certain value known as the “Zener Voltage ” or avalanche breakdown voltage. In order to operate in this region, the minimum Zener Current must be met in order to maintain voltage regulation.

According to the datasheet, this part can be either bipolar or unipolar, and if it is unipolar, one side should have a mark indicating the side that is the cathode

Here is the datasheet for reference: General Description (

Since this part’s reverse standoff voltage is 18V, the part will allow current to pass until it sees 18v, it will then start to restrict current up to 20V, and once it reaches 20V, the diode will prevent any current flow.

Here is another source that explains it a bit better:
Zener Diode Basic Operation and Applications (

I hope this helped you!

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Thanks for the warm welcome and all of the references and explanations, Nate. I really appreciate it! That answers my questions

Hello @oaldaw,

I do need to clarify as it appears I was a bit off in my original response.

Zeners won’t allow current through while reverse biased until the applied voltage exceeds the breakdown value, which in this case would be 18V.

for clarification, the part will not allow current to pass until it sees 18v, it will then slowly start to allow current up to 22.1V at 1mA according to the chart on the datasheet.

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