Voltage drop on a Zener diode

[requim]

What would be the expected voltage drop on a Zener diode marked for 6V 8A?  Also when looking for a replacement part on Digi-key I can't seem to find a diode that matches those specs, any suggestions on how to interpret the 6V 8A markings on the diode?

[IanB]

That seems odd.

If you passed 8 A through a 6 V zener the power dissipation would be 48 W! How big is this diode?

Zeners commonly come in standard voltages like 5.6 V or 6.2 V -- 6.0 V seems to be a special value. Also, the expected and maximum currents would normally be measured in mA rather than amps.

Does the diode have a type code written on it? That would be the best way to duplicate it.

[Rufus]

It is likely a 6.8v zener.

BZX85C6V8 would be a 1W 6.8v zener for example.

[IanB]

Ah, good point. "6V8" = 6.8 V

[amspire]

I agree - 6.8V and the "A" probably means 5%.  The package will give an idea of the power, but most zeners are used as a voltage reference, and for that purpose, the current is usually about 10mA maximum or 68mW power. So any zener of 400mW to 1W would probably be fine.

Richard

[requim]

Ah it seems I didn't get all the characters off the diode.  It's 6V8A9 ZN

So a 6.8 V .9 A? Zener Diode?

[amspire]

Ah it seems I didn't get all the characters off the diode.  It's 6V8A9 ZN

So a 6.8 V .9 A? Zener Diode?

No - I very much doubt it is 0.9A. What package is it in? If it is a small glass axial lead package, it is 1W maximum (probably half that), but you are leaving us guessing a bit.

[Hideki]

It means whatever the manufacturer wants it to mean. Such markings are not exactly standardized.

6V8 means 6.8 volts (they mostly follow the E24 series), but the A is usually the zener voltage range, like A, B, C, D or some other kind of binning.
See for example http://www.nxp.com/documents/data_sheet/NZX_SER.pdf

For NXP and Vishay, A is 6.4 to 6.7 V. Fairchild however says A is 6.29 to 6.63 V and ON semi's A is 6.46 to 7.14 V.
I have seen someone use A, AL, AM, AH to indicate the zener voltage range (possibly also for TVS diodes).

0.5 W is common but you can't know for sure just by reading the markings.

[requim]

Sorry wasn't trying to be vague or make this difficult.  Here's a photo.  It comes from a PS3 Slim power supply. If you would like the specific model power supply and the board location I can supply that as well.

[amspire]

I am sure any standard 6.8V 5% 400mW to 1W zener would be fine as a replacement.

[requim]

What's the expected voltage drop?  It seems like every diode and zener diode I have tested is between about .5 to .7.  Is that true of all diodes or have I just been dealing with subset of diodes?

[amspire]

The forward voltage drop will probably be a bit over 0.6V.  In most circuits, zeners are used only in the reverse direction, and so usually the forward voltage drop is not an important specification.

In the reverse direction, the voltage drop is 6.8 volts.

Richard

[requim]

What's the proper way to test a zener diode then? 

When I set my Fluke 87 V on diode mode it works fine in the forward direction, however in the reverse direction it gives me 0L.  And from what I understand you are saying it should measure 6.8V in the reverse direction.  Correct?

[amspire]

With a 6.8V zener, just get a 9v battery and a 270 to 560 ohm resistor in series with the zener (in the reverse direction). Measure the voltage across the zener.

[Bored@Work]

What's the proper way to test a zener diode then? 

Step 1: Understand how a zener diode works: http://en.wikipedia.org/wiki/Zener_diode

When I set my Fluke 87 V on diode mode it works fine in the forward direction, however in the reverse direction it gives me 0L.  And from what I understand you are saying it should measure 6.8V in the reverse direction.  Correct?

Step 2: Check the Fluke datasheet to learn how much voltage it uses in diode test mode.

Step 3: Combine the information from step 1 and step 2

Back to the question of how to measure a zener

Step 4: Drive it in reverse mode with a current limited source that can generate a higher voltage than the zener voltage (but doesn't because of the zener and the current limit)

Step 5 (luxury): Get or build a curve tracker and let it show you the I/V curve of the zener diode

[Lightages]

Please refer to the 6v8 specification as the "breakdown" voltage not the "drop". "Forward drop voltage " is used for a normal diode in the case that it is used to conduct current in its normal use. A zener diode is a diode that is used in reverse from normal and the junction breaks down at a certain voltage which can be adjusted through the manufacturing process. A normal diode will have the reverse breakdown voltage specified as exactly that, the "reverse breakdown voltage", and will usually be in the many tens or hundreds of volts.

It is important to not confuse terms when talking about electronics as it is confusing enough for new people without adding wrong terminology.

[amspire]

Voltage drop is a generic term for the voltage across a device. Using the word breakdown is more descriptive, but unnecessary.  The term drop has never been confined to the forward voltage drop of diodes.

I do understand what you are trying to say - use the more descriptive word if possible, but in many cases, "drop" is the better word, as the voltage drop is exactly the concept you are talking about.

If you are going to get pedantic, all zener diodes above about 6.8V are actually avalanche diodes as the avalanche breakdown process dominates the zener breakdown process. So a high voltage Zener diode is similar to all other common avalanche diodes, except it is manufactured with a precise breakdown voltage. The Zener breakdown only dominates below 4.7V, and in between, both processes are happening at the same time.  For convenience, we call both zener and avalanche reference diodes "zener" diodes.

Richard