why when i use my multimeter on diode test does a Schottky barrier dio 1

[richerdick]

why when i use my multimeter on diode test does a Schottky barrier diode measure only 1v in the reverse direction. forward biased it measures less than 0.2v
and it is for this low forward volt drop that i wanted to use it but was expecting it to measure o/c or 3v as is normally shown on my meter when testing a normal diode?

 

[itsmoked]

LOL. It's a Schottky!

You know nothing is perfect.. Schottkys have great Vf but horrible reverse leakage current values that can get up to tens of mA. Your multimeter will easily measure something in that direction as it wouldn't with a regular diode.

As you run up the PIV ratings on Schottkys the reverse leakage current plummets down to microamps. But, BUT! if the diode's temperature is raised the leakage will go up rapidly into the mA again.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[richerdick]

the application that i want to use it in is a small drive that derrives its reference from a plc in the advent of a power dip the plc on restart switches its analogue output to -10v before going to what ever level it is set to.
a diode was previously put in series with the drive to prvent this because it appears to fault the drive. as it likes to see a reference of 0-10v. this works but i have found recently that the opperating range of the reference is between about 1v and 3v this makes the volt drop quite critical 0.7v reduced to 0.2v sounds good enough for what i want to achive what do you think will the high leakage current prevent me from achieving what i am tring to?
thanks for the reply

 

[itsmoked]

Hmmm. Not exactly clear on what you're up to... When people talk references they usually don't mean they can wander around and be between X and Y but that they should be as exact as one can afford. This is why I'm confused.

But to move on, I understand that you want to drop less if you can.

Well the Schottky will definitely do that though often it is more than 0.2, often more like 0.3, or 0.4V. But anyway what will happen is during the reset there may be some leakage out of your drive back to the PLC during its poor behavior moment thru that Schottky. The question is will the current limiting of the diode be enough to prevent whatever is happening to your drive during these negative excursions.

Can you just try it? I suspect it will work fine.

What is the ambient temperature that this Schottky will be at? If it's less than ~35C it will likely work fine. If it is in a hot application you may have to do more. An important number to this all is the current the drive consumes into this 'reference'. Can you measure that? If it is really a small amount then you can add a resistor that limits the leakage more while not effecting the drive's reference. If there is a sizable amount of current then that won't work.

You may also be able to run a diode from your drive 0.0V (ground?) pointing to the reference line. Then when the PLC goes negative the drive will source current from its ground back to the PLC thru the original Schottky rather than having the drive's reference sourcing the current and not liking it.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[richerdick]

i have just tried a schotty in series with the reference as i described and it doesnt work. possibly due to the leakage current you describe but the drive sill sees -10v at reset. i will try both of your ideas itsmoked will the diode between ov and reference not short the plc output?

 

[Skogsgurra]

Hi, I just ran a BAT 42 shottky on my good o'l Tek 575 (see Engineering History) and it says that this particular shottky has .3 V fwd drop @ 1 mA and it doesn't leak more than .5 uA @ 30 V reverse. A lot more than an ordinary 4148, but not so bad. Mind you, this was at room temperature. So it may get a bit higher when things get busy.

But, still. That kind of shottky should work quite well.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[richerdick]

I am intrested in this concept of leakage current. i can measure the voltage going negative at reset even with the schotty in circuit is this as a result of leakage through the diode? is the leakage current the same as If test in the manusfacturers spec?

 

[Skogsgurra]

This is a simple introduction to diodes and their parameters.

http://www.powerdesigners.com/InfoWeb/design_center/articles/Diodes/diodes.shtm

Question: Is it a power shottky you are using? Those have rather high leakage, as smoked says. You should use a small one (same body as a normal 1N4148 etc) for your application. The BAT 42 is a signal shottky - not a power shottky.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Skogsgurra]

And here is a collection of data sheets on the BAT 42

http://www.datasheetcatalog.com/datasheets_pdf/B/A/T/4/BAT42.shtml

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[LionelHutz]

The analog input of the VFD should be a fairly high resistance, probably 10kohms or more. You could put a resistor in series between the PLC and the VFD analog signal line, maybe 100 ohms. Then, put a diode reversed biased between the ground and signal lines at the VFD to keep the voltage at the VFD from going to -10V.

The series resistor will create a voltage divider between it and the VFD resistance but it will be constant (10k VFD and 100 ohms series resistor give 99% voltage for example).

Make sure the diode and the PLC output can both handle the current that will flow when at -10V. This is dictated by the added resistor.

Then, you don't need to worry about the series diode drops or reverse leakage current. I hope that makes sense.

 

[richerdick]

some intersting points i think i may have been using a power schotty i have orderd a small signal one today will try it tommorow in a mock up righ i have made.
quite like the idea from lionelhutz will try this also as i am interested ion the outcome of both
thanks for your help im learning all the time!

 

[itsmoked]

That's what I said... Oh well.

Perhaps a picture.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[richerdick]

thanks for the help and the diagram having a bit of trouble sourcing the recomended diodes from our normal suppliers would be very grateful if you could take the time to look at the spec of the following

http://docs-europe.electrocomponents.com/webdocs/002f/0900766b8002f172.pdf

this is a bat86 diode seems to fit the low reverse leakage current and low forward volt drop that we have talked about
seem you were right the schotty i have been using has a comparitively high reverse leakage current 7ma at 25c
thanks again

 

[Skogsgurra]

Yes, that one is very similar to the one I tested (the BAT 42).


Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[richerdick]

again thanks for the advice

 

[LionelHutz]

Good picture Keith.

In the first picture it probably doesn't have to be a Schottky diode. Something as simple as a 1N4007 would probably work.

 

[itsmoked]

Thanks Mr. Hutz.

Actually his drive could have the input latching because of the protection diodes inside whatever the input device is. So you probably would want a Schottky in the hopes of your external one conducting before the internal one does. This is because the internal ones are always Schottkys.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com