Schottky barrier diodes testing

Schotty diodes inherently have higher capacitance at the junction in the reverse state. That one you have point to has over 1000Pf (1nf)/(0.001uf) Which seems to be rather high?

At higher voltages on the reverse side, this capacitance will be less noticeable of course. It drops off as the applied voltage goes up. In your case, you are using a meter which is applying very low voltage in comparison so you'll see the full effect..

Using the DMM impedance (10MEGS for most), you can create a RC network and calculate the time constant. t= R*C*5 = 0.050 sec in your case on the 10Meg DMM.

If you're getting much longer readings, may I suggest you apply a variable voltage source to test the break down? I think you may have a almost shorted diode..

Jamie

More than you ever wanted to know about component testing:

Some of my assorted digital volts guessers have a "diode" check position. The difference is that they draw some current through the diode, in order to eliminate the capacitive charge up effects that you're seeing.

Another way to test diodes is a build a crude curve tracer.

Find a sweep generator that will do a ramp or triangular wave. Put the diode in series with a current limiting resistor across the generator output. Put a scope across the diode. You should see the something vaguely resembling the diode characteristic curve. Something like this but with an audio generator instead of power xformer:

I always use the diode test scale on my DMM to test these. It should show a forward voltage of .3 to .4 volts. The reverse characteristics lead me to suspect it is bad. I'm lucky in that I have a variable power supply with current meter available to check reverse leakage. While this diode is rated at 60 volts, if it shows any measurable leakage at 20 volts I would replace it.

PlainBill

I also have two adjustable current power supplies like you mentioned. So in other words, what you're saying is reverse bias the diode across the supply, (cathode to positive) and look for leakage as read on the current meter? Lenny

I just pulled a suspect diode out of a dead board. It bears the numbers SR506. This seems to be a Schottky barrier diode. I haven't had much experience working with these at all. Normally when checking conventional diodes I would use my 260 on RX100 ohms. It's very simple. I read about 500 ohms one way and infinity the other. With this diode though just after pulling it I read in one direction about

***SB diodes have measurable leakage - I mistakenly binned a few in my learning years before I knew this. The forward drop is lower than a silicon diode and is quite variable depending on the power rating of the diode - very small SB diodes can have Vf as high as 0.4V, only just less than very fast silicon types, large SB rectifiers can have 0.2Vf, or even as low as

***A DMM with a diode check function that gives you a direct reading of Vf is pretty much indispensible, a little experience interpreting the readings is also quite useful.

***If you replace a silicon rectifier with an SB type; you may find it breaks down on reverse voltage, SB types start around 20V with ratings up to about 60V being fairly common - I've seen 90V SB rectifiers, but they cost more so manufacturers only use them if they have to.

Last time I checked, there were some 1200 volt Schottky types that came out not to long ago. The problem is finding a supply house that has some.

Jamie

Silicon Carbide Schottky by Cree. Up to 1700 volts now.

Allegedly stocked by Arrow, Digikey, Newark, and Mouser, but a quick check of Digikey showed limited supplies, probably because of the high price tag (about $20 in singles):

No offense intended Lenny, but ohms checking with an analog VOM (especially when using more than 1 ohm range, thus different currents most likely) is kinda crude.

Zener diodes might be a very good example.. both analog and DMMs will produce somewhat worthwhile readings, but a simple circuit will show the actual clamping voltage. A small project box that's big enough for a couple of bananna jacks or short leads with clips to hold the DUT, series resistor and meter leads going in, used with a small variable voltage power supply and either VOM or preferrably a DMM forms a complete test setup. The DMM will check the junction voltage drop with a diode test function.

I prefer analog meters for some things which they perform better for, but checking semiconductor junctions and various other components isn't very effective in terms of easily understood results.

For most semis that can be identified, checking some of the parameters listed on their datasheets will reveal all/most that needs to be known.. and the leakage aspect is often a significantly important test.. as PlainBill recommends.

-- Cheers, WB .............

Last time I checked, there were some 1200 volt Schottky types that came out not to long ago. The problem is finding a supply house that has some.

Jamie

Many of the fliers print bold that they have similar Trr to SB.

Correct, with precaution of using a current limiting supply or increasing the voltage slowly.

PlainBill

Beware using current limited supplies to test stuff, especially junctions. You can set the current to zero, but the energy stored in the output capacitor can zap the device. Suggest using an additional series resistor such that the open circuit voltage can support no more current than the device can handle.

When checking zener diodes I usually put a series resistance of about

But I'm still wondering then would it be a valid test to put this diode across my bench power supply reversed biased in series with say a 1000 ohm resistor and see if it draws any current? Will that prove if it's leaky? Lenny

at is the zener ratined voltage.and meter the dio more current than the device can handle.

When checking zener diodes I usually put a series resistance of about

###My homebrew zener measurerer is a little risky but works a treat.

### Its a pair of reservoir electrolytics from scrap monitors in series, with the tap to one AC leg of the bridge rectifier to make a voltage doubling rectifier, AC current is limited by a series pair of 68k/2W resistors (were the startup resistors for the UC3842 in a monitor PSU).

### In parallel with the test clips & sockets for the DMM probes is a shorting switch in series with an inrush limit NTC thermistor to dump any charge on the caps before connecting D.U.T.

###So far, after quite a few years use - only one D.U.T. has acted as a one-shot relaxation oscillator. Its satisfactory for checking the PIV of

###On the UK 230VRMS supply, the doubler/rectifier means I can check PIV upto about 600V - that's probably limited by leakage on the bridge rectifier I pressed into service.