Anyone have a really amazing multimeter (super high impedance)?

[midwayfair]

I need to know the resistance from cathode to cathode on two 1N4007 diodes (or similar) with their anodes connected together. I know it's well over 10M and so my multimeter won't measure it. I'm hoping it's similar or higher than the 300M across FET junctions.

[dadler]

I have a Keysight 34465A.

Unfortunately the closest I have is 1n4002.

I measured the resistance across the cathodes with the anodes tied together.

I get a reading for the first two seconds or so, and then it creeps up and goes over range. Here's a picture:

[midwayfair]

Hm. I got really excited when I saw the "1 ... G" (I was looking for a cheap sub for a 1G resistor) -- but if it's going above range it's possible it's not doing anything.

4002 is actually fine. I just needed anything with a reverse breakdown voltage above 35V.

[dadler]

The test current for this mode is ~.5uA. I imagine the junction capacitance charges and then the current goes down so low that it's way over range.

I have a Peak DCA Pro that can do curve tracing, maybe I will see what it shows (not sure if the data will be useful).

[Vallhagen]

I think i have an (old) MOhm-meter at work. I can give it a try tomorrow. I have 1N4001 and 1n4004 here. What voltages will you work with?

Why do you need this measure? It seems a bit funny to me, but i guess you have your reason? And also, it shall be possible to measure backwards on only one diode, the one that goes "forward" in your setup is really just a short in this case.

Cheers

[dadler]

Yeah-the results are the same with only one diode, as long as you get the polarity correct. The dual diodes just makes the (attempted) resistance non-polarized.

DCA Pro doesn't even know what to do with the dual diode, and of course with a single diode it behaves just like the spec sheet.

[midwayfair]

Hm, I'm thinking I might be misguided in my attempt to save $5. (1G are expensive ... some are $13 each.)

[dadler]

@100Hz I read 167 Mohm and @ 1kHz I read 103.6 Mohm impedance, but this is more by virtue of internal capacitance than actual "resistance".

These values will be really affected by the frequency and reverse voltage. The rated junction capacitance of the 1n4002 is 15pF @ 1MHz. I cannot measure that high of a frequency with this DE-5000.

Anyways, I imagine your use case was DC so not sure any of this is useful 

[Vallhagen]

Ah ok. you basically want a high-ohm resistance. Banzai have some 1G for 2.20 EUR (about the same in USD).

Anyway, i can see if i can do that measurement tomorrow.

Cheers.

[midwayfair]

Anyways, I imagine your use case was DC so not sure any of this is useful 

Hm. I think it's AC, actually. One of the 1G is a charging resistor for a mic capsule and the other is on a FET's gate. In both cases, they would see AC. I forgot to check the capacitance; it's only 4pF for a 1N4148, but a 4001-4004 has 15pF, which is actually ... kind of a lot.

[dadler]

Hmm- then the leakage/capacitance will be dependent on the frequency. Those quoted capacitance values are at 1Mhz according to the Fairchild datasheet.

I assume this is an audio circuit with standard audio frequencies?

The nice thing about resistors is that they are pretty much linear / frequency independent (well, at the low end). You could try some diodes and see what happens?  Might make for some interesting learning either way.

[dadler]

So since schottky's have higher reverse leakage, I tried two 1n5817 (only handles 20V reverse, I don't have any 1n5819 on hand) in the same configuration.

They start out around 130 Mohm and then ramp up slowly over time. But really slowly... It took about 3 minutes to ramp up to 1 Gohm. After 5 minutes it still hasn't gone over range and is floating around 1.2Gohm, and then dropped back down to 1.16 Gohm and floats around. Probably temperare dependent, my AC just kicked on. Of course we aren't really measuring resistance here, but leakage current.

Not sure this is useful but it was fun to measure 

[peterc]

Jon

I get my 1Gig resistors from RS Components who sell through Allied in the US.

This is the unit I get for about a $1.2
http://za.rs-online.com/web/p/through-hole-fixed-resistors/2477856/

Allied dont have this particular stock code (247-7856) listed, but if you call them with the code, you might get lucky.

Peter

[midwayfair]

Jon

I get my 1Gig resistors from RS Components who sell through Allied in the US.

This is the unit I get for about a $1.2
http://za.rs-online.com/web/p/through-hole-fixed-resistors/2477856/

Allied dont have this particular stock code (247-7856) listed, but if you call them with the code, you might get lucky.

Peter

Thanks! There are a few other things I can get from Allied too ... they're only about 40 minutes away from me but their shipping is extremely painful (it's over $8).

[Vallhagen]

Anyway, i can see if i can do that measurement tomorrow.

Just to follow up this: Nope, i couldnt do the measurement. But honestly i think the result would be ambigous (correct word?) anyway. As the currentflow (i avoid the word resistance here) through a diode backwards basically is dependent on the limit for the break-through voltage (which indeed is high) you cant rely on a measurement anyway. It is drifting and is unpredictable.

We have highvoltage diodes (say 200kV, look them up at Haefely) for some work applications, and they consist of several diodes (10-20kV each) in series. And when you design such "diode arrays", each diode is also in parallel with a high-ohm (say 10M) resistor, just to control/balance the leakage current and avoid flashover. So... a resistor is necessary to define resistance.

Maybe i try to explain something with too few words here, but hopefully the main point is stated.

Good luck with your project!

Cheers