Measuring forward voltage of diodes?

[Crookedtooth]

Hey all! I recently got a Peak DCA55 and have been measuring some things. Here something I am failing to understand...

BAT41s are supposed to have a forward voltage of about 0.38v, yet when I measure with my DCA55 it says the forward voltage is around 0.621v.

Am I missing something? Is the test current different than it needs to be using the DCA55? Is measuring forward voltage better with a multimeter?

I guess my real question is how do I find the forward voltage of my diodes? ?

I’m sure I will ask more questions regarding the DCA55, but for now that is my big one.

Thanks!

 

[Nostradoomus]

I usually use a multimeter for that, maybe compare the two?

 

[Chuck D. Bones]

Any idea what current the DCA55 uses to measure diodes? I'm guessing around 10mA based on your reading and the BAT41 datasheet.

 

[Nostradoomus]

I think it’s 5ma

 

[Crookedtooth]

@Nostradoomus @Chuck D. Bones Thanks for the input!
Sorry for the delay, but just got my new multimeter and wanted to have that for this little experiment.
I laid out 6 BAT41's that I have on hand, and a) hooked up to DCA55, ran test (the test current is 4.347 average), b) diode test on Extech, c) diode test on 115. Here are the results:

BAT41	DCA55	Extech ex330	Fluke 115
1 (Blue - schottky?)	.610	.308	.351
2 (Blue - schottky?)	.614	.309	.353
3 (glass)	.620	.314	.360
4 (glass)	.630	.316	.363
5 (glass)	.621	.314	.359
6 (glass)	.616	.315	.359

So what do you guys think?

 

[Chuck D. Bones]

Looks like they are probably legit BAT41s. Would be interesting to know the test currents on the Extech & Fluke. You can use one meter to measure the other.

 

[Chuck D. Bones]

Here's some diode data I collected last night with my curve tracer. I tested one of each part number.



Same data, plotted on log-linear scales. Now we can see the same S-shaped curve that we see in the BAT41 datasheet.

 

[Crookedtooth]

Whoa, interesting! Thanks for doing some tests.
So I am confused - and I am 100% not versed too well in electronics, so I am sure I am totally just missing something - but aren't diodes open or closed without much of a curve? I thought it would be more "on" or "off", like a square wave, once it hits the forward voltage value.
Would germanium diodes or other small signal diodes react differently?

 

[Chuck D. Bones]

It's a very simplistic view to think of diodes as ON or OFF. When you're designing power supplies, that might be good enough. A more accurate description of a diode's behavior is that the current is the exponent of the voltage (up to a point). With guitar pedals, we use diodes to re-shape the guitar's waveform and the shape of the diode curve makes a huge difference. It's not just about forward voltage (Vf), although that is an important factor because it sets the compression and volume of the signal. Germanium and Schottky diodes have the lowest Vf. Silicon rectifiers (1N4004, etc) are higher, small-signal diodes (1N4148) are higher than that and PIN diodes (BA283) are higher still. Then come the LEDs. I posted a set of LED curves somewhere here, can't find it at the moment...

There are other factors that influence how a diode shapes the tone in a dirt pedal, such as diode resistance and capacitance. Diodes in a dirt pedal is a very fertile topic. You can find many discussions on the FSB and DIY forums.

 

[Crookedtooth]

It's a very simplistic view to think of diodes as ON or OFF. When you're designing power supplies, that might be good enough. A more accurate description of a diode's behavior is that the current is the exponent of the voltage (up to a point). With guitar pedals, we use diodes to re-shape the guitar's waveform and the shape of the diode curve makes a huge difference. It's not just about forward voltage (Vf), although that is an important factor because it sets the compression and volume of the signal. Germanium and Schottky diodes have the lowest Vf. Silicon rectifiers (1N4004, etc) are higher, small-signal diodes (1N4148) are higher than that and PIN diodes (BA283) are higher still. Then come the LEDs. I posted a set of LED curves somewhere here, can't find it at the moment...

There are other factors that influence how a diode shapes the tone in a dirt pedal, such as diode resistance and capacitance. Diodes in a dirt pedal is a very fertile topic. You can find many discussions on the FSB and DIY forums.

Right on. That all makes sense the way you are describing it - thank you!
So do you think it's safe to just measure at least the forward voltage with just a multimeter for all diodes?
I'll troll some of those other forums you mentioned and am thankful to have this forum/space to learn about all of this. I'll see if I can find your LED graph too (was it maybe for that General Tso compressor LED?). Thanks!

 

[Chuck D. Bones]

Measuring Vf with a DMM will give you one point on the graph. You need to know the DMM's test current in order to know which point on the graph that is. If you know the diode's part number, it makes more sense to look up the datasheet and get the V-I curve from there.

 

[Crookedtooth]

Got an email from Fluke when I asked about the test current for their diode test and they said it was specified with a test current of <1.2 mA.

 

[Crookedtooth]

Adding to my confusion here - looking at Small Bear's inventory and he even lists using both a multimeter and DCA and getting different measurements: http://smallbear-electronics.mybigcommerce.com/germanium-diode-russian-d9b/

V fwd .25 to .3 V on my DVM, .35 to .45 on the PEAK DCA. The colored band is the anode on these.

So now I am even more confused which device to use.
For instance, klon(e)s are supposed to have the "magical" .35v diodes, right? So which device is measuring that .35v "correctly" to match that spec?

 

[Chuck D. Bones]

So it could be 1mA or 1uA and it would meet that requirement. If you want to know what the test current is, you can measure it with another DMM.

 

[Chuck D. Bones]

I don't know what to make of it when someone reports a diode voltage, for the reasons given above. My thinking is that the thing that makes the real Klon diodes "magic" is the shape of the V-I curve. Since Finnegan won't divulge his deepest design secrets, and why should he, we won't know how to discern the "magic" diodes from any other germanium diode. The best we can hope to do is find a diode pair that sounds good to us.

 

[Ginsly]

Count me in as another person stumped by this. Tested a Ge diode with a DCA55 & TC1, got around .38v on both.

Tested them with a multimeter and got .27v. Huh!

For some, in some situations, that would seem to be a big enough difference to wonder- which one should I pay more attention to? I came to the "close enough" conclusion with transistors, and I suppose I'll have to take diode reading with a grain of salt too!

 

[Chuck D. Bones]

Right. You shouldn't pay attention to either one.

 

[Ginsly]

Right. You shouldn't pay attention to either one.

Gotcha. But... maybe a little... right? I mean, people might need some frame of reference as to which diodes are closest to "optimal" for a given circuit. A lot of build docs I see call for 1N34As, and that would seem to indicate a Vf below a certain threshold... If I was subbing BAT41s for those, I'd need to test them and get as close to a 1N34As Vf as possible, I'd think. In that case, you have to test and you have to choose how to test.

Are you saying that you should just read the data sheet and mostly choose diodes based on their typical stats?

 

[Chuck D. Bones]

No, I'm not saying that because the datasheets rarely, if ever, provide enough info to tell us how a transistor, diode or opamp will work in a pedal circuit. It gets worse: using a certain part number Ge transistor, Ge diode or JFET is no guarantee of success because there is too much part-to-part variability. Professional pedal builders do one of three things about it:
1. They screen the parts to a criteria known only to them. We know that Finnegan did that. I suspect that smart guys like Brian Wampler do it too.
2. They build a bunch of pedals and scrap or rework the ones that don't sound right. I don't know for sure that anyone does that, but I have my suspicions.
3. They build a bunch and let the customers figure out if they got a good one or not. This was common practice with germanium FuzzFaces back in the '60s.

I test parts, audition them on a breadboard and/or socket them on the PCB. I will also tweak resistor values as req'd to dial-in the bias on JFETs & Ge trannys. Trimmer resistors are not a panacea. If you have to go to the extreme ends of the trimmer, then you have the wrong transistor installed. You've probably seen me write: "not all pedal circuits are plug-n-play." If you want easy, then stick to Si transistor and opamp circuits. IMO, the rewards for getting a Ge or JFET circuit to sound right are worth the effort.

 

[Ginsly]

using a certain part number Ge transistor, Ge diode or JFET is no guarantee of success because there is too much part-to-part variability.

If you want easy, then stick to Si transistor and opamp circuits.

This makes total sense, and even with Si I've found that it isn't a case of blindly plugging something in because the build doc said so. Some sound awful, some sound good, and some sound awful in a good way.

When creating a circuit, I completely get what you're saying. I guess I'm just talking about someone like myself trying to populate a PPCB board to-spec and not knowing which data to trust before the auditioning can even begin! Ha... Sounds like everyone is kinda in the same boat with this, at least.

It really seems that just because a certain hfe/Vf/leakage/etc is indicated in a schematic/build doc, that doesn't mean that there is an objective way to arrive at those indicated parameters, necessarily.