Big Muff diodes forward voltage

[yeeshkul]

Did any of you guys actually measured the forward voltage on Big Muff diodes?
I mean some old model - Triangle or Ram's Head.

[nocentelli]

I thought they were just the standard 1N914 diodes

[Mark Hammer]

My own experience is that there is variation in forward voltage within the same part number, so there may be something about the forward voltage in a particular pedal, but in all likelihood nothing distinct about a model or issue.  Or at least nothing with so much impact that it supercedes differences in transistors, cap-value variation, and 5% emitter-resistor variation, in any audible way.

[yeeshkul]

Kitrae refers to 0.5-0.6V forward voltage (i suppose on-board measurement). 1N914 measure around 0.4V.

[teemuk]

An expert in solid-state manufacturing processes could likely provide wealth of information about this topic. Anyway, I'm pretty sure there's somekind of "binning" strategy going on: Generic parts, like diodes within certain reverse voltage rating etc. may come from the very same manufacturing lines, but the devices are tested and then divided to different "bins" based on their performance. Devices in certain bin meet specs of 1N914. Devices in other bins are closer to specs of other diode models, and so on.

Even if you look at datasheets, it specifies certain tolerances for just about everything. So yes, there is definitely device-to-device variation going on. Not to mention, generic diodes like 1N914 have been manufactured for few decades by several different manufacturers in various factories. I'm pretty sure modern manufacturing techniques are not even trying to mimic older ones. But if the final diode meets specs of 1N914 it gets thrown in the bin.

In that process I'm pretty sure exact forward voltage and knee of the characteristic curve aren't top priorities. In most diode applications "curvature" is pretty much insignificant parameter and forward voltage rating probably falls within certain tolerance close to what you'd expect from a silicon diode. (Which in practice will vary).

Doesn't really matter in a power supply, yet applications where such things matter are niche so there's probably not much incentive to develop a manufacturing process for 1N914 that would lead to super predictable and tight paramater tolerances. Those are other diode models. That naturally cost more. Anyway, good designs should support great deal of device and component variation because in practice it's always something that one will encounter.

[teemuk]

1N914 measure around 0.4V.

Well, that is a "nominal" rating at best. In practice the parameter varies so much it's most effectively indicated as a graph. Here's one example, which also seems to be a measurement:

Looking at 1N914 graph, forward conduction seems to start at about 0.4V but full "saturation" happens around 600 millivolts. The forward voltage also depends on diode current and temperature, and as said the spec'd tolerances really aren't too tight. For example, the range of the graph ends around 4mA forward current, while nominal Vf for many diodes is specified at higher current, usually around 10 mA. At that forward current we see that forward voltage has risen to about 600 millivolts and does not show a trend to increase significantly any more.

Let's look at few examples of "1N914". For forward voltage most datasheets display a characteristic curve and/or a table of minimum, typical and maximum values.

Vishay's datasheet specifies -only- maximum value, which is 1V @ 10 mA. Characteristic curve shows forward voltage of about 800 mV @10 mA. At very low currents the forward voltage is slightly over 400mV but at higher currents (scale ends to 1A) forward voltage has increased to about 1.6 volts. An interesting feature of the graph is that it indicates "scattering limits", manufacturing tolerances. The forward voltage looks to vary device-to-device much more at higher diode currents than at lower ones.

EIC's datasheet specifies again only maximum value, which is again 1V at 10 mA. Curve shows 0.8V forward voltage at 10mA, around 500mV at very low currents and about 1.2V at diode currents slightly below ampere.

Fairchild's datasheet is more interesting since it lists, not only 1N914, but also 1N914A and 1N914B. Specs are also given at different diode currents as follows:
914B min 0.62V, max 0.72V @ 5mA
914 max 1V @ 10 mA
194A max 1V @ 20 mA
914B max 1V @ 100 mA
Characteristic curves are shared by each. Scale starts from 10 mA at which forward voltage is about 750mV. At forward current of around 150 mA the forward voltage has risen to about 1V and at 800 mA it is about 1.4 volts.

You can probably figure a certain trend from that. One thing for sure is that specifying forward voltage is somewhat vague unless the specified value is "nominal", which is vague by itself. Otherwise we need to indicate at which forward current the forward voltage was measured. Curve probably tells a great deal more than a few "nominal" numbers but in many applications of 1N914 even many of those numbers are somewhat insignificant. Yep, it will conduct somewhere below 1V... that's all I need to know.

914 is not exactly a "precision diode" so expect that "loose" specs indicate that plenty of differences in performance at forward regions exists from device to device.