Measuring forward voltage of diodes?

[Chuck D. Bones]

The designer should have an idea of what part parameters are important. As DIY builders we need to know that too if we want our stuff to perform like a production pedal.

Robert has started putting parametric data (HFE, lkg, Vp, Idss) on some of the Pedal PCB schematics. My understanding is these values were measured on devices from a production pedal. We still don't know how important each parameter is or how much margin we have on the stated numbers, but it's a decent target to shoot for. When I post schematics, I'll indicate critical voltages in the circuit and the notes usually indicate how to dial them in.

 

[Chuck D. Bones]

The problem with doing that is a single data point does not make a curve. The diode currents in a BMP peak out at around 120μA in the first clipping stage and around 55μA in the 2nd clipping stage. Unless the TC-1 measures at those currents, it's not clear to me how its measurement is relevant. FD100, FDH400, 1N4937 & 1N914 all measure close to 580mV at 5mA, but they're all very different down around 100μA.

When you measure HFE in a BC549B, do you have a pass-fail criteria? IOW, how close to 335 does a transistor need to be to pass muster? This is akin to a question I used to ask a lot at work: "once you have collected this data, what are you going to do with it?"