Electronic – the best way to measure current and voltage through a diode

diodesimpedancemeasurementmultimeterpower electronics

I am reading the book Learning the art of electronics and on a part about diodes, the author pointed out that in characterizing a diode(I, V relation ) there is an unavoidable error due to the ammeter and voltmeter.
Two arrangements are given:

  1. A voltmeter in parallel to series arrangement of a diode and ammeter.
  2. An ammeter in series to a parallel arrangement of a diode and voltmeter.

Further, the author hints that the value of the voltmeter impedance relative to that of the diode makes one of the arrangements have a lesser error effect than the other.

I understand that in case 2, a voltmeter with an impedance comparable to the load will lead to a significant amount of current going through the voltmeter than the diode(load). Thus the second arrangement is better with a voltmeter of high impedance.

My questions:

  1. In what case will the first arrangement be preferred over the other?
  2. What would be the best arrangement in a case that you do not know the values of instrument impedance's and diode( load ) impedance?
  3. Based on the impedances of commonly available cheap multimeters, which way should i use to characterize a diode?

Thank you in advance for your answers.

Best Answer

Short answer: non of the above.

the author pointed out that in characterizing a diode(I, V relation ) there is an unavoidable error due to the ammeter and voltmeter.

If this has to be done with any accuracy (and why shouldn't it), before any mention of measurement techniques you should consider how to apply current to a diode so that it doesn't get warm. You may think that this isn't important but, any reasonable characterization should include how the device works at different temperatures BECAUSE the forward volt drop of a diode not only is affected by current but quite significantly by temperature.

Because of this, the best test set-up (or if not the best then one I would consider to be adequate) is to design a highly accurate current source capable of applying calibrated current pulses to the diode. The pulses should not be of any significant length of time in case the device warms more than (say) 1 degree. We're probably talking about pulses of no-more than 1 ms.

In what case will the first arrangement be preferred over the other?

So, neither of the questions arrangements are adequate other than for teaching the VI characteristic to basic beginners. This is because an ammeter isn't needed because the source itself should have been designed to produce a darn stable current (on demand) AND, if you tried to use an ammeter it would have to take readings at a specific instant in time and "hold" that reading.

This also means that a standard voltmeter is also not capable of recording the forward volt-drop because it would not know when the 1 ms current pulse was applied.