What is the best way to measure current and voltage through a diode

Question

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.

Answer 1

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.

Answer 2

Most portable DVMs have 10 megohm input resistance on the voltage ranges while the current ranges usually have about 200-400mV burden when at full scale. (Often the input resistance on the 200-300mV scale will be much higher, maybe even 1Gohm, as this is often the native input to the analog to digital converter)

The diode leakage current in the reverse direction will be nanoamps or microamps while the forward drop will be a few tenths of a volt up to 1-2V.

If you are measuring the leakage current use arrangement 1 so that the operating current of the voltmeter does not affect your measurement. The voltage across the current meter will not affect your measurements significantly.

If you are measuring the voltage drop in the forward direction use arrangement 2 to avoid the current meter affecting your voltage drop measurement - the current for the voltmeter will be insignificant.

Be careful about the voltage drop in the test leads when measuring the forward voltage drop as there can be several hundred millivolts drop there. Measure the voltage across the diode directly at the diode when measuring the forward voltage drop.

Answer 3

Any parasitic draw the meters would have can be accounted for and used to correct the measurement. The argument is silly.