EEVblog Electronics Community Forum

Electronics => Beginners => Topic started by: k_sze on September 03, 2016, 12:58:37 pm

Title: When do I need high voltage drop diode test?
Post by: k_sze on September 03, 2016, 12:58:37 pm
In his basic lab setup video, Dave suggests to get a multimeter that has a diode test of 4V drop, if possible.

But I notice that the Fluke 87v (which seems to be Dave's go-to DMM) can do a whopping 7.9V drop. So my question is: when do I need that kind of voltage drop for diode test? (You can tell that I don't know much about diodes, basically.)
Title: Re: When do I need high voltage drop diode test?
Post by: Connoiseur on September 03, 2016, 01:21:10 pm
Such high diode voltage drop test is useful for testing LED's, especially white and UV ones.

Actually diode voltage drop is an undesirable phenomenon in most of the circuits (to be specific; power converters). It causes power loss = Vfd X I
where Vfd is diode drop and I is the current though the diode.
Therefore diodes with lesser forward voltage is preferred for switching supplies. This is also the reason why most entry level multimeters (and even some high end keysight stuff) don't have such a high voltage diode tester.
Title: Re: When do I need high voltage drop diode test?
Post by: Alex Eisenhut on September 03, 2016, 01:47:16 pm
Perhaps high voltage diodes. They are actually a stack of lower voltage diodes, so it stands to reason that they will have a higher forward voltage. But there is a lot less equipment that needs high voltage diodes these days.
Title: Re: When do I need high voltage drop diode test?
Post by: David Hess on September 03, 2016, 02:45:01 pm
I am sure they raised it to handle higher forward voltage drop LEDs but now with a 7.9 volt compliance, those Flukes will also damage bipolar transistors when they reverse bias the base-emitter junction.
Title: Re: When do I need high voltage drop diode test?
Post by: Mechatrommer on September 03, 2016, 04:13:41 pm
Perhaps high voltage diodes. They are actually a stack of lower voltage diodes, so it stands to reason that they will have a higher forward voltage. But there is a lot less equipment that needs high voltage diodes these days.
i wonder why there is no "renez" diode. low voltage zener such as 3.3V is insensibly worst Izt, imagine for low volt PSU input or battery powered application, 20mA just to get a good quiescent regulation is nuts. now say if i have a 6V PSU or battery input in power sensitive application, i just want to drop it to 5V or 3.3V with minimal quiescent current requirement for mcu or logic chip, a good regulation is not important criteria but board space is, i can just use "renez" diode to do the job, if the manufacturer cannot make a specific and precise forward "renez" voltage  drop, like zener has (3.3Vr, 5Vr, 10Vr etc, Vr = renez voltage), at least they can make multiple of Vd drop such as 2Vd, 3Vd,5Vd etc in one small footprint, Vd=normal diode drop 0.7V. why? to save board space instead of using 5x diodes in series to get the desired drop, we can just use one renez. before someone ask why not use LDO regulator chip, i say price, instead of using $2-5, or even the modern $20 LDO, i can use $0.02 renez. there are other applications for this, such as bjt or darlington's base voltage biasing where sometime you will see 2-4x diodes in series will be used.
Title: Re: When do I need high voltage drop diode test?
Post by: David Hess on September 03, 2016, 04:57:25 pm
Ii wonder why there is no "renez" diode. low voltage zener such as 3.3V is insensibly worst Izt, imagine for low volt PSU input or battery powered application, 20mA just to get a good quiescent regulation is nuts. now say if i have a 6V PSU or battery input in power sensitive application, i just want to drop it to 5V or 3.3V with minimal quiescent current requirement for mcu or logic chip, a good regulation is not important criteria but board space is, i can just use "renez" diode to do the job, if the manufacturer cannot make a specific and precise forward "renez" voltage  drop, like zener has (3.3Vr, 5Vr, 10Vr etc, Vr = renez voltage), at least they can make multiple of Vd drop such as 2Vd, 3Vd,5Vd etc in one small footprint, Vd=normal diode drop 0.7V. why? to save board space instead of using 5x diodes in series to get the desired drop, we can just use one renez. before someone ask why not use LDO regulator chip, i say price, instead of using $2-5, or even the modern $20 LDO, i can use $0.02 renez. there are other applications for this, such as bjt or darlington's base voltage biasing where sometime you will see 2-4x diodes in series will be used.

The temperature coefficient and zener impedance for a low voltage zener maybe bad, but it is many times better than a bunch of series connected forward biased diodes and the difference becomes even greater at lower currents where the dynamic impedance of the low voltage diodes increases even faster.

If you want a simple but accurate low quiescent current voltage drop, then use a resistive voltage divider and a follower made from an operational amplifier or a pair of bipolar transistors.  If you want a micropower regulator, then use an operational amplifier and bandgap reference.