[Boatanchors] Diode voltage drops

[Barry L. Ornitz]

Rolynn Prechtl, K7DFW, wrote:

> No it's not constant, but what's the ARRL have to do with 
> a universally accepted junction voltage?
> 
> My experience and training has come from the USAF, IBM 
> and Tektronix, Inc.  They all use 600 to 700 mV for the 
> expected junction voltage of a silicon device (with 
> reasonable currents).
> 
> This ball park figure plays in actual use. It works at 
> the current levels most of us see and use. It works when 
> troubleshooting and repairing.

It may be a reasonable assumption for the base-emitter drop 
of small signal transistors but it is not a good assumption 
for power transistors and rectifiers handling more than a 
few dozen milliamps.

> If the ARRL is wrong, then semiconductor manufacturers 
> have followed.
> 
>  http://www.americanmicrosemi.com/faq.htm#3
> 
> "Vf is the forward voltage drop of a semiconductor. It 
> consists of two components; an intrinsic offset voltage 
> (0.3 volt for germanium ; 0 .7 for silicon) plus If x R, 
> where R is the resistance of the semiconductor and
> leads multiplied by the current, If. Vf is undesireable 
> and is minimized whenever possible as it adversely 
> affects the efficiency of the circuit and results in a 
> power loss, that loss is Vf x If."

Sadly, most of the information on the Microsemiconductor 
site is vastly oversimplified.  If you never get beyond 
this, you may someday be in for a rude awakening as I will 
relate below.

> There are children listening so why confuse them.
> 
> Remember, "it's for the children".

This is exactly my point.  You need to know the real 
relationships before you can apply simplifying 
assumptions.  Otherwise you will rely on the assumptions in 
situations where they are totally inappropriate and get 
yourself into trouble.  

A good example is with diode selection and the design 
of capacitor input filters.  Hams have had trouble with 
this for years, but the definitive article on rectifier 
operation with capacitor input filters was published in the 
1940's.  [O.H. Schade: "Analysis of Rectifier Operation" 
Proceedings of the I.R.E., July 1943, pp.341-361.]  The 
ARRL has published part of Schade's graphs since the 1970's 
in their Handbook which shows output voltage of a capacitor 
input filter as a function of the product of the load 
resistance and filter capacitance with the ratio of series 
resistance to load resistance as a parameter.  But they 
have left out similar graphs showing the ratio of peak 
diode current to average current, and RMS diode current to 
average current using the same arrangement. [At least as 
late as the early 1990's this was the case.  Perhaps the 
new Handbooks have included this information.]  Even with 
average currents well within a diode's ratings, the use of 
very large values of filter capacitance can produce peak 
currents that will destroy the diode.  A good example was 
when Motorola initially introduced the HEP-170 diode as 
part of their universal replacement line.  This was a diode 
that was rated at 1 kV PIV and 2.5 amp average current.  
What was plainly published (albeit in small print), but not 
understood by most hams, was that the diode had peak 
current ratings no higher than ordinary 1 amp diodes.  When 
used in many ham linear amplifier power supplies using 
capacitor input filters, these diodes would fail.  It was 
the excessive peak currents causing the failures but all 
the ARRL seemed to worry about was the initial turn-on 
surge.  If they had reprinted Schade's other two graphs, 
the problem would have been better understood.  Strangely 
while I have seen many reprints of Schade's voltage graph, 
only the Motorola rectifier handbook republished the full 
set of curves.  [Ever see the vacuum tube audiophools add 
excessive capacitance to their power supplies and wonder 
why they get such short rectifier life?]

But before anyone accuses me of ARRL bashing, I have been a 
member for over 37 years and will likely remain one.  And 
yes the ARRL has a few excellent technical people on their 
staff.  But their modern publications are oriented toward 
the lower technical expertise of most hams today.  I find 
it a pity that as a group, we afficianadoes of vacuum tube 
gear tend to know more electronics than the rest of hamdom.

To go back to why I believe you need to teach the full 
theory first (perhaps not in great detail for rank 
beginners), and then teach simplifying assumptions AND 
WHERE THEY CAN AND CANNOT BE USED RELIABLY, can be shown by 
the example of the friend who shared an office with me in 
graduate school.  He was from Istanbul, Turkey, and was a 
brilliant student.  One day we were comparing the answers 
to some class problems and I noticed his answers were very 
close to, but consistently different from mine.  We then 
went to a few other graduate students whose answers agreed 
with mine.  Digging into the difference, we discovered that 
he used a value of 22/7 for "Pi" in all his calculations.  
He said he was taught that this was the exact value of Pi 
by a teacher many years earlier.  Somehow all of his later 
mathematics courses had failed to convince him that Pi was 
an irrational number and that 22/7 was only an 
approximation.  While 22/7 might be a good approximation 
for some problems, it is certainly not accurate enough for 
all.

So to conclude, please look at the graphs I will be happy 
to provide, and then understand where the approximations 
work and where they do not.
    
        73,  Dr. Barry L. Ornitz     WA4VZQ     ornitz@tricon.net