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Selenium Diodes in instruments
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You will find quicky in forums, how terrible old Selenium diodes are, how dangerous, and what strange trouble people go through, to replacing those by Silicon diodes. They are trying to simulate something with similar characteristics. Generally, anything having to do with diodes, is a field of knowledge, where fully inexperienced people feel competent to advice others about how things should be done. I stressed this point at some other places of this website, when using vacuum tube diodes. People make every silly error you can think of, being fully resistant against good advise. They have this idea of a diode being a switch, which is on and off, depending on the voltage polarity. Not understanding what the implication of dynamic resistance is, of the type which by itself is not a fixed number, but a number depending on the diode current.
Forums are nice on the one hand, but it generally goes wrong when it's just gut feeling what they are telling about, and they are saying so as if they are specialists. Go to any forum and ask for sound difference of new made 300B vs genuine Western Electric NOS 1930. You will quickly be advised by people who never even had a Western Electric NOS 1930 in their hands. But they will tell you how it sounds. This is a fully ridiculous advice of course, when it comes from somebody who has not just only never heard a genuine 1930 NOS Western Electric tube playing, but also never had one in his hands. But for replacing Selenium diodes, it's exactly this kind of "knowledge" they ventilate at you. I do not have the real number for this of course, but I guess that 95% of the Selenium Diode talk is by people who never held one in their hands. And from the remaining 5%, I dare say that virtually nobody took the trouble to make a forward voltage vs. current plot. So what are these people talking about? It's forum babble, that's all it is.
For the same reason, when investing my savings on stocks, I made a good habit to refuse taking any advise whatsoever, from people who do not own a substantial amount of stocks themselves. Not when it comes to the benefit of owning stocks, and not when it comes to the risk. Initially this may seem arrogant towards to the "advice giving" person, but it saves me from confusing reality with opinions.
In the same way, any opinions about circuit design, coming from people who never had such components in their hands, is of little practical use. Strangely however, inexperienced people, most of the time seek advise on this level. Even more strangely, when advised by people who really know better, they begin to dispute this. This is the strange world of forums.
When is an diode ideal?
That answer can be given quickly. At a rated forward current, it has no forward voltage, and in reverse condition it has endless high breakdown voltage, and no capacitance. This knowledge doesn't get us any further :)
For power supply applications, today's silicon diodes are close to ideal. You can buy such for 1 cent, which have a peak reverse voltage of 1000V, and forward voltage of 0.7V at 1Ampere. Just the large types, so 25 or 100's of ampere are very bulky. So, not really ideal, but still very good.
For RF applications, we need them with low capacitance. Such exist, but the low capacitance comes from small chips, with the drawbacks of this.
For small signal applications, it becomes more difficult to understand.
Here, the diodes are usually intended for changing AC voltage to DC. WIth large signal that is easy, but how will you rectify a 20mV signal? This can never be done with silicon.
Myself, I have several pieces of old equipment with Selenium diodes, and they all work good. Initially I was reading the forum babble too. So yes, I wanted to replace those bad, dangerous, poisonous, and unreliable items. But with instruments, of course the meter scale, and the whole functioning takes into account what curves Selenium diodes have. So there is quite some risk to compromise the quality of the instrument.
So looking into curves of Selenium diodes, I can only say, those curve were always EXCELLENT. Though these Lucas branded, early type silicon diodes, in the AVO Mk4, I sometimes had to replace.
For small signal, Silicon diodes are the worst choice. To understand this better, throw overboard all that forum babble, you are probably infected with that as well. Of course it has to do with the higher forward voltage of Silicon, but that is not the real issue. Germanium has not only a lower forward voltage, but unlike Silicon, there a rectifier function BELOW the forward voltage as well. And not just a little bit below, but at EVERY voltage. That is why you can use a Germanium diode, to detect (and rectify) a signal if just a few millivolts. Whereas a Silicon diode is simply dead below 0.4 Volts.
Now here comes the good thing with Selenium. These diodes have this interesting property as well, to begin working much below their rated forward voltage. To rectify small signal, this makes them very good as well. Not for radio, because of their capacitance, but for AC voltmeters they are ideal. Not as ideal as Germanium, but quite close.
Selenium diodes are just two metal plates with some Selenium and other composition coated on it. And voila: A real semiconductor, and a good one! Only maximum reverse voltage is not very high, but resistance is much lower than vacuum tubes. You can even take them apart, there are dark layers on the metal. Pile it back together, and they still work. I tried that with an old Siemens Selenium bridge rectifier. I do not intend to do so with my AVO (Made by LUCAS) diodes. But the simplicity of such diodes is amazing.
It's just there is a high voltage drop at high current. We all know that. So though they appear low voltage, high current, if compared to vacuum tubes, they do get very hot at high current. But what happens at low current? They do not get hot, do lifetime expectancy is good. There there is no large voltage drop any more, and here comes the advantage vs Silicon: So no voltage drop, and they work already at very low voltage.
With Silicon diodes, if you want to use them a little below 0.7V you can do so, but current flow will be small. You can increase the current flow by putting several in parallel, this needs no explanation. But a quicker way to achieve this, is using a very high current Silicon diode. Like a 6 Ampere diode, and use this to rectify small signal. This is exactly why you will find such (6A) diodes in parallel of the meter of the Mk4 tube tester, to protect it. (And not a small signal diode, as the forum babblers will suggest you).
The Selenium diode on the AVT two panel tester, is not used to protect the meter. This meter is not very fragile anyway. It's purpose is only to generate the negative grid voltage, for the "set zero" knob. It means, in this tester you can replace it by a Silicon diode if needed, and when the Selenium diode seems to work good, you can just leave it in. The forward voltage of this (LUCAS) Selenium diode is appr 1.4V at small signal, so replacing it could be by two silicon diodes in series, and that's all you need to do. In equipment were forward voltage does play a role however, you need to be more careful what you are doing.
This Silicon Diode , red and black leads attached to it, was used by some repair man, long ago. It was to repair this supposedly bad Selenium diode. Now believe it or not, but the man reversed the panel meter by mistake. Which had no effect on the function, as he swapped all other connected too, as he found their polarity was "wrong". It's crazy, but such people exist. Just a little problem remaining with the he negative grid voltage, because that was now positive. So he though the Selenium diode was bad, and he replaced it with a Silicon diode. And of course reversing it, so to get negative grid voltage again. He happily put it back together and the tester worked well ever since. Until I had to clean the meter, and I found out it was put in reversed. Believe me it took me some time to understand this mess, and repair it all back to normal.
What was nice of him, he let in the old "bad" Selenium diode, but when I connect it to a curve tracer, there is nothing wrong with it. Indeed after re connecting everything correctly, the tester worked normal, with it's original Selenium diode.
Note here, the scale of the HAMEG Curve tracer is 3V per division. So the Selenium diode as well as the Silicon replacement diode, have 1.4V forward voltage. Unfortunately I lost the Silicon diode, but it was a LUCAS High Voltage diode, with two diodes in series, in one epoxy housing.
This is the old LUCAS Diode. Texts have come off a little bit, but the readable part is: .../6A. This is either a .6A or a 6A type.
Read my report about the Tektronix 576 curve tracer, go to the part about testing diode curves.