RCA Institutes quiz

Thread Starter

AnalogKid

Joined Aug 1, 2013
9,494
From a 1969 ad for RCA Institutes, a mail-order electronics training course. There is no trick or secret, the resistor value doesn't matter, it's just a straight-up beginners question. What is your answer?

upload_2018-5-30_23-54-48.png

ak
 

WBahn

Joined Mar 31, 2012
26,398
To a first order, 21.4 Vpp.

That's making some pretty obvious assumptions about things like the resistor being sufficient to keep keep the reverse-biased Zener in breakdown but below it's max current and also assuming nice, nearly vertical characteristics in both forward conduction and reverse breakdown.
 

BobTPH

Joined Jun 5, 2013
4,051
0V, because the upper output is connected to the shadow, not the circuit. (What’s that about?)

Okay, I’ll play along. Looks like 21.3 to me.

Bob
 

Thread Starter

AnalogKid

Joined Aug 1, 2013
9,494
Answer portion of the same ad, upside down in the original.
upload_2018-5-31_14-19-26.png
Again, ***RCA*** Institutes.

Globus carnis sugat.

ak
 
Last edited:

WBahn

Joined Mar 31, 2012
26,398
They are using an even simpler model, namely that a zener diode has no forward voltage drop. I don't know how reasonable that is, since by the time students are introduced to zener diodes, they should be quite comfortable with the notion that physical diodes exhibit a more-or-less constant voltage drop when forward biased.

I assume that they gave full credit to people that included a forward drop in their answer.
 

Hymie

Joined Mar 30, 2018
1,003
They are using an even simpler model, namely that a zener diode has no forward voltage drop. I don't know how reasonable that is, since by the time students are introduced to zener diodes, they should be quite comfortable with the notion that physical diodes exhibit a more-or-less constant voltage drop when forward biased.

I assume that they gave full credit to people that included a forward drop in their answer.
Although not directly related to this question, during my early years of circuit design, I would consider by default that a diode had a forward voltage drop of around 0.7V.

It soon became apparent to me that this was not necessarily the case – especially where the diode was within a low current circuit and therefore the resultant diode volt drop might be closer to 0V.
 

WBahn

Joined Mar 31, 2012
26,398
Although not directly related to this question, during my early years of circuit design, I would consider by default that a diode had a forward voltage drop of around 0.7V.

It soon became apparent to me that this was not necessarily the case – especially where the diode was within a low current circuit and therefore the resultant diode volt drop might be closer to 0V.
It would have to be VERY low current to get close to 0 V for a typical silicon diode. Even with an ideality factor of 2, it would be around 120 mV/decade of current.

I generally use 0.7 V unless it's a power device in which case I'll use 0.6 V or even 0.5 V. For higher than normal currents I might use 1 V.

But I can't recall ever having to discover that it wasn't some fixed voltage. I think my first exposure to diodes was in 10th grade when I started taking a ham radio class. My memories are a bit vague, but I seem to recall him drawing an exponential curve and then talking about how the voltage increases rapidly up to the knee voltage at a very low current and then stays pretty much constant over the typical range of current, two or three orders of magnitude, and can be approximated well by 0.7 V.
 
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