# RCA Institutes quiz

#### AnalogKid

Joined Aug 1, 2013
9,636
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?

ak

#### #12

Joined Nov 30, 2010
18,223
21.2........

#### 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.

#### AnalogKid

Joined Aug 1, 2013
9,636

#### BobTPH

Joined Jun 5, 2013
4,289
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

#### crutschow

Joined Mar 14, 2008
28,545
What if the diodes were silicon carbide.
It states the diodes are 10V zeners.

#### AnalogKid

Joined Aug 1, 2013
9,636

Again, ***RCA*** Institutes.

Globus carnis sugat.

ak

Last edited:

#### WBahn

Joined Mar 31, 2012
26,398
It states the diodes are 10V zeners.
But it doesn't state that they aren't 10 V silicon carbide zener diodes.

However, it's a beginner's question and thus perfectly reasonable to assume the standard beginner's model for zener diodes.

#### 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
1,014
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.

#### MrAl

Joined Jun 17, 2014
8,615
Hello there,

Ha! The output is 0vpp because the input is not connected to anything

#### 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.

#### ElectricSpidey

Joined Dec 2, 2017
2,027
The output is 50 volts because the crappy Chinese Zeners failed the second the power was applied.

#### Kelsey Fitzgerrel

Joined Oct 2, 2017
3
Y'all are overthinking don't you have anything better to do .

#### KeepItSimpleStupid

Joined Mar 4, 2014
5,090
Y'all are overthinking don't you have anything better to do .
You need multiple choice and "Chose the best answer".

In a college class I challenged some question on a an exam. I told him, my answer was correct. he finally said OK, "Your not supposed to know that yet"

#### crutschow

Joined Mar 14, 2008
28,545

#### MrAl

Joined Jun 17, 2014
8,615
Y'all are overthinking don't you have anything better to do .
Hi,

Yes, but we did that stuff already and had some time left

I played a chess match at the old RCA building near here long time ago. So long ago i almost forgot.