Zener diode operation

[HenryAnthony]

Hi All!

First post here and I am hoping someone will entertain my simple question. I am a vintage motorcycle enthusiast and am engaged in a discussion about Zener diodes. These old bikes use them to prevent high voltage from entering the circuits and ruining electric equipment.

My position is that the Zener converts excess current into heat which is then dissipated via a heat sink. The reduced current is then sent on its merry way to ground.

My adversaries conclude that the heat is a result of the Zener dumping the excess current to ground.

I say that if the excess current is dumped to ground, then the Zener would be of no use.

Who is correct?

Thanks for any replies!

Henry

 

[Skogsgurra]

A zener works like a brim overflow. When voltage gets higher than desired, the zener diode starts to conduct and diverts the flow (current) to ground.

The resulting loss power, which is defined by the product of actual zener voltage and diverted current is dissipated as heat.

So, you are right about the heat and your friends are right about the operation.

Why do you think that a zener would be of no use if the excess current is dumped to ground? The current needed by the load is still available for the load. And also goes to the load.

If the zener didn't dump the excess current to ground, that current would flow into the load and cause damage there.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[HenryAnthony]

Gunnar,

Thanks so much for your reply. I think my lack of detailed knowledge in this area has led to somewhat of a misunderstanding due to lack of agreement of what is meant by "excess current". What I mean by "excess current" is what you refer to as "The resulting loss power". My statement about the zener being of no use is, I think true but poorly stated. I was trying to speak to the zener converting the higher voltage to a lower voltage and the resulting loss of power being converted to heat. Then the lower voltage excess current is dumped to ground. If the resulting loss power was not converted to heat, the zener would simply be passing the higher voltage current through to the circuit. Sound better?

Thank you for taking the time to educate me!

Henry

 

[Skogsgurra]

No, you still haven't got it.

The zener diode has only two terminals. It is connected across the device you want to protect and the voltage across the zener is exactly the same as across the protected circuit. It can, therefore, not convert anything. What it does is just what I said - it diverts current to ground when voltage gets to high. It is simple as that. No more to it. Your rephrasing does not change anything.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[LionelHutz]

There really is no such thing as "excess current" and a zener isn't capable of "converting" anything either...

The zener simply begins to conduct when the voltage gets too high. The bike stator has a certain resistance and the zener draws enough current that there is enough voltage drop across the stator resistance to keep the output voltage steady. Or at least reasonably stable since the zener is not a perfect device.

Both the stator and the zener are wasting energy in the form of heat using this scheme. The design is very primative.

You'd likely have to change to something like a buck/boost switching power supply to really reduce the circuit losses.

 

[Skogsgurra]

I start to understand that the OP thinks it is a three-terminal regulator. Like a 7805 or 7812 or similar. I also think that he doesn't see the difference between voltage and current.

I played his game when using the term "excess current". I did it to make things a little easier - but I shouldn't have done that. I conserved the misconception, I believe.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[IRstuff]

"I was trying to speak to the zener converting the higher voltage to a lower voltage and the resulting loss of power being converted to heat. "

No, the zener does no "conversion" at all. The zener can be considered to be a resistive on/off switch. When the zener is conducting, it acts as a resistance in parallel with the load that it's protecting and reduces the overall resistance in the circuit. This reduces the voltage drop across the power supply output because of the load is in series with the output resistance of the power supply. If you had an ideal power supply with zero output resistance, a zener would do absolutely nothing, except to burn out.

Because the zener conducts resistively, it dissipates power, period.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[rmw]

I consider this question to be on topic:

OK, so maybe I can get some help for this before I leave this life. I had thought it might be in the nest life that I would find out what has been a mystery to me for ~50 years now.

When I was a teen I had a crystal radio complete with cat whisker and crystal rock which worked well when the cat whisker was aimed just right on the right spot, but which lost its signal when someone walked across the wooden floor or bumped the thing or jiggled it in any way. It was a PITA.

There was also at the time a gadget called a "rocket radio" which was some type of crystal radio without a crystal and cat whisker but it had a diode in it that for some reason I have always thought was a zener diode. Being a pre-ME, I didn't otherwise know anything about electricity that would have otherwise introduced me to a thing called a zener diode and I have never really known whether or not it was or what it was so I found this thread interesting.

What I did was to take the (thought to be) zener diode out of a rocket radio and put it in series across the catwhisker to the crystal base and the radio worked 24/7 without ever losing the signal.

I loved that radio for 2 reasons. First, I always had radio on my pillow to listen to at night and second, it ran my mother nuts. She would come into my room and lift the earphone and hear the radio playing and rod me out yet once again for leaving the radio on and wasting electricity (which as poor as we were we could ill afford). No amount of trying to explain to her that it didn't use electricity would appease her - she could hear it playing and KNEW that it was using (wasting) electricity. That went on for years. Maybe she, now in her next life has been set straight.

My question is:

Could that have really been a zener diode as I have always thought, and if so what was it doing in that circuit?

rmw

 

[Skogsgurra]

Beautiful story, rmw.

No, no zener diode. Just an ordinary signal diode. Probably an 1N914 or an older brother. Today, you can still buy them in any 'radio shack'. The name is more likely 1N4148 these days.

Get one and test in your crystal radio. Which, I hope, you still have.

What it did in your radio? Same thing as your Galena crystal and the cat's whisker. It rectified the high-frequency radio signal (hundreds or thousands of kilohertz) and 'detected' the envelope, which was the low-frequency audio information that your ears cold hear and sent the audio signal to the headset (or the 'horn' if you had one).

No electricity used that your meter could know about. And just a tiny fraction of a milliwatt that the radio station paid for.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[VEBill]

Crystal radios of a certain era (1960s to 1980s) typically used 1N34 or 1N34A germanium diodes. The lower Vf of germanium (about 0.3 volts, as compares to 0.7 or so for silicon) helped with sensitivity. The Radio Shack stores even carried them in stock for this purpose.

 

[Skogsgurra]

Of course! That's a big difference. Would shottky diodes work?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[rmw]

Gunnar,

Thanks for the explanation.

Unfortunately, she was also a throw-away-a-holic. I think she wreaked her revenge on me when I went away to University. Lots of things of that nature from my childhood that I would dearly love to have today, including my BB gun just "went away".

Shortly after the era where that occurred, the transistor radio appeared, so I am sure I had graduated to one of those as well. I was so early in on those that people gawked at me riding my bicycle with a wire coming out of my pocket to a 'thing' in my ear. My generation's version of an IPOD I guess.

rmw

 

[IRstuff]

"put it in series across the catwhisker to the crystal base and the radio worked 24/7 without ever losing the signal."

If you put it "across" the cat's whisker, then it was in parallel, not in series, which would explain why its reliability increased. That would be like the first radio I built in junior high.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[VEBill]

...Schottky diode...?

Like many (ha ha) I wanted to try one in my crystal set, but it was actually easier to procure a nice multiband portable radio than to get my hands on a Schottky diode.

 

[IRstuff]

Hmm, don't recall anything being mentioned about Schottky diodes for my junior high project, but it was quite sensitive, particularly if the antenna connection was attached to the phone dial hook ;-)

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize