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Author Topic: Is 36 volt zener necessary on you DIY QRP output stage?  (Read 4253 times)
AK0B
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Posts: 267




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« on: October 11, 2017, 04:53:01 AM »

Do you really need a 36 volt Zener to protect your output transistor on you DIY QRP design? I have never blown the output transistor on my designs. But of course I never operate my xmtr into an open RF output connector or disconnected antenna. It has been my feeling by the time the Zener switches most RF transistors would already be blown.

I do tune my antenna tuner by listening to the noise on the receiver so I am near match before I press the key.

Would like your opinion on the subject.   In addition, I am in the process of converting a Pippin from vector board to PCB.  It is more of a super Pippin since I can run over 3 watts: but the basic idea is the same as an original Pippin using a NPN and PNP for the two stages. The transmitter draws no current until the key is pressed. Or at least nothing even in the milliamp range.

My goal is a survival radio set up.  Something simple.  The world now days is always on the edge of an potential EMP pulse.  And if I believed the news we will all be destroyed by the end of the month anyway by an invisible asteroid from outer space or the planet hidden on the dark side of the Sun.  lol

Stan AK0B
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KA9UCN
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Posts: 92




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« Reply #1 on: October 11, 2017, 05:52:40 AM »

You have not given enough information. What transistor or transistors are used as finals? What is the total output potential in watts for this stage? How is it biased? Is it transformer matched to the antenna?
A schematic would be a great help.
Joe KA9UCN
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K0UA
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Posts: 1341




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« Reply #2 on: October 11, 2017, 06:52:54 AM »

And I want to know more about this asteroid...Smiley
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AK0B
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Posts: 267




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« Reply #3 on: October 11, 2017, 07:44:32 AM »

chuckles  the asteroid  is 2012 TC4    fly by Thursday about 5 GMT I think  I listen to way to much late nite radio. the planet is the one they claim we can see.   

The Pippin circuit is on the web - goggle it.  I am using 2n3053 and 2n4036 both heavy duty but cheap.  But in general I never have used a zener for either transformer coupled or capacitor coupled LPF.  I am sure a 2N2907 would also work.  I always worry about power dissipation and use 45 to 60 vdc breakdown if possible.

Most circuits from GQRP do not use zeners seems the ones from the usa like to use them. 

Stan

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K8AXW
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Posts: 6305




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« Reply #4 on: October 11, 2017, 09:36:31 AM »

Quote
But of course I never operate my xmtr into an open RF output connector or disconnected antenna

Have you ever tried to transmit into a brokenantenna or have one that breaks in the middle of a QSO? 

Most safety devices are there "just in case," just as you are preparing for an EMP pulse that might never happen. 
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AC7CW
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Posts: 961




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« Reply #5 on: October 11, 2017, 11:06:27 AM »

I worked on OEM power supplies in the '70's. They had a 2N3055 pass element. Overvoltage protection was a zener that would go short given enough time and it worked fine.

I did a test on a zener once. I had a 5volt 100amp adjustable supply and a 3.3volt zener. I turned up the voltage and left it up: the zener had a glass case and it melted!
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Novice 1958, 20WPM Extra now... (and get off my lawn)
W1VT
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Posts: 2480




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« Reply #6 on: October 11, 2017, 12:07:50 PM »

OCT 1978 - QST (PG. 34)
On Solid-State PA Matching Networks   
(Technical Correspondence)
Author: Lewallen, Roy, W7EL
Article: QST Archive [PDF]
Keywords: DESIGN MATCH NETWORK

Roy explains that it is the capacitance of the zener diode, not the voltage breakdown, that protects the final transistors.
A highly inductive matching network can cause 50 volt spikes on the collector of a 12V QRP transmitter.  Alternately, one can also use 28 volt RF devices and run them off 12 volts to survive 50 volt spikes.

Zack W1VT
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WB6BYU
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Posts: 17042




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« Reply #7 on: October 11, 2017, 03:04:55 PM »

Quote from: W1VT

.. Alternately, one can also use 28 volt RF devices and run them off 12 volts to survive 50 volt spikes.




Which includes the popular 2N3866 and 2N3553, which were developed for 28V aircraft transmitters.
But not the 2N4427 or many of the CB output transistors that often get used.
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LUCYAJONES
Member

Posts: 16




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« Reply #8 on: October 11, 2017, 09:15:45 PM »

Do you really need a 36 volt Zener to protect your output transistor on you DIY QRP design? I have never blown the output transistor on my designs. But of course I never operate my xmtr into an open RF output connector or disconnected antenna. It has been my feeling by the time the Zener switches most RF transistors would already be blown.

I do tune my antenna tuner by listening to the noise on the receiver so I am near match before I press the key.

Would like your opinion on the subject.   In addition, I am in the process of converting a Pippin from vector board to PCB.  It is more of a super Pippin since I can run over 3 watts: but the basic idea is the same as an original Pippin using a NPN and PNP for the two stages. The transmitter draws no current until the key is pressed. Or at least nothing even in the milliamp range.

My goal is a survival radio set up.  Something simple.  The world now days is always on the edge of an potential EMP pulse.  And if I believed the news we will all be destroyed by the end of the month anyway by an invisible asteroid from outer space or the planet hidden on the dark side of the Sun.  lol

Stan AK0B

Furthermore, I need to find out about this space rock.
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AC7CW
Member

Posts: 961




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« Reply #9 on: October 12, 2017, 10:05:33 AM »

OCT 1978 - QST (PG. 34)
On Solid-State PA Matching Networks   
(Technical Correspondence)
Author: Lewallen, Roy, W7EL
Article: QST Archive [PDF]
Keywords: DESIGN MATCH NETWORK

Roy explains that it is the capacitance of the zener diode, not the voltage breakdown, that protects the final transistors.
A highly inductive matching network can cause 50 volt spikes on the collector of a 12V QRP transmitter.  Alternately, one can also use 28 volt RF devices and run them off 12 volts to survive 50 volt spikes.

Zack W1VT
Crazy!! Capacitance of active devices gets overlooked. An engineer was using fets to switch small capacitors in/out of a phase linear small signal amplifier. We could not get useful results out of the thing. It went on for weeks. One day I picked up a stray fet datasheet and it said that as the thing was driven to saturation the capacitance became huge, it was orders of magnitude greater than the capacitors we were switching.
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Novice 1958, 20WPM Extra now... (and get off my lawn)
W1VT
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Posts: 2480




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« Reply #10 on: October 12, 2017, 02:30:50 PM »


Which includes the popular 2N3866 and 2N3553, which were developed for 28V aircraft transmitters.
But not the 2N4427 or many of the CB output transistors that often get used.

28V parts were more common than CB parts when I was growing up in Honolulu.

Zack W1VT
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WB6BYU
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Posts: 17042




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« Reply #11 on: October 12, 2017, 02:43:35 PM »

True, but with a lot of TO-39 parts being discontinued, they aren't as common as they used to be.
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AK0B
Member

Posts: 267




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« Reply #12 on: October 12, 2017, 09:32:40 PM »

I found a place with reasonable cost even with no shipping most of the time.   utsource

I am calculating the inductance now and hope to run some destructive tests to see what happens

stan

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KB1GMX
Member

Posts: 1496




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« Reply #13 on: October 15, 2017, 10:01:11 AM »

Oddly I've not used the zener.  I have however done work to insure the match is correct
between the device and the antenna.

As to worrying the no/shorted/very bad antenna thing, I do not.  Over the years I've
done horrific things to finals in transceivers and amplifiers some accidental and some
very deliberate.  In all cases the failure of the final was not the result.  this is for what
I'd call finished designs.  During development I've cooked and fried many as testing
for instability and all is part of that.  Even then if the device was adequate for the
voltages and currents it usually survived.  The yabut is that output stages that
are unstable with loading can zap themselves under unusual loads and if the stage
was built to remain stable its less likely to rare.

Guidelines I've used.

- A device with a voltage rating 3-4x the applied voltage. Not less than 2x for class
  A and AB(1) devices.
- Peak current rating well above the actual planned level of use.
- Enough cooling to reach maximum power capability of the device without melting
  even if its used at lower power. 
- test at 0, 25, 50, 75, 100 and 150 ohms load  plus open as that coves most reasonable
  cases out there.  Real loads are often 30 ohms and reactive or 75ohms and reactive.

One last thing.  If you cannot assure the load is reasonable add a absorptive SWR
indicator (resistive bridge) and add a tuneable output circuit to correct that.  that will
at least help optimize power transfer and will help protect the output device and may
help the RX as well.

If all this is seems too technical, buy a spare part, as they are often cheap at the time.
Also to my experience having a spare seems to insure its longevity.  Wink

Allison
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AK0B
Member

Posts: 267




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« Reply #14 on: October 15, 2017, 11:47:51 PM »

Thanks Allison,  I think that is good advice.

I wonder what percentage use a decent antenna tuner for QRP?

It is so easy to tune the tuner for maximum noise on the receiver before one ever supply's any xmit power.

I like the Z-match design. 

Stan  AK0B

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