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Old 25th February 2012, 04:23 PM   #1
WILD1 is offline WILD1  United States
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Default Zeners and voltage regulation

I have a circuit with a power supply source of a little over 650 volts. It has a 100K ohm resistor and 8 zeners totaling 353 volts in series connected to ground. The circuit supplies current to the anode of one tube and the control grid of another. With the tubes out everything seems fine. The voltage across the zeners is 353 volts and the voltage across the resistor is 315 volts which is what I want. But if I put the tubes in it falls apart. The voltage across the zeners drops to 261 volts and the voltage across the resistor shoots up to 387 volts. What is going on? Is it that the resistor is bad and failing under the increased load from the tubes? It checks out when I take it out of circuit.

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WILD1

Last edited by WILD1; 25th February 2012 at 04:29 PM.
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Old 25th February 2012, 05:04 PM   #2
WILD1 is offline WILD1  United States
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Come on guys. I am getting zenerphobia out here!
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Old 25th February 2012, 05:43 PM   #3
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The shortest answer is that your resistor is too big. The slightly longer answer is that the voltage drop across the resistor is large enough so that the zener is no longer properly biased. The still longer and big picture answer is that a resistor-zener "regulator" is not suited for main B+ supply IMHO.
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Old 25th February 2012, 05:52 PM   #4
M Gregg is offline M Gregg  United Kingdom
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If this is an existing circuit IE not built,

Then you are drawing to much current so the VD across the resistor is to high so the zeners cannot regulate..Similar to Leadbelly says this is similar to the value of R being to high VD to high.


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Old 25th February 2012, 05:53 PM   #5
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Your tube circuit probably pulls more current than your zener regulator can deliver.
650v supply - 350v zener = 300v across 100k resistor.
300v / 100k = 3mA.
If your load demands more than 3mA, the zener falls out of regulation 'cause there's no more current left for the zener...
Your data suggest that you need some 4...5 mA (360v-260v)/100k=approx. 4mA.
So you have to use a lower ohm resistor.
Lets say you want some headroom - 10mA that means R = 300v/0.01A= 30k ohm > make it 33k (4...5 Watt requ'd !)
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Old 25th February 2012, 05:55 PM   #6
DF96 is offline DF96  England
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Ohm's Law tells me that even when no valves are present the zener string is only getting 3.15mA, probably barely enough to maintain the zener voltage. Take a bit of current (I assume the valve anode takes some current?) and the voltage will drop.

BTW if you want to guarantee a response in under an hour you had better engage an electronics engineer on a retainer contract and pay him well.
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Old 25th February 2012, 05:59 PM   #7
WILD1 is offline WILD1  United States
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Tell Audio Researches engineers that. This is a circuit out of one of their power supplies. That's why I think something has failed hear. If a resistor is a current limiting device and it fails wouldn't the voltage increase. E=IxR
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Old 25th February 2012, 06:01 PM   #8
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by DF96 View Post
BTW if you want to guarantee a response in under an hour you had better engage an electronics engineer on a retainer contract and pay him well.
LOL... Nice one


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Old 25th February 2012, 06:03 PM   #9
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by WILD1 View Post
Tell Audio Researches engineers that. This is a circuit out of one of their power supplies. That's why I think something has failed hear. If a resistor is a current limiting device and it fails wouldn't the voltage increase. E=IxR
V=I x R...

Is the value correct to the drawing..if no drawing is it correct to the other side? Unless its common to both?
If you measure the resistance is it correct to the marking?
The value could be changed as long as the max current through the zeners is not exceeded..
Then again if this is a design value it should work unless something is wrong with another part of the circuit..


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M. Gregg
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Last edited by M Gregg; 25th February 2012 at 06:12 PM.
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Old 25th February 2012, 06:10 PM   #10
WILD1 is offline WILD1  United States
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Sorry DF96 this has been bugging me since yesterday. LOL what values? Extrapolate. The values I gave are what my meter gives and I gave you the correct values'
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