Zener diodes in series as a voltage regulator

[dbishopbliss]

I am trying to use a series of five 62V zener diodes as a simple voltage regulator, but I don't think its really regulating.  I was hoping to get 283V as my final voltage, but I'm getting 300V instead.

Here's an low tech diagram:



372V ---+--2.2K--+--+--2.7K--+--+ 283V
        |        Z  |        |  |
        |        Z  |        |  |
        =        Z  =        =  8 10mA (that's supposed to be a current symbol)
        |        Z  |        |  |
        |        Z  |        |  |
 GND ---+--------+--+--------+--+



I thought the five diodes would clip the voltage at 310V (62*5). The voltage after the 2.2K resistor, but before the diodes is 318V (if I recall).  Is this too close to my regulated voltage for it to actually work?

[JC]

I've always found Zener circuits to be a little hard to predict, since they essentially act as a variable load on the supply.  With your circuit as shown, what are the Voltage drops across the 2.2k resistor and the 2.7k resistor?

[braubeat]

One potential problem you have is that a resister in series with a shunt regulator and a capacitor in parallel form an oscillator. In that arrangement you have, the cap should be smaller than 1uf or put a resister in series with the cap. Actually I am oversimplifying, the circuit needs to be analyzed more carefully.
Hope this helps

Michael    

[Grainger49]

David,

I think with a diode "regulator" you should have a Pi filter before and no additional resistors.  Then you regulate to the total of the zener voltage.  There are a lot of different voltage rated Zeners.  You can tweak to the voltage you want.

[dbishopbliss]

David,

I think with a diode "regulator" you should have a Pi filter before and no additional resistors.  Then you regulate to the total of the zener voltage.  There are a lot of different voltage rated Zeners.  You can tweak to the voltage you want.

I am using 62V diodes to get me close.  I want to have an additional pi filter after the diodes to remove any noise that they might cause.  This is where I'm a little fuzzy... what input voltage should I have for the diodes to actually regulate.  They don't appear to do anything now?

[Grainger49]

I could be way off base here but I see the Zener string like an OD3.  The FP III has a small bypass cap after the OD3 and nothing else.

If the Zeners are doing nothing now you might not have more than the total Zener voltage coming in.  If the voltage isn't exceeded they do nothing.

[braubeat]

Seriously guys, if you put a large capacitor after a shunt regulator it will oscillate. It may very well have destroyed the zeners. A small capacitor is all you can have. Also Grainger is correct, in that putting a pi filter after the regulator compromises the regulation. Oscillation is one of the major headaches you run into when designing your own circuits. If you look on the data sheets of gas regulator tubes you will usually see a warning about this exact problem. I once destroyed some tubes this way which is why I finally read all the warnings.

Michael

[Paul Joppa]

...
I thought the five diodes would clip the voltage at 310V (62*5). The voltage after the 2.2K resistor, but before the diodes is 318V (if I recall). ...

The Zeners are usually 5% devices, so you should expect anything from 295 to 325 volts. Which you have.

Now imagine that the input voltage, 372v, is +/-10% - which is what the power company promises.  If the power line is 10% low and the Zener voltage is 5% high, calculate the available current through the 2200 ohm resistor. Do it again for the power line 10% high and the Zeners 5% low. Then you will understand why we reduce 400v to 150v in the Foreplay regulator circuit.

[dbishopbliss]

...  if you put a large capacitor after a shunt regulator it will oscillate... A small capacitor is all you can have.

My cap is 56uF so is that large or small?  Are you saying it should be  small like 0.01uF or small like 2 uF?  Compared to 10,000uF it seems small, but perhaps not small enough. 

I have the parts for a non-regulated version of the power supply.  What will it sound like if it oscillates?  I will build that and see if I can hear a difference.

... If the power line is 10% low and the Zener voltage is 5% high, calculate the available current through the 2200 ohm resistor. Do it again for the power line 10% high and the Zeners 5% low. Then you will understand why we reduce 400v to 150v in the Foreplay regulator circuit.

I will do the math in the morning and see if I understand.  :-)

[dbishopbliss]

I will do the math in the morning and see if I understand.  :-)

I did the math, but I don't understand.  What am I supposed to get from the following?

Power line 10% low = 372 - 37 = 335V
Zener 5% high = 310 + 15 = 325V
Current = 335 - 325 / 2200 = 4.5mA

Power line 10% high = 372 + 37 = 409V
Zener 5% low = 310 - 15 = 295V
Current = 409 - 295 / 2200 = 52mA

I am using a 2.2K 3 watt now, but 52 * (409-295) / 1000 = 5.9W?  That's a pretty big resistor if I double or triple its rating for safety.

If I were to decrease the 2.2K resistor to 1K?  If I were using no regulation, PSUD predicts the voltage after the 2.2K resistor to be 318V.  If I were to use a 1K resistor, the voltage is predicted to be 342V.  In theory, will the regulator be able to clip 342V down to 310?  I guess the problem with a smaller resistor is that the current increases so I would need a resistor with an even larger power rating. 

If I were to use a 3.3K resistor the voltage goes below the regulator value of 310V.  This means I need to start with a larger voltage.  Hey, maybe I do understand why you reduce 400V to 150V.  Am I on the right track?

[braubeat]

Check out this data sheet for an 0b2wa shunt regulator tube. Look at the application notes section.  I couldn't swear it is exactly the same for zeners but I wouldn't be surprised if it was similar. It basically says don't use a shunt capacitor larger than .1uf.

As far as the voltages are concerned you should leave yourself enough headroom to account for the worst possible scenario. Say the zeners are 15 volts higher than predicted and the power supply 15 volts less than predicted and you have only 8 volts of headroom. The components in your amplifier can vary more than 8 volts with a small change in temperature. So you calculate the highest possible voltage of the zeners and the lowest possible voltage of your power supply and make sure your headroom is more than that. Also you must calculate the lowest possible voltage of the zeners and the highest possible voltage of the power supply to make sure the zeners and the resisters have enough wattage. For example using the 2.2k resister if the voltage drop is say 120 volts (probably a minimum in this situation) the current is 54.5 ma and the dissipation is 6.54 watts for the resister. Each zener is 62 volts so they could be dissipating as much as 3.4 watts each. You should always use components rated well over exactly what you need. As you can see the fact that circuits seldom behave exactly as predicted when using imperfect real world parts makes designing with them quite complicated. But fortunately it's also lot of fun.
Good luck   

   
Michael

http://frank.pocnet.net/sheets/138/0/0B2WA.pdf

[dbishopbliss]

I understand your point about the capacitor, but I'm still trying to grasp the whole regulator thing before I begin to worry about oscillations, etc.

The voltage drop across the 2.2K resistor is only around 60V.  The circuit draws around 20mA total and I am using a 3W resistor.  Based upon your numbers, I believe that each zener is dissipating around 1.25 watts - they are rated as 5W so I have plenty of head room.

However, I don't think I have a large enough transformer to use a zener regulator based upon my calculations above (unless I ignore the extreme cases and assume that my power will not fluctuate that much). 

[JC]

OK, maybe this goes to what I was trying to get at above: IF the 2.2k resistor is dropping ~60V, then the entire "circuit" after the 2.2k must be drawing 27+mA, mustn't it?  So, how much of that total is the circuit the supply is feeding, and how much of it is the Zeners sinking current?  IOW, your ultimate goal is to supply 283V at ? current to your tube circuit?

With the circuit as shown, you are getting 300V at the top of the Zener stack (which is within the tolerance of the Zeners, I suspect) , is this correct?  And, you are still getting 300V at the output of the supply?  Is that also correct?

[Paul Joppa]

...  Hey, maybe I do understand why you reduce 400V to 150V.  Am I on the right track?

Yes, you seem to be! Notice the 4mA isn't enough to power the circuit, much less the Zeners. And 52mA means the Zeners will dissipate 3.25 watts.

If you do the calculations for a higher voltage, say 600v, you will see a much smaller variation in current - but the resistor becomes huge. That's what is in the Foreplay - two 10-watt resistors in parallel.

Now what you don't know unless you read the data sheet with a very suspicious mind, is that Zener diodes of the "barrel" construction dissipate heat through their leads. In order to meet that "5-watt" rating, you must keep the leads to no more than 3/8" long and attach them at that point to a heat sink that will keep the temperature at that point to 75 degrees C. That's actually a pretty good-size heat sink, not just a couple square inches of stamped clip-on but finned and extruded or cast aluminum. And you need one for each lead. Seriously, this is totally unrealistic! I try to keep maximum Zener dissipation below 20% of the so-called rating for that reason, and I'd try for 10% if it was to run continuously inside the chassis of a tube amp.

[dbishopbliss]

I think I'm going to bail on this whole zener diode regulator idea.  :-(  At 36 cents each, I knew it had to be too good to be true.  I think I will see if I can shoehorn a choke in my enclosure instead.