The AM Forum

I have a 100 volt 50 watt zener and it states in the specs Izt of 120 Ma.
Is this what the diode needs to draw for it to be operating correctly?
Thanks,
Don W9BHI

Don, Izmax would be 50watts/100V or 500mA.  Izk is the current at knee voltage and Izt is the test current. I don't know what Izk is but sure it is below Izt.   You want to select some where between Izk and Izmax and factor in a power derating. Izt could be used or maybe slightly higher. Gonna depend on the application.  What is your plan for this zener?

Don,
Izt is the bias current required for the zener diode to be withing in specified voltage tolerance band.
Based on the impedance of the diode, the voltage will vary outside of the voltage range with different current levels going through the zener.  The current set by the current limit resistor into the zener should be the Izt current plus the load current.   Doing that, at least with a steady state load will guarantee that the Izt current through the zener is according to its specification.  If the load is a varying load, one must assure that under light loaded conditions which will redistribute the current through the zener will not over dissipate the rating on the zener diode.  With fixed loads, its never a problem. 
Hope that helps!
Joe-W3GMS             

This is a power supply for a Gates BC-1H solid state audio driver.
It is taking the +660 volt power supply from the Gates down to 150 volts through a 5k 50 watt resistor and a pair of 75 volt 50 watt stud mount zener diodes connected in series as well as taking the -260 volts from the Gates down to -182 through a 1k 25 watt resistor and a 100 volt and an 82 volt 50 watt zener diodes in series.
The 150 volt zener pair are drawing 54 Ma. by themselves and the total current drawn by the driver and zener on the + supply is 112Ma.
The 182 volt zener pair are drawing 22 Ma. by themselves and the total current drawn by the driver and zener on the - supply is 88 Ma.
Are these zeners drawing enough current to be working properly?
The + supply is only 139 volts (s/b 150) and the - supply is only 175 volts (s/b 182).
Thanks,
Don W9BHI

Don,

Look up the IZT on the 75V and 82V zeners and I will do the calculations for you.

Joe-GMS

Joe,
The Izt for the diodes are:
75 volt = 170Ma.
82 volt = 150Ma.
100 volt = 120Ma.
Thanks,
Don W9BHI

Don,

Lets discuss the 150V supply first. 

The zener only current, with no external load connected does not match the math. 

For the resistor of 5K fed from a 660V supply going into two series connected zeners the voltage across R1 becomes: 660V-150=510V.  510V across 5000 ohms the current is 510/5000 which is equal to 102MA.  So first we need to figure out whats different between what you observing and the math.   

Your saying that the zeners all by themselves are only pulling 54MA from the 660V supply with no load connected, so something is different.   

Joe-GMS     

This whole scheme seems a little crazy.  Better to build a proper supply for the piece of equipment.

Fred

Take a look at the schematic and see if it makes any more sense.
I need to take the measurements again.

Interesting. I wonder why the zener regulation on the rail supply to the TIP54s.

These days a high voltage Mosfet and a low power zerner to make a simple regulator will do more than fine...
no need for the high power zeners... (you could use a high voltage transistor too). imo.

The regulation is required because the bias needs to be stable, it's not a PP circuit on the output that would tend to give a good PSRR,
and you don't want excess noise or hum...

Looking at the schematic, the reason for the 5k 50W resistor R47 is that it is supplying more current for +150 regulation and current via R49. I.e., Current for an external circuit (not shown), plus zener current, plus current for the regulated +150V for the TIP's. You're dropping more voltage for the +150V supply than you are for the -182V supply.

R48, the 1K 25 resistor is only supplying zener current and regulated current for the -182V rails.

Izmax is 540 mA. Izt, the test current, is 170 mA.

Q1 is a simple common-emitter voltage amplifier, while Q2, Q3, Q4 form a DC coupled power amplifier for the Mod grids. Mod grids are biased at -70V so what you have is audio jiggling above and below -70V.

Each half is fed an out-of-phase signal from T1.

Phil - AC0OB

Izt is Current at Zener Threshold, is it not?

--Shane
KD6VXI

Edited to correct autocorrect

R49 going to TB1-7 is for a voltmeter and TB1-8 in not connected to anything.
This driver board is connected up to my BC-1G and has tons of clean modulation.
I was just trying to figure out why the 5k resistor gets so hot and that the specified voltages were running a bit low.
Doing the math, I see that the 5k 50 watt resistor is dissapating about 48 watts.
I think both of the supplies are not supplying enough current to the zeners and the board.

Don W9BHI

(112mA^2)X5k = 63 Watts, which is greater than 50 Watts, so something isn't right here.

A current of 112 mA should be dropping 560 volts across R47.

Are you putting your meter across R47 when taking measurements?


Are the -260 volt and +660V supplies really producing those voltages?

(I now see after printing this sheet that R49 is a 300k and not a 30k).

The common elements here for the -182V and the +150 are the TIP's Q3 and Q7, so I would look at the voltages on those two transistors as well. Possibly one or more are leaky.

It is my experience that for 50W zeners anything > 15 mA will make them conduct.

And, zeners can get leaky as well.

Phil - AC0OB



 

I also just noticed that going to pins 3 and 4 of TB1 there are Left and Right "bias" adjustments, respectively.

Are these bias pots adjusted correctly?

Phil - AC0OB

Bias pots are adjusted correctly to give 40Ma. idle current on each 833.
The unit is functioning properly.
The current measurements that I gave are not correct.
I will be taking the correct measurements tonight.

Don W9BHI

Izt means Zener Test Current, fwiw.

Well I checked the voltage drop across the resistors and did the math, the numbers were very close to what they should be except the 50 watt resistor is dissipating around 50 watts.
I will change that to a 100 watt unit.
The zener diodes that are in this have a +/- tolerance of 10%.
The pos voltage is 7.4%low and the neg voltage is 4% low, both in spec.
I guess that all be well after the new resistor gets installed.
Thanks for all of the help, you guys are the best.
Don W9BHI

That driver circuit is designed to fail. :P

In all the years I serviced stations that had the BC-1H1's  never saw one that wasn't previously repaired.

I fixed enough of them as well, it was always attributed to either 'gremlin spikes' or mod tube shorts.

A wise idea was a spare, ready to go.

They did proof out well, and sounded as good as an AM rig could.

Dennis

They did have good sound.

At one Rock station we even took out T1, but kept R1, R2, R22, C1, C12,

Then we breadboarded a balanced input circuit using NE5533AN's and fed audio to the junction of R2-C1 and R22-C12.

Sam and Dave's "Soul Man" never sounded so good.  ;D

Don, glad to hear the audio driver is working as it should.

Phil - AC0OB

I love it...    Audiophile AM!    ;D