Basic Zener Vreg but still confuse...

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zamproject

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May 11, 2010
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1,290
Hi all

Just fixing various issues on a elka rotary speaker cab
I get about everything going on and fix various minor fault, but I'm still puzzled with the reverb driver and the associated regulation that was initially faulty.
Very basic, audio supply +/-40v, no reg, just fullwave and caps
Then the separate pcb for reverb driver around an old TAA611 "power" IC regulated by a 11V zener via a 270ohm from the +40V rail.
Initially supply at this pcb measure dead, 500mV IIRC, I check the zener and obviously was short.
I change the zener, check the 5W resistor and voltage was very high, well over +/-5% and increasing, at about 12.8V at DMM after let say a minute, I cut the power.
I also measure the V+ rail and it goes around 45V in place of 40, I check primary and there is no option as it's an old 70' fixed 220 single primary (my wall is more around 235)
My quick math for the zener and 270ohm give a close to limit for dissipation at 40V and suspecting heat issue in the zener, so I decide to replace the 270 with a 330 (just what I have in stock @5W) and the voltage still go too high I think, at about 12.6V (loaded and unloaded...)
But I don't get why I'm so far from Zener voltage
I realize I don't use zener that often and probably miss thing about it ...

Best
Zam
 

Walrus

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Jul 18, 2006
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883
Location
London, UK
What Zener did you get?
They used to be available as either 5% or 10% tolerance, so it could easily be more than 12 volts.
 

zamproject

Well-known member
Joined
May 11, 2010
Messages
1,290
So this is power overload drifting ?

What's the zener wattage? 45V - 11 = 34 / 270 = 0.126A * 11 = 1.4W but you want to double that at least. So it needs to be 3-5W.
as I say, at start it was already on the edge...probably the zener burn since long time...
initially it was a 1W 11V 1N4741 and within 40V "nominal" it's already 1.2W...designer definitely do it wrong...considering the supply before that is not regulated and now outputting 45V..
I put a BZX85 1.3W which now have to handle more or less the same power with the 330ohm under 45V
But the current should be splited/shared with the load ? so I was gessing it will be ok, which is not...

What Zener did you get?
They used to be available as either 5% or 10% tolerance, so it could easily be more than 12 volts.
BZX are 5%
The TAA611 is given for 12V max, I'll like to not fry it as its obsolete and expensive (for what it is...)

Thanks to both of you. I'll instal 3 or 5W as soon as I get some...

Best
Zam
 

zamproject

Well-known member
Joined
May 11, 2010
Messages
1,290
Ok...think I got it...
For some reason it was counter-intuitive for me yesterday, but obviously if there is same current in the zener with or without load it's that the load take noting...I have sound at reverb output so not expectic to measure few mA at load for the whole rev circuit. At scope the TAA611 give no gain at output, just same as input signal, the "power" pushpull of the ic should be dead
this is why the zener take all the power...
I'll catch a replacement for the amp

Best
Zam
 

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