Hi,

I'm going fast and furious into building things, which is the reason I am
posting
so much. Latest project is a variable power supply.

Situation: Radio Shack wall transformer with 11 VDC unregulated output is
going into an LM371. I want to shunt with a pair of 5.1 V , 1A Zeners in
series
so that my adjustable output voltage is repeatable. Presently my Zener
series resistor is 180 ohms.

Problem.. I'm frying my zeners!!

What should I be doing differently?

Thanks in advance,

The Eternal Squire


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The Eternal Squire wrote:

Hi,

I'm going fast and furious into building things, which is the reason I am
posting
so much. Latest project is a variable power supply.

Situation: Radio Shack wall transformer with 11 VDC unregulated output is
going into an LM371. I want to shunt with a pair of 5.1 V , 1A Zeners in
series
so that my adjustable output voltage is repeatable. Presently my Zener
series resistor is 180 ohms.

Problem.. I'm frying my zeners!!

What should I be doing differently?

What are you shunting with the zeners? Describe the circuit after the
11 volt DC raw supply.

--
John Popelish

Situation: Radio Shack wall transformer with 11 VDC unregulated output is
going into an LM371. I want to shunt with a pair of 5.1 V , 1A Zeners in
series
so that my adjustable output voltage is repeatable. Presently my Zener
series resistor is 180 ohms.

Problem.. I'm frying my zeners!!

What should I be doing differently?
======================
Would you please indicate where the 2 Zener diodes (in series) are located
in the circuit .
Before or after the LM317
If before the LM317 why ? The LM317 provides excellent regulation even with
fluctuating 'upstream' voltage.
What is the LM317's adjustable output voltage range. And which max current
are you drawing ?

Are you sure the zeners are 5.1 V,1 Ampere and not 1 Watt. If the latter is
the case the permissible current must not exceed 200 mA. .


For getting best reliability ,I would suggest you load LM317 with a current
less than 0.4 Ampere .
If more current is required use a 'wrap around ' PNP power transistor.
This is described and shown on page 1.15 of the book : Experimental Methods
in RF Design ,by W7ZOI,KK7B and W&PUA .

Frank GM0CSZ / KN6WH

You need to first know the maximum and minimum current that your circuit
will draw, then you can design the zener part. While this is over
simplified, it should give you a start in the right direction:

The zener is sort of a current buffer in that it will absorb the current NOT
taken by the load when the load is drawing its minimum current. Start
thinking like this:

When the ckt being regulated is drawing maximum current, the series resistor
in the zener leg is sized to provide a low zener current. When the ckt is
drawing minimum current, all that extra current (the difference between the
maximum and minimum current) goes through the zener. If it is too much you
either need to get a higher dissipation zener, go to an amplified zener or
some other regulator type.

--
Steve N, K,9;d, c. i My email has no u's.

"The Eternal Squire" wrote in message
...
Hi,

I'm going fast and furious into building things, which is the reason I am
posting
so much. Latest project is a variable power supply.

Situation: Radio Shack wall transformer with 11 VDC unregulated output is
going into an LM371. I want to shunt with a pair of 5.1 V , 1A Zeners in
series
so that my adjustable output voltage is repeatable. Presently my Zener
series resistor is 180 ohms.

Problem.. I'm frying my zeners!!

What should I be doing differently?

Thanks in advance,

The Eternal Squire


---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.564 / Virus Database: 356 - Release Date: 1/19/04

Situation: Radio Shack wall transformer with 11 VDC unregulated output is
going into an LM371. I want to shunt with a pair of 5.1 V , 1A Zeners in
series
so that my adjustable output voltage is repeatable. Presently my Zener
series resistor is 180 ohms.

Problem.. I'm frying my zeners!!

What should I be doing differently?

Are you sure the zeners are in the correct polarity ? Also I have not
looked at the way zeners are speced lately but aren't they specified as
Watts instead of Amps ?