ampage
Tube Amps / Music Electronics
For current discussions, please visit Music Electronics Forum. New: view Recent Searches.
New: visit Schematic Hell!
The sunn still shines online!

 
Listen to great tunes streaming live right now!

ampage archive

Vintage threads from the first ten years

Search for:  Mode:  

Modifying Power Supply


 
1/9/2003 2:15 PM
Mike
email
Modifying Power Supply
Is it safe to step down a 12V 500mA AC/DC wall adapter down to 9V by just placing a 1W 9.1V Zener diode across the output? Already cracked it open, it appears there is 13V zener across the output now. Would I need to add an additional resistor too?
 
1/9/2003 2:57 PM
Jakob Erland
Hi Mike,  
 
No, just adding a zener without a resistor would not work. The zener that is already there serves as protection only, I think.  
 
If you use a zener and a resistor to drop to 9V, you'd loose 3V@500mA = 1.5W in the resistor/zener. This would also give you a voltage quite dependent on the current drawn from the PSU.  
 
A much better thing to do would be to use a LM7809 voltage regulator. It's VERY simple to use, and will give you stable 9V voltage, short-circuit protection and over-temperature protection.  
 
 
Jakob E
 
 
  Sunday
Book Of The Day The Ultimate Tone, Volume III by Kevin O'Connor
Have you ever wondered if there is a better way to build a Bassman, Champ, Plexi, an 800, AC-30, Bulldog or Portaflex? Or you wanted to build an SVT with off-the-shelf parts? How about a master-volume amp that doesn’t change tone with the master setting? Everything you need to know is right here, including: proper grounding techniques, wiring methods, and mechanical considerations. Eighteen chapters cover the “iconic” amps everyone knows and loves, with schematics and layouts for each, along with the technical history of the product. Eyelet-board and chassis-mounted tube socket construction is used throughout, for easy servicing and modding. TUT3 is very accessible even if you cannot fully read a schematic and is a "must have" if you are going to build an amp for your self.

Note: The Ampage Archive is an Amazon Associate site. A small commission is paid to the site owner on any qualified purchase made after clicking an associate link such as the one above.
 
1/9/2003 4:39 PM
Mike
Would I need additional input and output caps for the 7809?
 
1/9/2003 4:43 PM
anonymous
Yes - you might even consider simpley changing the existing zener.  
Just make sure it has a suitable power rating.  
Look here under "Zener diode regulator" for some insight into the design procedure...  
 
http://www.physics.uoguelph.ca/phys3100/lecture19.PDF
 
1/9/2003 5:29 PM
Jakob Erland
Naa, you should'nt really mess with the over-voltage protection Zener.  
 
And yes, you'll need a small (like 10uF/25V) electrolytic on the output of the 7809 regulator. If there's a capacitor already in the unit for smoothing the DC - which i think there should be - you don't need another capacitor on the input.  
 
Jakob E
 
1/9/2003 5:42 PM
anonymous
This is silly.  
Zeners are used as regulators all the time.  
What is being referred to as an over voltage protection zener - is a regulator! We're moving from one damn zener to two caps, a regulator and a pwb maybe? Get real.  
 
This is a very simple thing - don't make a wall wart into rocket science.  
 
Do a google search on "zener regulator" and leave this 7809 nonsense behind - really.  
 
Here's another way to go ...  
 
http://www.jt30.com/jt30page/micKcircuits/Zener-Voltage-Drop.html  
 
This method is endorsed by our very own RG Keen.  
Can there be a better recommendation?
 
1/9/2003 6:27 PM
Rick Erickson
email
Re: Annonymous
quote:
"This is a very simple thing - don't make a wall wart into rocket science."
 
 
Sure, hang a 9V zeener off the output of a 12V "wall wart" power supply and tell me how long it takes before the zeener shorts and takes the transformer with it.  
Your reference to zeener voltage reduction in an amplifier power supply has nothing to do with the zeener regulation application the original poster is refering to. In one application the zeener is in series with the supply, in the other it is in parallel. Can you figure out which is which, and why the parallel arrangement will burn out unless a series resistance is added to the circuit?  
Please try to give information only when you know what you are talking about. Hiding behind an annonymous lablel doesn't help your credibility any.  
As a low voltage positive regulator the three terminal 7809 (7909 for neg. reg.) is far superrior to a simple zeener arrangement, however 12V input is on the low side to insure good regulation at 9V output. These reg's will "drop-out" if the input voltage is within 2 volts or so of the output voltage, causing hum and noise.  
 
RE
 
   Page 1 of 3 Next> Last Page>>