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Modifying Power Supply

1/9/2003 2:15 PM
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
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1/9/2003 4:39 PM
Would I need additional input and output caps for the 7809?
1/9/2003 4:43 PM
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...
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
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 ...  
This method is endorsed by our very own RG Keen.  
Can there be a better recommendation?
1/9/2003 6:27 PM
Rick Erickson
Re: Annonymous
"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.  
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