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Old 5th May 2010, 12:49 PM   #1
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Default How to drop voltage and current

Hi all,
I have on of those power supply, and I want to use it for powering a small amp and fm radio. The rated output is 12V DC 1.3A.
The power supply is suitable for the amp, but probably not for the radio, as this circuit is designed for a 9V battery. My need is to drop the voltage to 9V and also to drop the current because the radio only draws 8 mA, and feeding the circuit with a 1.3A would probably damage it.
What is the best way to have 9V 8mA from my power supply?
Thanks, Ralf
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Old 5th May 2010, 01:01 PM   #2
Glowbug is offline Glowbug  United States
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Pretty simple circuit

Something like this: Portable CD Player Adapter For Car

Don't worry about dropping the current...the load will draw the current it needs.
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Old 5th May 2010, 01:34 PM   #3
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I also was thinking of a L7809, but why on the circuit posted there are 4 polarized caps while on the datasheet there are only 2 non polarized caps?
Ralf
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Old 5th May 2010, 02:19 PM   #4
81bas is offline 81bas  Germany
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I think it is possible even to trim this psu to deliver 9V without any external components... In the photos in ebay I see the trimpot inside. Very possible, that by rotating of this thing you will get the desired voltage
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File Type: gif trim.GIF (90.6 KB, 303 views)

Last edited by 81bas; 5th May 2010 at 02:22 PM.
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Old 5th May 2010, 02:43 PM   #5
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Probably I wasn't clear. I would like to use one supply for both the amp and the radio, and the voltage is right for the amp. I was thinking to drop the voltage only for the radio circuit.
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Old 5th May 2010, 02:52 PM   #6
81bas is offline 81bas  Germany
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Quote:
Originally Posted by giralfino View Post
Probably I wasn't clear. I would like to use one supply for both the amp and the radio, and the voltage is right for the amp. I was thinking to drop the voltage only for the radio circuit.
Oh, sorry, I have skipped some points reading your post...

Also, you can simply connect a 3.3V Zener Diode between 12V power supply and your radio, to get approx 9V This will be about 8.7V, but this should be ok...

Last edited by 81bas; 5th May 2010 at 02:57 PM.
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Old 5th May 2010, 03:25 PM   #7
AndrewT is offline AndrewT  Scotland
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you can run the little power amp directly after the rectifier of the 12Vac 1.3Aac supply.
The DC voltage after rectification and smoothing will be about 16Vdc to 18Vdc.

If you really meant 12Vdc then run the amp direct off that supply.
From the same supply attach a lm7809 to give your 9V for the radio receiver. Download the datasheet for the lm7809 and follow the advice accurately.
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Old 5th May 2010, 04:06 PM   #8
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Mmm, never used a zener, how do I connect it?
Will this one be good (zener from rs-components)?

Anyway, it will cost me less to use a L7809, if I only need the 2 non polarized caps depicted in the datasheet (0.1 and 0.33 uF), because I already have them.

Regarding the current drop, anybody knows for sure that my supply won't do any harm to the radio circuit based on a TDA7000, that needs only 8mA?

Thanks, Ralf
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Old 5th May 2010, 04:15 PM   #9
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Glowbug View Post
Don't worry about dropping the current...the load will draw the current it needs.
Glowbug is telling you the truth.
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Old 5th May 2010, 04:18 PM   #10
AndrewT is offline AndrewT  Scotland
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The Zener solution is the cheapest.
You need a 180r to drop voltage from ~12V to ~9V.
A 9V 400mW or 500mW Zener.
A capacitor across the Zener. Anything from 100uF 16V electrolytic upwards.
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