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Old 23rd November 2003, 04:50 AM   #1
avenger is offline avenger  United States
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Default How to lower supply voltage

Hi Guys, I just finished my power supply for my bridged clones, but the thing is that I have around 45Vdc at the output, what would be best way to lower that to 35Vdc? I was thinking of the LM388K or the use of a high power zener....what you guys think? What would be the best way to go about this. Thank you in advance for your time and concideration.
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Old 23rd November 2003, 06:50 AM   #2
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a series pass regulator would be the easiest way for someone to do that with some experience....tons of examples out there for that....if you need further advice there are examples on the forums or I can point you to some good examples

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Old 23rd November 2003, 09:56 AM   #3
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There is a very simple method of uprating the power of a 1.5 amp regulator like standard 317 or low drop out 1086.

The method is stolen from a common circuit for 12v power supplies. In these supplies they use a 2n3055 NPN bipolar transistor. These are used because they are very tough and reliable.

Unregulated plus in goes to the collector of 2n3055 and also to in pin of reg. Voltage devider between out and ground, centre of which connected to adj. Out connected to base of 3055. Regulated output taken from emitter of 3055.

Voltage devider calculation is: Use 100ohm for upper resistor (R1), that is out to adj. Lower resistor use a 5k preset and adjust for output required.

Output from 3055 becomes output voltage of reg plus 0.6v.

In the actual 12v supply zenners are used and a small signal transistor to drive the 3055. My way is simpler I think, and allows complete adjustment. I also use a zobel on output of reg (2.7r and 0.1uF tant). There are many refinements to the reg, I have just given you the simple way.

You have to stand this circuit below ground for the - rail of your supply, in which the + above refers to ground, and the - to the - supply. The whole thing is floating.
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Old 23rd November 2003, 11:09 AM   #4
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Quote:
There is a very simple method
please post some scematics.....
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Old 23rd November 2003, 11:29 AM   #5
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it's just a standard linear regulato with a series pass transistor...I think you can see it in the datasheet of a LM7812...go look for it on National's site...
www.national.com
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Old 23rd November 2003, 04:35 PM   #6
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i never saw what was wrong with some series imedance on the primaries.
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Old 23rd November 2003, 04:44 PM   #7
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Quote:
Originally posted by theChris
i never saw what was wrong with some series imedance on the primaries.

Well, don't we normally want supplies with low variation with load? Secondary series impedance would work against that.

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Old 23rd November 2003, 06:13 PM   #8
ronc is offline ronc  United States
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It would look o me like 1.5A supply would be adequate for a supply from the LM317 as long as an adequate H/S was used?
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Old 23rd November 2003, 07:05 PM   #9
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Quote:
Originally posted by janneman



Well, don't we normally want supplies with low variation with load? Secondary series impedance would work against that.

Jan Didden

not at all... explain why everyone insists on 1000uF filter caps. you get large ripple under load. As i see it you can use series impedance or an attenuator. the attenuator could be a better approximation as the fully loaded and unloaded voltages would be closer.

also, i'd use the series impedacne on the primaries, less current, no worries of matching.

in any case with any linear regulator, the voltage drop -- current product will determine heat dissipation. for 10V of drop and 1 amp of current you dissipate 10 watts. for 2 amps you'd drop 20 watts. now if your amp only draws 1amp from the secondaries it's feasible to use a linear regulator. i'd bet from the discription that this is a subwoofer amplifer. if so you will draw more then 1 amp from the secondaries, and things will begin to get hot.
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Old 23rd November 2003, 09:00 PM   #10
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Quote:
Originally posted by theChris
not at all... explain why everyone insists on 1000uF filter caps. you get large ripple under load. [snip]

Not " everyone insists on ...". And why do SOME insist on it? Well, this one guy reported that it improved his music, or so he thought. The rest, as they say, is history.

But yes, you can use secondary series resistors, but usually they are used to smooth the cap rippel, kind of an erzats choke. But if you want to use them to lower the secondary voltage I think you very quickly get a very hot resistor. I would, if feasible, wind some windings on the core (well insulated of course) and put that in opposite phase in series with the primary. I have used that often, always with complete succes. You need to experiment a bit to get the right # of turns. Start with 10 and see what you get. Then you know the voltage per turn. The rest is downhill.

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