# How to reduce dc voltage ?



## ]|[ GorE

i have a 17V o/p AC-DC adapter whose output i want to feed to a voltage regulator ic whose max safe input voltage is 16 V.

 How to reduce the 17v to say 15V ? I am sorry for forgetting all my electrical basics !


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## dsavitsk

If you know the current that will be drawn, use a resistor in series before the voltage regulator. V=IR to figure out what size to use.

 If you don't know the current, use another voltage regulator first (LM317, for example.)


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## ]|[ GorE

The AC-DC adapter has output current of 2.5A.

 So V=IR

 17=2.5R
 Therefore,R=7 Ohms ?


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## dsavitsk

No, it goes the other way. The 2.5 amp rating means that the PS is capable of supplying up to 2.5 amps. The actual amount that flows is determined by what the circuit draws. So, to use the resistor method, you need to figure out how much current the circuit is going to draw. Generally, you can do this by looking at datasheets, etc. Note, however, that depending upon the circuit it can change, and if it does, the amount of voltage dropped by the resistor will also change (to a point where if the circuit is off and there is no draw, the regulator will see the full 17V) which may not be ideal. If it were me, and I was worried about toasting the 16V max regulator, and I was using a circuit that does not have a constant draw, I'd put an LM317 in front of it.


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## dsavitsk

Quote:


  Originally Posted by *]|[ GorE* 
_The AC-DC adapter has output current of 2.5A.

 So V=IR

 17=2.5R
 Therefore,R=7 Ohms ?_

 

Also, for V, use the amount you want to drop.

 So, assume a current draw of 500mA and you want to get the voltage down to 15V, that's a 2V drop, so 2 = .5R or 4 Ohm

_Edit: I hope nobody saw the first bit of math I did 
	

	
	
		
		

		
			



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## tangent

Quote:


  Originally Posted by *dsavitsk* 
_use a resistor in series before the voltage regulator_

 

While this may technically work, it is only useful in trivial cases. Almost always, increasing a power supply impedance is a bad thing. A voltage regulator gets you the voltage you want, along with a nice low output impedance.


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## mono

What type of AC/DC adapter is this?

 If it's unregulated and floating at 17V with no load, it may easily be well below that once implemented. If it's not unregulated, if you have access to the interior it might be easy enough to make it regulate down the extra volt, and/or just put a diode on the output to drop another half a volt or so.


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## ]|[ GorE

Quote:


  Originally Posted by *mono* 
_What type of AC/DC adapter is this?

 If it's unregulated and floating at 17V with no load, it may easily be well below that once implemented. If it's not unregulated, if you have access to the interior it might be easy enough to make it regulate down the extra volt, and/or just put a diode on the output to drop another half a volt or so._

 

Yeah ,its 17V floating with no load.The Adapter belongs to my Cambridge SW DTT2200 (POS) speakers; so if i were to attach say 2 diodes in series after the adapter output , that would drop the voltage by almost 1 volt right ?


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## ]|[ GorE

Dont caps have a resistance too ? If i were to put a cap after the output ?


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## ATAT

putting a cap in series would be kinda pointless.. it'd remove all your DC component from the power supply =p
 putting it to ground would act as smoothing ripple.. that'd be bad

 Yes the cap has a resistance, it's theoretically infinite.

 I'd suggest a simpe LM317 circiut


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## skyskraper

i think you're over complicating this. what regulator are you planning on using? a 7812 will regulate down to 12v and has a maximum input of 35v. its a fixed terminal regulator, only needs a couple of caps on either side of it and is dirt cheap. similarly, the other regulator people have mentioned already will take an input voltage of up to 40v and regulate it down to whatever you need. slightly more complex, but not by much. both should be easily and cheaply attainable.


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## mono

Glancing around the rest of head-fi, it seems you might be wanting to use this for a CMOY, is that right?

 This is a bit of an overkill for a CMOY. Any typical 12V wall-wart putting out 200mA is even overkill for a CMOY, which needs a few dozen mA, if that. 

 SOund quality on a CMOY also seems sufficiently low enough that there'd be diminishing or no return on a fancy regulator either, it would be quick and easy to just throw an LM317 behind a smaller wart and reuse your nicer and higher current parts for some other project.

 Given how little current you're pulling, I'd just put an LM7815 in front of the 2nd regulator, use a different power supply or different regulator.

 With the regulator you have now, with it's max input of 16V, it's not really as useful for headamp projects, as post-regulated it'll be under 15V at best, with many people quickly wanting to go higher than 15V on more advanced projects. You could get by with sub-6V on a CMOY though, how low depending on the opamp- I dont' mean to imply otherwise.

*Edit:* Above should read "... could get by with *sub-16V*", not sub-6V.


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## ]|[ GorE

Its an Analog Devices 3367 Voltage Regulator with max safe input voltage of 16.5V.Not 16v
 I was thinking,maybe it is safe to connect with dropping teh input voltage since under actual working conditions of cmoy,surely the output of the adapter would output less than 17V right ?


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## mono

Not much less, CMOY just isn't but a trivial load on a 2.5A transformer.


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## ]|[ GorE

Quote:


  Originally Posted by *mono* 
_With the regulator you have now, with it's max input of 16V, it's not really as useful for headamp projects, as post-regulated it'll be under 15V at best, with many people quickly wanting to go higher than 15V on more advanced projects. You could get by with sub-6V on a CMOY though, how low depending on the opamp- I dont' mean to imply otherwise._

 

So is about 15v too less for the cmoy ?


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## ]|[ GorE

is 15V less for the cmoy ?


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## mono

15V would work for CMOY.

 I was thinking ahead, that if this CMOY is not your last project, if you ever wanted to reuse the power supply you made with a better amp, odds are pretty fair that you would then want the supply to be able to output higher than 15V for that OTHER/next amp.

 Since you don't need 2.5A capacity, and since a higher voltage wall-wart or transformer of suitable capacity for a headamp might not cost but $2-10, I'm suggesting that if you're going to the trouble to build a power supply yourself, it would benefit you more to build one that can supply higher than 15V, even though 15V will run a CMOY.

 Given that CMOY is so low current, you could just put a resistor in series on the transformer output and it would probably do fine, presuming you had a few hundred or more uF capacitor between the transformer and regulator. A couple diodes would drop the voltage some too.

 Personally, I don't try to hit only the absolute max value a part will tolerate even if that max is spec'd a little margin built-in. If a regulator were spec'd for 16.5V max, i'd try to hit lower than 16.0V, but that's just me? Also if/when any AC line surges occur, with this type of supply the output of the transformer reponds linearly to the surge and puts out the higher voltage to the regulator during this surge event.

 Anyway IMO, your regulator and transformer are better for another project than powering a CMOy even though you could make them work if necessary.


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## rickcr42

Quote:


 While this may technically work, it is only useful in trivial cases. Almost always, increasing a power supply impedance is a bad thing. 
 

Yes and no.If this higher impedance precedes a regualtor and the regulator has a low output impedance it is the regulator impedance that will be seen by the active circuit and not the resistor.
 there is also the argument _for_ some series resistance at the power pins of the active device which will raise the impedance seen but at the same time reduce RFI which in the end may be more of a determining factor on if or if not the stage actually sounds good.

 finally my triode amp has RCL "taps" to obtain three different voltages from the single main voltage and is not only good sounding method but damn near a universal method to obtain mukltiple voltages from a single power supply buss.Properly designed a resistor voltage divider will not be a detriment in this paricular case and way better than any alternatives for ease of implementing.


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## ]|[ GorE

Quote:


  Originally Posted by *mono* 
_15V would work for CMOY.

 I was thinking ahead, that if this CMOY is not your last project, if you ever wanted to reuse the power supply you made with a better amp, odds are pretty fair that you would then want the supply to be able to output higher than 15V for that OTHER/next amp.

 Since you don't need 2.5A capacity, and since a higher voltage wall-wart or transformer of suitable capacity for a headamp might not cost but $2-10, I'm suggesting that if you're going to the trouble to build a power supply yourself, it would benefit you more to build one that can supply higher than 15V, even though 15V will run a CMOY.

 Given that CMOY is so low current, you could just put a resistor in series on the transformer output and it would probably do fine, presuming you had a few hundred or more uF capacitor between the transformer and regulator. A couple diodes would drop the voltage some too.

 Personally, I don't try to hit only the absolute max value a part will tolerate even if that max is spec'd a little margin built-in. If a regulator were spec'd for 16.5V max, i'd try to hit lower than 16.0V, but that's just me? Also if/when any AC line surges occur, with this type of supply the output of the transformer reponds linearly to the surge and puts out the higher voltage to the regulator during this surge event.

 Anyway IMO, your regulator and transformer are better for another project than powering a CMOy even though you could make them work if necessary._

 

Appreciate the lengthy reply.So this capacitor you speak of : more than 100 uF
 ,it should be connected in parallel(thats between the + and ground of the Adapter output) right ?

 And would somewhere around a 5ohm resistor do ?


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## tangent

Quote:


  Originally Posted by *ATAT* 
_putting it to ground would act as smoothing ripple.. that'd be bad_

 

No, reducing ripple is *good*. This is part of the reason the power supply caps in the CMoy exist.

  Quote:


 Yes the cap has a resistance, it's theoretically infinite. 
 

If you mean the cap's equivalent series resistance then theoretically it's *zero*, and in practical caps it is some small value, rarely more than a few ohms, and frequently down in the 10's of mOhms, depending on the cap type and value.

 If you mean the cap's impedance, then it is frequency dependent. Theoretically, it is zero at infinite frequency, infinite at DC, and varying in between. That's why we use the term "impedance" -- it isn't resistance, exactly.

  Quote:


  Originally Posted by *rickcr42* 
_If this higher impedance precedes a regualtor and the regulator has a low output impedance it is the regulator impedance that will be seen by the active circuit and not the resistor._

 

Sure, but the resistor will increase pre-regulator ripple, which reduces the effectiveness of the regulator. 

 If the resistor is small, such as a few ohms as part of an RCL ripple reducing filter, that's fine. If the resistor is there to drop many volts, I still don't like it, unless the amp is operating in class A: it's going to create current-modulated rail ripple otherwise.

  Quote:


 there is also the argument for some series resistance at the power pins of the active device which will raise the impedance seen but at the same time reduce RFI 
 

Yes, this can be helpful, but it's better if the R is followed by a C (such as a bypass cap) so the impedance at the power pin remains low, yet you keep HF rejection.


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## ATAT

heh Tangent thanks for catching that.. I meant that's bad to the first part, not the second part.. reducing the ripple is a good thing =)

 Cliff notes:
 Parallel cap = Good, less ripple
 Series cap = Bad, no DC


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## rickcr42

Quote:


 If the resistor is small, such as a few ohms as part of an RCL ripple reducing filter, that's fine. If the resistor is there to drop many volts, I still don't like it, unless the amp is operating in class A: it's going to create current-modulated rail ripple otherwise. 
 

The only real harm in casting off a lot of volts is wasted power and generating heat so the resistor wattage needs to be up to the task of full time duty.This is actaully standard practice in even the ultra high end of audio gear and has really no bad effects if _before_ a regulator which like an audio stage has a high input impedance and low output Z (if well designed).
 The option of multiple regulators also asumes voltages in the range of the V-regs minus the 1.7 volts which if exessive will make for a short lived and noisy regulator so again would call for a simple voltage divider.Solid state devices dont like to be run perpetually hot and like with a high bias pure Class-A amplifier stage the devices will get noisy before their time and will have a far shortened life span unless the heat sinking is way beyond normal considerations.Lie 200 watt heat sinks for 10 watts power.

 Another way is LCR stages for each "tap" which are also lossy networks but here it gets way more tricky on the resistance/reactance/attenuation front and means actual listening and tweaking once the ballpark figure is reached.Not recommended for the blind build.

  Quote:


 Sure, but the resistor will increase pre-regulator ripple, which reduces the effectiveness of the regulator. 
 

meaningless in actual use.totally non audible and it is sonics that count.

  Quote:


 If the resistor is small, such as a few ohms as part of an RCL ripple reducing filter, that's fine. If the resistor is there to drop many volts, I still don't like it, unless the amp is operating in class A: it's going to create current-modulated rail ripple otherwise. 
 

May seem so but a tiny amount of ripple at the pins with an opamp nothing due to how opamps handle power sources.The elimination of a path for RFI IS audible,the other not.Most do not even know it is getting in and chalk up a "bright" sound to opamp sonics or cap selection while all along it is another thing easily remidied by totally isolating all paths for entry.

  Quote:


 Yes, this can be helpful, but it's better if the R is followed by a C (such as a bypass cap) so the impedance at the power pin remains low, yet you keep HF rejection. 
 

Pretty much where I go with all power inlets.Total CLR isolation.the RF bypass caps are a given and already there in any common sense design.The R and L choice.

 Some evils are more evil than others so with audio the trick is to limit the evils that effect sound and not worry abou the ones athat effect electronic theory.
 I am of the _opinion_ that even an amp limited to a 30hz-15khz range but done right will beat the pants of any wideband design head to head on the sound merits.Better to have greatness in small doses than mediocrity in full force.

 Again,just my personal opinion folks


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## tangent

Quote:


  Originally Posted by *rickcr42* 
_totally non audible_

 

Unless the amp is designed to protect against it (e.g. full dual mono) increased rail ripple will often show up as increased inter-channel crosstalk, which reduces the width of the soundstage. With speakers, you might indeed not notice. With headphones, it doesn't take much crosstalk to be obvious even with "swap the cables" type A-B listening tests.

  Quote:


 Some evils are more evil than others 
 

I agree. I'm not wildly against what you're saying. I'm just saying that what with all the effort we go to to decrease power supply impedance -- better regulators, fiddly bypass cap schemes, short and thick wiring -- adding a series resistance to the power supply seems like a last-resort kind of thing.


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## PinkFloyd

Quote:


  Originally Posted by *rickcr42* 
_totally non audible and it is sonics that count.
_

 

Rick makes a very good point there. Not everything that measures bad actually _sounds_ bad and in some cases things that don't measure up on paper can actually sound "better" to our ears and "measured" distortion can actually make the amp sound good...... there are no "absolutes" when it comes to measurements, just because they all add up doesn't mean you've automatically got a good sounding amp.


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## minhal110

hi i m new here...i have the same problem.i need to charge my lamp which requires 6v and 1.2A max charging current...i have a dc power supply of 12 V and 3A max current...how can i reduce it to 6V and 1.2 A current???
 thanx in advance


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## BrianDonegan

Quote:


 i have a 17V o/p AC-DC adapter 
 

Two simple solutions: 

 1) use a higher voltage opamp (LME49860 perhaps?)

 2) pick a different wall wart.


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