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How to reduce dc voltage ?

post #1 of 26
Thread Starter 
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 !
post #2 of 26
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.)
post #3 of 26
Thread Starter 
The AC-DC adapter has output current of 2.5A.

So V=IR

17=2.5R
Therefore,R=7 Ohms ?
post #4 of 26
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.
post #5 of 26
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
post #6 of 26
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.
post #7 of 26
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.
post #8 of 26
Thread Starter 
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 ?
post #9 of 26
Thread Starter 
Dont caps have a resistance too ? If i were to put a cap after the output ?
post #10 of 26
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
post #11 of 26
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.
post #12 of 26
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.
post #13 of 26
Thread Starter 
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 ?
post #14 of 26
Not much less, CMOY just isn't but a trivial load on a 2.5A transformer.
post #15 of 26
Thread Starter 
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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