18v out of a 19.5v laptop power brick

[alparent]

I have a 19.5v 3 amp laptop power brick giving out about 19.90v

Can I use a 7818 to get 18v or is 19.9 and 18 to close?
Is there a better way?

I'm making a power station that will give me 18v, 15v , 12v, a bunch of 9v, an adjustable 9v and a -9v.

[FastJunkie]

This is a really shitty way to do it and you probably shouldn't listen to me but I suppose you could get it down to the 18V using some rectifier diodes. Although the best way would be to obviosuly use a voltage regulator(s).

[R.G.]

You can do it. There are low drop out (LDO) regulators which can work with less than a volt of headroom.

But as a curiousity, do you *have* to have 18.000V? Would 17.5V work OK? 17.0? The LM317 is adjustable, flexible in application, and very cheap.

[Mark Hammer]

And RG's query, Alain, is because the regulator needs to see 2V more at its input than it presents at its output.  So an LM7818 regulator would not work properly with an input of less than 20vdc.

I have a spare 317 that came with some drawers full of regulators a friend gave to me.  I'll bring it in to work on Monday.  It even comes taped to the cardboard backing with the schematic Radio Shack had on the packaging! 

[alparent]

But as a curiousity, do you *have* to have 18.000V? Would 17.5V work OK? 17.0? The LM317 is adjustable, flexible in application, and very cheap.

You tell me! Your the experts! 

I want to make something like this http://www.muzique.com/power.htm

I've notice the 18v popping up in a couple of threads....so I figured it was a good value to have (but would the supplied 19.9v be as good?)
And are 15v , 12v even worth having? (I have the regulators for them..but If I don't need those values.....why bother?!)

Would 19.9v, 9v, -9v and adjustable 9v by more them good enough?

Thanks

[R.G.]

I believe 18V was used because that's the nominal value of two 9V batteries in series. Actually, a battery is some voltage between 9.4V (most fresh alkaline PP3's) and 7.0V, where most battery makers think they're exhausted.

I think that labeling a 17V or 17.5V output "18V" is no worse than what most advertisers do, and should do whatever you want to do with it.

...er, what DO you want to do with it? There's no real advantage in having a power output you don't use, and using on the wrong pedal could pop the internal caps.

Some commercial pedals have 10V rated electrolytics. We had to go to 25V rated caps because people insisted on applying 18V or more to pedals that were designed for 9V. It was simpler than telling people that they had fried their own pedals. We used to use 16V rated.

The internet is great for telling people "Hey, do this neato mod and your guitar will sound like God throwing flying saucers at Jupiter" or some such. I suppose that could be a form of self-paced education, but it seems like a drastic way to do it.

[alparent]

I just measured another brick I have.....I read 18.9v...close enough!

I've guest some effects like the electric mistress are 18v...so having 18.9v might be useful......if I ever need it?

But I think 15v and 12v wouldn't be that useful.

Thanks for all your input.

[DavenPaget]

I just measured another brick I have.....I read 18.9v...close enough!

I've guest some effects like the electric mistress are 18v...so having 18.9v might be useful......if I ever need it?

But I think 15v and 12v wouldn't be that useful.

Thanks for all your input.

Some digital effects have 12V . Remember to slap on a heatsink for each and every Vreg .

[R.G.]

As one other caution - laptop supplies are switching power supplies that are optimized to charge batteries. They may not (   ) be quiet and well behaved when powering high gain analog circuits without much power supply rejection capability. The majority of distortion pedals fall into this last category, unfortunately. Watch for whining and noise problems if you use them straight into an analog effect.

[alparent]

Watch for whining and noise problems if you use them straight into an analog effect.

What could I do to make it quiet and well behaved?
I'm I better to use a standard wall wart? Or would I have the same problems (or all new ones?)

Just found a wall wart marked 9.4v 400ma...but when I measure it I get 13v..?

Excuse my ignorance   and thanks for the help.

[R.G.]

What could I do to make it quiet and well behaved?

All power supplies which make DC from AC or use switching processes to make DC have some residual ripple. The classical answer it "filter it". That's good, and works, but filtering gets decidedly into black magic at high frequencies where a short wire has so much inductance that it makes for an inductor, a tuned circuit, and a radio antennal at the same time. Modern switching power supplies tend to run at 100kHz to 1MHz, and switching produces harmonics well into the many MHz, which work *fine* for radio transmission. So "filters" comes to mean "the entire layout and placement of everything that touches the power supply. This is why there is a ferrite lump on so many power cables these days. Ferrite tends to be dissipative to RF in many cases - it eats up RF as heat.

I'm I better to use a standard wall wart? Or would I have the same problems (or all new ones?)

Energy efficiency regulation has been effective in outlawing the low frequency wall wart. I'll try really hard not to get off into a rant about how stupid that is, but it's the way it is. Low frequency (50 and 60HZ) wall warts have the advantage that the ripple they make can be reduced with simple filters that do not need the black arts (that is, RF design procedures) to be designed and tested. But none of these are being made any more.

Switching type power supplies *can* be made reliable and quiet. The company I work for sells one that is. However, it requires constant effort to maintain quality and consistency. A generic one may or may not be quiet. If the basic switching frequency were the only issue, the ripple would be well above audio and not detectable. However, whenever there is a second RF source around, the two frequencies can cause the difference between them to appear as an audio whine.

Just found a wall wart marked 9.4v 400ma...but when I measure it I get 13v..?

Classic issue. It really means "this thing puts out 9.4V ONLY when it's loaded with 400ma; we made it 13V with no load so it would sag down to 9.4V under full load".

Excuse my ignorance and thanks for the help.

Ignorance is curable, and you're trying to cure it. That's worthy of compliments. What is bad is being ignorant and not trying to cure it. You're well past that. Ask questions. That's the way to learn.

[alparent]

Is there a way to test if my power source is stable and un-noisy?

I can try it on the couple of effects I've made.......but how do I really know?

Wouldn't want it to be a source of problems down the line.

[R.G.]

Is there a way to test if my power source is stable and un-noisy?
I can try it on the couple of effects I've made.......but how do I really know?
Wouldn't want it to be a source of problems down the line.

That's a problem. Yes, there are ways, but you are most unlikely to have the high frequency equipment to do these tests.

As a practical matter, trying it on a few effects and then keeping this in mind in case there is trouble is all you can realistically do without some lab equipment. As always, practicality rules. If you try it on a couple and it's OK, then use it. If you start getting mysterious noises at some point with a new effect, remember this may be the cause. At that point, you're back to where you are today, but you got to use the thing OK in the meantime.

[petey twofinger]

i was planning on building something like this for my work area as well . this post was exactly what i needed to read , thanks . but not sure i would need anything more than a 12 , a few 9 's a 6 and maybe a 4.5/5  and a 3 . i play with smaller toys and things too .

i ordered a few different 78xx v regs and built one circuit with a diode and 2 caps last night . is that 7809 limiting the current to 100 ma ? i read something like that , but i thought on the data sheet it said one amp , how much do they "put out" ?

i made a lil heat sink , about 3 mm thick aluminum , about one inch high , the width or the 7809 . is that too small ? i mounted it so it covered the entire meatl are an the back . i plan to run just a few pedals , and maybe my sustainer off it . do i need any of that white goop ? what is that stuff , lithium paste ? whats a good solution for easy to make cheep heat sinks ? the pdf said "if more than 300 ma cut 3 inches from the soda can " ! does blood conduct very well ?

i also got some smaller 5 volt vreg , i have to look at those too , they are small  .

i was thinking of running these 78xx off of sla 12 volt batts , even a deep cell , any issues with trying that ?

what about running a "multi Vreg box"  off of a power supply from a pc ? thats what i had planned to try for my "bench" .

i do have some older wall warts that are higher voltage , 14 plus volts 1 amp or more   , some lap top supplys too .

when looking through my boxes of wall warts i noticed "type 2" power supply on a few . what does that indicate ? whats a type one , two , three etc ?

my attempt at obtaining this knowledge is long over due . thank you .

[alparent]

Glad my ignorance couls serve others 

[petey twofinger]

my post was too long .

what i am most curious about is this , if i were making one of these units , a multi Vrg box , with 4 78XX in it , , what would be the wisest choice to power it ,

a an old pc power supply

b and old wall wart , or

c a lap top power supply

thanks .

[tempus]

I want to 2nd RG's suggestion - try it on some effects (regulated to 9v of course) and see if it's noisy or not. I recently tried to use laptop PS (they're so small and high current I couldn't resist) and found that, without any extra filtering, it was almost as quiet as my regular PS. The almost part was a tiny little hash sound (bad description I know) that I only noticed when I cranked my amp and didn't actually play my guitar. It was barely noticeable. I didn't end up using it in the end, partially because of the (albeit tiny and almost negligible) extra noise, but also because what I was really hoping to do was use it to power my analog (i.e., Boss) effects and my digital (Zoom) effects at the same time. For some reason the Zoom didn't work properly without its own dedicated supply.

So it may be dead quiet, may be quiet enough for your purposes, or may be way too noisy. The only way to find out is to try it.

[petey twofinger]

i plan to try a three but   , i have a painful permanent back injury , can only work for a little bit then take a day or two off . so i try and research a lot to keep my tinker time down . 

i over did it last week , this week end , so the fast few days have been agonizing . this type of "work" keeps me from going stir crazy so it really is healthy for me in the long run .

[tempus]

I see. I wasn't criticizing, I've just found (from my own experience) that in many cases, in the time I've posted questions and waited for answers on forums, I could've just tried a bunch of alternatives and got the answer myself. That being said, using a forum can be a huge time (and work) saver.

Again echoing RG's question, do you really need all the various voltages you're designing for? The 15 and 12 seem totally superfluous, although if you're designing your own stuff you could use them. The 18 might come in handy for some stuff, and the 5 for logic chips (although you'd use an internal regulator if it was for a regular pedal), the -9?, I don't know.

[petey twofinger]

i do need that many different voltages , cause this is not just for pedals , i do a lot of monkeying around with all sorts of crap , one of my multi voltage adapters died , i have repaired the other one at least 4 times , i figured i would just "go nuts" , and then have all my basses covered in my work area .

i am guessing i will end up with this smaller pc power supply i got , its kinda cute , really small , so i will start with that . i am thinking i will put a 2.1 mm connector for the input , then i could try a few different sources whenever i need something different .

about that - 9 volts arent there pnp pedals that need a "reverse voltage ? i am guessing i could take the -12 off the pc power suppluy and run that thre a "regular" voltage regulator and that would work , but .

really kinda confused about - voltages .