9VDC AND 18VDC Together from an adapter???

[Kleber AG]

Hey all  
I'd like to build a DC power supply with 9VDC AND 18VDC for simultaneosly use(at same time), say 9Vdc for a overdrive box and 18vdc for an EQ...

Is it possible with only one transformer?
Does that 78L09 series of regulators just go as high as the 78L12??? Or is there a 18Vdc version?

umhmhmhh... Lm317s???
How must I do that? Please!

Note: I can easy get (buy)a big variety of transformers from a factory near here, lets say 12V,24V etc... 300mA, 500mA, 1A...

Thanks
Kleber AG

[brett]

Simply use a multi-tapped transformer.  In this case I *think* it would be 24V AC and 12V AC (otherwise known as 24VCT for centre-tapped).  I'm no expert on the regulation system, but you could use a bridge, big cap and 7809 to regulate the 12V to 9V.  Another bridge, cap and 7809, referenced to the 9V line (like a pseudo-earth) would give you 18V from the 24V line.  There must be stuff about this on the web.

And don't forget that the primary side bites hard!

cheers.

[Kleber AG]

Thanks Brett!
I'd like to make it as small as possible, but maybe with 1A.

Would that kind of transformer be biger or smaller than a simple 24V transformer?

Looks like that exists 78XX for 7809, 7812, 7818, 7824...?...
BTW: What would be the differences between 7809 versus 78L09

Sorry I'm totally new to that...

Please, if someone have great experience with this things, could you point me to the best way of achieving that??? Reliable??? Size??? etc..

Thanks a lot for anymore info, please!  
Kleber AG

[SoundTech]

The 78xx series is typically in a TO-220 type package, and can handle around 1A of output current.  the 78Lxx series is typically in a TO-92 type package, and can handle around 100mA of output current, which is plenty for a single effect (for a few, actually), and is much smaller.  I orefer to use the larger voltage regulators, but I usually plan on powering a LOT of stuff with it.  You really don't need to use a multitap transformer, and it'll keep the project smaller if you don't.  Just get a nice 24v transformer, rectify the output, and parallel the DC+ to a 78L18 and a 78L09.  Make sure to do proper filtering to make it nice and quiet.

[niftydog]

center tapped ±12V transfomer.

ground the "negative" wire, then you'll have +12V and +24V to regulate.

78XX series regulators come in 5,6,8,9,10,11,12,15,18 and 24V

LM317s have all the info you need included in the data sheet which can be easily found by googling "LM317.pdf".

[Kleber AG]

 Thanks everyone!

Well, I "guess" I'll make it from a common 24V transformer (would it be smaller than the centre taped option) like SoundTech said:

Just get a nice 24v transformer, rectify the output, and parallel the DC+ to a 78L18 and a 78L09.

Thanks by the way...
Well but changing the 78L18 and 09 for the 7818 and 7809 as I'm gonna probably have around 1A for it (is it too much?)

Will have 110V/220V option swicht...
I thinks it's a good idea to "fuse" it too?
Also is there some way to protect it against DC output short-circuits? Like when it beats at the boxes chassis???

All of that is for my rig that sometimes gets too much used/abused on the road...

Thanks for anymore tips and hints and thoughts and whatever you think it's gonna help me
Peace
Kleber AG

[aron]

I've paralleled 2 voltage regulators to get 2 different voltages. It works well, but remember to use a heat sink if you are drawing a lot of current.

[brett]

A fuse is essential for safety.  0.25A should do.

[niftydog]

I'll make it from a common 24V transformer (would it be smaller than the centre taped option) like SoundTech said:

the difference would be pretty marginal between a "normal" and a centre tapped, I would think.  A multi-tap is a whole different story however.

7818 and 7809 as I'm gonna probably have around 1A for it (is it too much?)

They are current limited to 1A. At or near 1A you'll need some good heatsinking to keep them operating.

Also, consider this;

24VAC (transformer is probably rated in RMS volts) rectified will produce around 32VDC, depending on a few factors. Feed 32VDC into any 7809 regulator and it will try to dissipate 21VDC. Now, you're planning on drawing 1A or there abouts?

P = VI = 23 x 1 = 21 Watts!  :shock:

That's a lot of watts!

Even the 7818 will be cooking @ 12 Watts!

Also, 32VDC is pretty close to the maximum input spec for this device.

If you're going with this transfomer, a better option would be NOT to parallel the regulators, but to STACK them.  Using the +18VDC as a "ground" for the +9VDC regulator will mean it isn't working as hard. Unfortunately, this also means that you have two different ground potentials, and you can't afford to mix them up!  :shock:

If you want 9VDC from a regulator, the ideal situation is to be feeding it with 11VDC.  To get 11VDC from a transformer, you don't want too much more than 9VACrms. (14VACrms for 18VDC)

Since 9V transformers aren't exactly common, 12V is about the next best thing.  But 24V is just OTT.

Also is there some way to protect it against DC output short-circuits? Like when it beats at the boxes chassis???

the regulators provide just such protection.

Definately fuse it.  If not for protecting the power supply, for protecting your precious equipment!

[Kleber AG]

Thanks    Many more info to ponder...

Does that transformer "center tapped ±12V transfomer" is also called 12+12V If so I found it at the factory near here, and because of that power dissipation/wattage issue/problem may it be better to use the 12+12V

If so let me see if I really got it: 12+12V has 3 secondaries taps, one outer tap has +12V, the center tap 0V, and the other outer tap -12V ?
Then I have to ground the -12V and use the +12 outer tap that actually will provide 24V and the 0V center that actually will have 12V

Sorry if I'm confused, I know almost nothing about this relations... :oops:

I really apreciate all help!
Kleber AG

[Mark Hammer]

I'm not sure why this has gotten so complicated.

When you power a pedal with a 9v battery, sometimes you want a voltage for biasing purposes that is half the supply voltage.  So, you use a resistive voltage divider to chop the supply voltage in half, and stick a cap from 4.5V to ground to smooth out any line noise so that those things which use 9v don't influence those things that need 4.5v and vice versa.  One basic supply but two different voltages.

Same thing works when deriving multiple supply voltages from the same transformer.  The only real difference is that a DC voltage to serve as starting point has to be derived first.  Once it is rectified and smoothed out, though (usual methods with diodes and big caps), there is no reason why it cannot be "sent" to two places: one that regulates for a higher voltage, and one that regulates for a lower one.  As long as each regulated output has a nice-sized cap to decouple each output from the other, there should be no problem.

Three-pin regulators generally want at least 2V more coming in than going out or else they misbehave.  An 18v regulator will want 20vdc coming in to do its job right.  If such a regulator is fed the rectified output of a 15vAC transformer, you end up with a little over 21vdc, which is fine.

The Motorola MC7818 will give you up to 1A of regulation at 18vdc and easily accommodates input voltages up to 35v.  The same 21vdc can also be fed to a MC7809.

There is no realneed for a centre-tapped transformer.

[niftydog]

You are correct about the transformer, Kleber.

All of these ideas will work, but some aren't particularly suited to your requirements.

For instance; the last idea, using a voltage divider to create the 9VDC from a regulated 18VDC...

consider what you're asking of the resistors in the voltage divider circuit. You want them to drop 18VDC to 9VDC, so there's 9VDC accross each resistor. Now you want a 1A supply...

Power = 1A x 9VDC = 9 Watts. Got any 9W resistors lying around?!

Now consider this; the more current that flows through the resistor, the more voltage drop there is accross them.  So you really want a small value resistor, because at 1A the voltage drop will be equal to the value of the resistor. Got any 9W resistors less than 1 ohm lying around?

Now, you want to draw 1A from the +18VDC and 1A from the +9VDC? Well, a regulator is limited to 1A total.

For ease of design and efficiency, your best bet I believe is a centre tapped tranny.

BTW, if you want a 110/220 option, you'll probably need to buy an appropriate transformer with dual primarys. Do you really need this option?

[Mark Hammer]

Whoops.  I may have left some things a little too much in the air.  You are absolutely correct.  This is NOT a job for a resistive divider, and I should have specified that more clearly.  What does remainsimilar, though, is the notion of starting off with a higher DC voltage and "tapping" it in ways that yield lower voltages, which is essentially what a resistive divider does.  Of course, when the current in question is only a couple of milliamps at most, resistive dividers are MUCH more appropriate as a quick and dirty solution.

[Kleber AG]

"center tapped ±12V transfomer" is also called 12+12V

Tomorrow morning I'm gonna go buy that.

One last question:
May I buy a lower 9+9V transformer, as after rectified it would provide more that 18 or 9 Vdc??? Would it be safe with the regulators? Or must I stay with the 12+12 Vdc???

Thanks!!!!!!!!!!!!!!!
Kleber AG

[niftydog]

after rectification, a 9VAC transformer will provide somewhere around 11VDC, which is JUST enough for a regulator to regulate. But, a ±12V is probably easier to find in the shops.

[Kleber AG]

Man, I got all the parts but for some hours now I can't be sure how to wire it  :oops:

Could someone draw or describe step by step that?

I have a 12+12 transf. a bridge rect, caps, fuse, box, wires...

Thanks
Kleber

[Kleber AG]

ooops, also I got the 7809 and 7818

[Kleber AG]

Is it right?
Can I use the same ground (points A/B) for both 18 and 9 vdc? Puting them together? Or should I have 2 separated grounds?


Thanks
Kleber AG

[niftydog]

Hope you can follow this diagram! Warning, it is untested... but it should work, assuming I haven't made a boo boo.  :shock:

Basically, you wire the + and the centre tap transformer terminals into a bridge each. Then, ground the - terminal. Now, you effectively create two independent single voltage supplies.  Large electrolytics for filtering, smaller caps (as shown in the datasheet for the 78XX series regulators) to prevent unwanted spurious signals.

Code Select Expand

                                  ____
       .-----.                   |    |
-. ,---| ~/  |-o---o-------------|7818|-o---+18VDC
 )|( o-| / = |-o   | +           |____| |
 )|( | '-----' |  ###              |   ---
 )|( |         |  ---    ____      |   ---
 )|( | .-----. |   |    |    |     |    |
 )|(-)-| ~/  |-)---o----|7809|--o--)----)---+9VDC
 )|( o-| / = |-o   | +  |____|  |  |    |
 )|( | '-----' |  ###     |    --- |    |
 )|( |         |  ---     |    --- |    |
 )|( |         |   |      |     |  |    |
-' '-o         o --o------o-----o--o----o---o
                                            |
                                           ===
                                           GND

[niftydog]

looks like you got it.  Use the same ground point for both.

Oh, and fuse in the primary side active line. be careful around here, only touch it when it's completely disconnected from the mains!