# Single power supply for multiple devices

Thread Starter

#### deefactorial

Joined Jun 11, 2008
33
I have several devices that I would like to have one power supply for.

The various device power requirements are:
Device Number 1: 12V @ 4A
Device Number 2: 5v-30v @ 500ma
Device Number 3: 5v-30v @ 500ma
Device Number 4: 12v @ 40ma
Device Number 5: 3.6v-5.0v @ 22ma

I was thinking that I would be able to get a wall mount power supply 12V @ 4-5A, then make a simple little circuit to regulate the voltage for device number 4 and 5.

I noticed that there are some AC to DC converters on Digikey with multiple outputs but I was not able to find one with the right power profile.
this one is the closest I could find.
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=393-1040-ND

How many Amps should my power supply be ?
What components for the little circuit would do the job the best,
I'm thinking a diode, cap, voltage regulator, cap would do the job.

Thanks for the expertise.
Dominique

#### studiot

Joined Nov 9, 2007
4,998
You need to add up the maximum voltage multiplied by the current for each use.
This will give you the total power requirement. Multiply this by 1.2 for a safety margin.

1) 12 x 4 = 48 watts
2) 30 x 0.5 =15 watts
3) 30 x 0.5 = 15 watts
4) 12 x .04 = 0.48 watts
5) 5 x 0.022 = 0.1 watts

Total 78.6 watts.

This will be comfortably covered by a 100 watt transformer 9-0-9 volts ac centre tapped or 15 volts ac secondary.

All the supplies can be derived by rectifying and smoothing this.

Thread Starter

#### deefactorial

Joined Jun 11, 2008
33
the current power supplies for device 2 and 3 are 6V @ 1A so they would take 6 Watts. So the total would be 60.58 watts.

I can't find any power supplies with +9v, 0v and -9v if that is what you mean.

I've found a couple Wall and Desktop Transformers AC DC 12v and 9v power supplies greater than 72 watts.

would one of those do ? ie:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=EPS337-ND
or
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=271-2436-ND

Also how would I go about rectifying the voltage while maintaining a ripple voltage less than that needed for the low amperage devices?

#### studiot

Joined Nov 9, 2007
4,998
I did say you need to size your supply for the maximum demand.

This means the maximum voltage times the current.
You stated you wanted two 30 volts (max) half amp supplies.
that is 15 watts each

That's the laws of physics I'm afraid.

I suggested a 9-0-9 volt transformer, not a +9 -9 dc supply.
Using a centre tapped transformer like this means you only need two rectifier diodes, although you will notice the voltage needed is slightly higher than with a bridge rectifier and a single secondary.

Thread Starter

#### deefactorial

Joined Jun 11, 2008
33
I'm sorry if you miss understood me. What I meant was that it can accept anything from 5v to 30v, The power supply that came with the device is 6v 1 amp, why would the supplier send a power supply that is under rated.

I don't understand why I would try and make a power supply from scratch when there already exists power supplies that I can modify for my needs.

I had a look a the AC center trapped transformers;
how do you get one that will be able to accept 120v and 240v, north American and European AC.

So what I hear that your are suggesting that I do is create a AC to DC converter by taking an AC transformer, Rectifying the AC to DC using two Diodes or a bridge rectifier. Adding various size Caps, and voltage regulators to get the desired voltage and amperages for the various devices.

My theory is that If I use a wall mount AC to DC power supply rated for 12v, that it would be able to supply the power for at least three of the devices right out of the box, then it would be just a matter of regulating the voltage for the other two devices.

#### Wendy

Joined Mar 24, 2008
23,396
Sounds like a PC power supply would work, if you can live with the ripple of a switching power supply.

Thread Starter

#### deefactorial

Joined Jun 11, 2008
33
PC power supplies are typically rated 300 - 500 watts, which is a little overkill for this project.

I was thinking that something like a laptop brick 80 - 100 watts would be better suited.

#### studiot

Joined Nov 9, 2007
4,998
Yes I did misunderstand that the supplies could be anything from 5 to 30 volts. Since 12 volts falls within this range, no further conditioning is necessary.

That takes care of 4 of your 5 supply needs 12 volts.

A small 5 volt regulator will certainly satisfy the last one.

I'm thinking a diode, cap, voltage regulator, cap would do the job.
I certainly read this statement to mean that you were contemplating 'rolling your own' ans sized accordingly.

However you can certainly obtain 12 volt supplies in the range 5 to 7.5 amps which is what you need. Many auto battery chargers run up to 6 amps which would be ideal. they would also give you a safety margin but would require smoothing capacitors. You can also get wallwart switched mode supplies in this range, although they are rather costly, they are alreasy smoothed and regulated.

Thread Starter

#### deefactorial

Joined Jun 11, 2008
33
Thank you Studiot for your reply, it affirmed that I was on the right track.

Do you think that I would need to regulate the ripple voltage for the 12v 40ma device?
The input voltage for that device can be anywhere between 833 Vdc.

I was thinking of ordering this wallwart:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=EPS337-ND

the ripple voltage is +-1%

correct me if I'm wrong the calculation for the ripple voltage would be
6A * 0.01 = 0.06A
+-60ma
= 120ma ripple voltage.
what size of cap would I need to smooth this out?

#### jpanhalt

Joined Jan 18, 2008
11,087
The DigiKey item isn't going to give you your 5V. Have you looked at Jameco? It has lots of 5V/12V power supples for far less than the DigiKey one.

John

Thread Starter

#### deefactorial

Joined Jun 11, 2008
33
I had a look on Jameco for some power supplies and the closest I could find was one that had three outputs one 5v 3A and two 12v 3.4A

The min quantity is 10, so the price is higher than that of digikey.
I only need one.

I find it really hard to search the Jameco website. Is there a specific power supply you had in mind ?

#### jpanhalt

Joined Jan 18, 2008
11,087
Jameco is not my first choice for anything, and their web site is hard to use. Nevertheless, it has a good selection of power supplies at prices that are generally less than DigiKey. When I go to Jameco, I use its paper catalog, then check on line if it is in stock. I had no specific supply in mind. For years, I used a wall wart with +/- 12V and +5 volt from them for my bench supply that was very cheap.

John

#### Wendy

Joined Mar 24, 2008
23,396
Just a suggestion, before you spend the money, compare the prices between the computer power supply and the brick. A computer unit may be overkill, but price should also be a factor. Besides, overkill can be better sometimes.

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Thread Starter

#### deefactorial

Joined Jun 11, 2008
33
I'm finding the Jameco website to be impossible to find what I'm looking for.
I have not been able to find a wallwart power supply with 12v 6A and 5v 22ma.

I think I'm going to try the digikey wallwart 12v 6A supply, going into a bread board with Analog Devices 5v 250ma voltage regulator. And a 10uF Aluminum Electrolytic cap after the voltage regulator. Then I would solder the appropriate connectors for the various supplies. Then put it in a plastic box with all the cables going out of it.

Here is the voltage regulator I found.
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=ADP667ANZ-ND

I'm curious if I should add any capacitance before or after the voltage regulator. And If I should add a capacitor before the 12v 70ma device.

In terms of application; saving power is more important that saving money on the purchase of a power supply because this product will be running on solar or a generator. And in the long term saving power does save money.