Using 2 wall warts for + and - supply

[wannabegeek]

Hi all,

I have a detection circuit that needs a positive and negative voltage supply.

I happen to have two wall warts, one 9 V and the other 15 V.

The circuit is forgiving with voltage and this combination will do IF, I understand how to use them !

I need the 15 V for a positive supply and the 9 V as the negative.

What is the standard way to connect them ?

My first thought is to hook them up in series. Then for the pos voltage, take a lead of the 15 V positive end and center tap. And then for neg, take a lead off of the neg end and center, calling the center the 'high' side.

Is this OK to do with random WW ?

15 V has I = 300 mA
9 V has I = 100 mA


TIA !

wbg

 

[daddles]

First, make sure they are DC output wall warts, not transformers. Then connect the - of wall wart 1 to the + output of wall wart 2. This becomes the reference potential in your circuit. Then the + output of wall wart 1 gives the positive voltage and the - output of wall wart 2 gives the negative voltage. This can be done because the wall warts have transformers in them to isolate them.

 

[wannabegeek]

Thank you daddles....that makes sense....I have learned from reading that this is done with transformers and that is what a center tap does... WW are dc output I check first off.

So now my ref voltage will be 6 V....not gonna work for my application....I am detecting a serial card output that swings from ~ -10 or -6 V to ~ 5V.

Everything is setup to work with a zero crossing...oh well, I'll look for another 15 volt WW......

 

[davelectronic]

dual voltage

Hi there.
Or build this and take the side you want down a couple of volts.

http://www.sentex.ca/~mec1995/circ/741-ps.html

 

[wannabegeek]

Hi Dave,

I might just build it....I have been collecting parts to do so for while...I need to find out the voltage and current/power ratings on the transformer I pulled out of an old stereo amp.

My scope is an ancient 2 MHz Tektronics N210 and it has issues....
so I'm not too keen on making a supply without a better scope.

I guess I could use my meter in RMS mode and figure out the peak to peak voltage.

What about a fuse ?

 

[davelectronic]

Detection circuit

Hi again.
I dont know the current you need for your circuit, but at a guess a good size stereo transformer should cover that easy, if wall warts do.
I dont own a scope yet, i will get one soon as most of the stuff i build is psu's the ripple out is going to be very very low, look at the regulators data sheets for the parameters.
Following the circuit should be fine. Ive used stereo transformers to power my CB radios before, look at what stereo it came from and you should be able to work out roughly the transformers current rating, a multi meter will do the rest.

Ive pulled windings off a stereo transformer to get the voltage i wanted, but taking into account the wire size as a clue to the current rating for a given voltage, higher the voltage generally lower the current, remove turns lowers voltage giving more current, ive not killed one yet. Dave. PS sorry yes fuse the transformers primary, and fuse the output before the circuit connection, and ground the transformers chassis or the case if its metal.

 

[daddles]

Feel free to build something, but I always recommend that folks head to a local second-hand store because you can pick up wall warts from 25 cents to a buck apiece. Even at $2 each, that's still a steal. I've done some characterization of wall warts and have found that some switching HP wall warts used with laptops were superbly regulated little power supplies (12 V @ 2.5 A); I wish I had a box full of them... This compares to no-name warts that have quite poor regulation. And you certainly can't tell this behavior from looking at the labels! BTW, it's interesting to stress the warts beyond their ratings and watch how things degrade on a scope (I have a commercial DC load and this makes it easy to do this kind of testing; if I didn't, I'd make one from an op amp and power MOSFET).

 

[wannabegeek]

Thanks Dave for the words of wisdom...!

I am gonna try to make the WW work since I think it's a cool solution that can be done with give away parts...and I'm excited to build my first real power supply with
your suggestions in mind.

cheers,
wbg

 

[wannabegeek]

@ daddles.... to make such a load...am I making a follower and using R=V/I to
make a really high impedance...?

I have some OP AMPs and an IRF 510 sitting around....my scope is hard to trigger
and is just a mess sometimes...if I get a job this summer I might have to buy a newer
'old' scope even if it's just 20 MHz.

 

[davelectronic]

wall wart power

Hi guys. yes go with the WW idea, iam all for bargains, some recycle, and like ideas of using stuff for other than its intended purpose. Plenty of wall warts on my travels to, and as you say can be very versitial. I build psu's as thats what iam into mainly linear at the moment, the MOT is one i want to try next, 1Kw of 12 to 13 volts of linear power for RF power amps, as the purchased units cost a fortune.

 

[davelectronic]

wall wart power

Dam phone only gives 512 charecters, ive pulled 30 amps from a moded atx with no problems, i love the idea of modding stuff, and re use recycle. Loads of goodies out there to play about with, ive seen an old battery charger in a flea market, iam after the tranformer its huge, but the price is to high, £ 25 quid or so, to much i think. I will watch see if price comes down, keep up the modding. Dave.

 

[Resqueline]

So now my ref voltage will be 6 V....not gonna work for my application....I am detecting a serial card output that swings from ~ -10 or -6 V to ~ 5V.

What do you mean 6V? You get -9V +15V = 24V - with the center tap at 9V.
In addition to all else said, be aware that most WW's give a considerably higher output when lightly loaded.

 

[wannabegeek]

I didn't write my post very well...I mean that the two WW
would have a common ground, other wise how else could I get
a negative voltage...

I guess daddles meant that too when he wrote that "we can do that because they are isolated..."...? Can't voltage sources be
used in series in general ....?

I am getting confused too...

And thanks for the warning, I did check my WW's with a meter first thing....

wbg

 

[Resqueline]

Yes, you connect them in series and connect the center tap/point to common "ground".
I just didn't get where you got the 6V figure from. End-to-end you get 24V.

Some laptop PSU's have mains ground connected through to the negative output.
You couldn't connect those in series, doing that would short out one of the supplies.

 

[wannabegeek]

@ Res

The reason you didn't get my numbers is that I was WAY-OFF in my thinking

I was drawing the circuit with and without the common earth ground and thinking of it as the same...I was SUBTRACTING because I was thinking in physics V=Delta V b/w
two points and I should sub and get the right sign.

I was using a model of two equal batteries in series and thinking there was 0 V at the center tap. Then, b/c I didn't always draw in the common earth ground, I was thinking
to sub to get the 0 V.....pretty noob mistake....

I just finished my BS in physics and I often fall into trouble with simple stuff since I studied all kinds of stuff a little, but not too much of one thing.

thanks everyone for your input !
wbg

 

[jackorocko]

Yes, you connect them in series and connect the center tap/point to common "ground".
I just didn't get where you got the 6V figure from. End-to-end you get 24V.

Some laptop PSU's have mains ground connected through to the negative output.
You couldn't connect those in series, doing that would short out one of the supplies.

couldn't you just cut one of the grounds and all would work ok? I mean if they are hooked in series then wouldn't the ground for one PSU work for both?

 

[Resqueline]

couldn't you just cut one of the grounds and all would work ok? I mean if they are hooked in series then wouldn't the ground for one PSU work for both?

Mmm, maybe, but it might not necessarily be good enough for full safety, & you might have to break open the case(s) and do some surgery inside, not very practical.
But I just picked it as an example, I'm sure there are other both easier & harder to solve cases one can encounter out there.

 

[wannabegeek]

I'm glad this thread is still alive, it means that this topic is not as simple as it appears.

For a CG, is this correct ?
To make a common ground connection: I am NOT using the negative terminal of either one of the WW, since they are floating.

I don't have an updated electrical outlet in my basement bench area with a third ground in the socket.

Therefore, I must take a wire from the earth ground copper rod in my garden, and run it in as short a path as possible to my circuit.

 

[Resqueline]

Don't confuse common "ground" (which is just the common supply wire from the junction between the series connected ww's) with mains ground..

 

[wannabegeek]

I think I got it now....thanks!