Converting PC PSU to Bench supply

[Pete B]

I'm in the process of converting a 300W Liteon ATX power supply and have spent many hours absorbing 'How-to's' from all over the Web and largely have an understanding of what I am doing.

Whilst I am settled on a 12V supply using the yellow wire (x3) and a 5V supply using red (x1), the bit I am stuck on is the rating of the permanent load on the 5V rail.

I understand that I need to install one or more resistors. I have heatsink compound to improve the heat transfer to the casing, so I think I am ready to go. A red and a black wire have been set aside for this.

Opinions vary, depending on which account one reads, as to which value of resistor is used. As a result I have the following available:

2 x 10W 1R;

5 x 5W 10R

I note that some installations use just one resistor, others use them in series or parallel

It is 40 years + since I scraped a Physics 'O' Level and I cannot for the life of me correlate the figures to understand what is happening in the resistor

If I=V/R, using the 1R resistor, then I = 5/1 = 5A

Using the 10R, 5/10 = 0.5A

What relevance does the figure of 10W or 5W have for each resistor? Am I just one calculation away from a simple answer?

Would anyone be kind enough to gently take me through the calculations, preferably using words of one syllable or less, please?

Pete

[John Privett]

OK, let's have a stab at this one...

First off, on my ex-PC power supply I have a car stop/tail bulb connected to the 5v supply and it seems to do the job well enough. From my reading at the time, the power supply just needs some sort of load on the 5v in order to function and to give a decently regulated supply on the 12v line. I don't think I ever saw an answer as to how much of a load was required on the 5v line - it seems fairly non-critical.

I think in my case it's the tail lamp filament that's actually connected - the bulb would have been one I replaced on my car, and the brake filament usually goes first. As well as giving a load on the 5v line, the bulb glows dimly - a bit like a glow plug but a bit brighter - and if nothing else it reminds me that the power supply is on.

Now then, getting on to your resistors... As you rightly say, using Ohms law (V=IR) you can work out what current will flow through a resistor given the voltage across it. (Or in a different situation, if you knew the current flowing through it you could calculate the voltage dropped across it.) So, as you said, sticking a 1ohm resistor across a 5v supply would result in a current of 5A flowing. (Gut feeling already is that's probably a bit higher than you want/need.) Using a 10ohm resistor instead will give a 0.5A current. My guess is that the 0.5A current should probably do the job.

But what about the wattage of the resistor? Well that's an indication of how much power the resistor can handle without coming to any harm (ie. overheating!) Multiply volts x amps to get watts, so your 1 ohm resistor drawing 5A across 5v will be dissipating 25W. If you use a 5W or 10W resistor then it's going to get rather hot very quickly and you'll detect a sort of "burning" aroma.... On the other hand the 10 ohm resistor is only dissipating 2.5W - much more like it, though it's still likely to get a bit warm.

So what about putting resistors in series or parallel? Putting two resistors in parallel is going to give a total resistance less than either one on its own. The formula is (and apologies, it;s awkward to write formulae without having subscripts available in the character-set);

1/Rt = 1/R1 + 1/R2

where Rt is the total effective resistance, R1 is the resistance of one resistor and R2 is the resistance of the other. (This just continues for 3, 4, or more resistors, so 1/Rt = 1/R1 + 1/R2 + 1/R3 + 1/R4...... etc.) Plugging some values in, then using two 10ohm resistors gives;

1/Rt = 1/10 + 1/10. So Rt = 5 ohms

Anyway, I could go on (and on, and on, and on...) but that's probably more than enough for now! I'd try a single 10ohm one for starters, if you have problems with it then may be worth trying 2 of them in parallel - that would give 1A load on the 5v line.

[John Privett]

I might just add that if you put resistors on series then you just add up the resistance. So two 10 ohm resistors in series in a resistance of 20 ohms.

[Biggles' Elder Brother - Moderator]

Great post John, very well explained. I was waiting for how you were going to explain resistors in parallel - but you got there nicely!

BEB

[Pete B]

Many, many thanks, John, I now know what to do and understand how you reach that point but, TBH, when it comes to the algebra and maths involved in working out the reciprocals, my brain starts feeling soggy.........

However, a bit more research has come up with this phrase:

If a number of identical parallel resistors are connected, the total resistance will be the resistor value multiplied by the reciprocal of the number of resistors.

I can see that, in this case, the figures are 10 x 1/2, which makes 5.........

You wouldn't believe how long it took me to get my head around that!

Anyone point me towards an idiots guide to maths???

Pete

[John Privett]

You're welcome Pete! I don't know of any idiots guide to maths sites, and it's very nearly 40 years (actually 'only' 38 years) since my O level physics too, though I did go on and do a degree in the subject...

Even so, that's longer ago than I care to think! My younger daughter is doing A level physics and it's been an "interesting" experience trying to get back up to speed with it to help explain some of the concepts she hasn't quite grasped at school! Her elder sister breezed through it (and maths) without any help from me and is now off at uni studying engineering.

[Pete B]

Hmm, these genes are strange things. I have a 12-year-old grandson who is gifted at maths - I can take no credit whatsoever for that part of his DNA!

Pete

[Pete B]

Just an update - it works.....

John's helpful response gave me the confidence to start snipping and soldering today and I fitted just the one 10 ohm 5W resistor, which became reasonably warm. This gave a supply of 11.98V.

Having read that the voltage in the 12V circuit can be increased by adding resistors to the 5V circuit (now that has me scratching my head), I tried it with 2 x 10 ohm, which gave me two rather hotter resistors and 12.02V.

Adding a third, it all became hot rather quickly and raised the voltage to 12.05V.

I also tried the 1 ohm 10W resistor which became very hot, so I gave up with that before testing the voltage.

All measurements are with a non-calibrated inexpensive DVM so shouldn't be taken as highly accurate but 11.98V should be fine for the charger. I don't think the gains from adding resistors are worth the extra heat generated.

The single 10 ohm resistor has now been fixed to the body of the PS with thermal compound, in the draught from the fan, so cooling shouldn't be an issue.

I've now assembled and tested the PS and all seems to be working well. I paid €3 for the PS from a boot sale and the components came to about £4, so for under £7 for a potential 20+A power supply (not that I'll ever need that much), it's been a worthwhile exercise

Pete

[John Privett]

Posted by Pete B on 23/05/2012 23:09:00:

I also tried the 1 ohm 10W resistor which became very hot, so I gave up with that before testing the voltage.

Told you so! That would have been dissipating 25W. I have a couple of soldering irons that run on less than that!

Anyway, great to hear you've got it working - thanks for letting us know.

[Antiquated]

One thing you need to check is the wattage allowed on the 12V rail. Although the PSU is described as "300W", that is the total load allowed.

Many PC PSUs have much lower limits on individual rails (5V, 12V etc) than the advertised limit. This will partly depend on the version of the ATX standard used by the PSU, as well (at one point the 12V rail was legacy only, so limited to 50W or less, now it seems to have become a main rail again). If you are lucky there will be a label on the side that lists the maximum load per rail.

Edit: a quick googling seems to reveal that 5-6 year old Liteon 300W PSUs allow between 15-19A on the 12V rails, so I'm not sure that you will be able to manage 20+ amps.

Edited By Antiquated on 24/05/2012 09:23:54

[Ben B]

Your output wattage may be limited by the output only being 12v. Hopefully not but usually to get high power outputs you need a higher output voltage.

[Pete B]

Thanks for that, A. The +12V rail max is 19A, +5V is 25A. As I have only a 50W charger at present, I'll not be coming close to the limit just yet.

My existing power supply, a universal laptop supply, gives a max 67W at 15V, which has been adequate for my Accucell 6.

I'm considering getting a 200W charger, so I've fettled this in anticipation.

Pete

Just seen your post, Ben. I have time on my side so getting the maximum charging rate isn't the main criteria. It's been an interesting exercise to get iiut working and give me a more powerful PS than I have at present. If I run it at something less than its full capability, I'll be happy with that  

Edited By Pete B on 24/05/2012 09:38:17

[Olly P]

Must look at the one in the basement....

[RichFR]

Just a bit of info for anybody else looking to do this I used a fairly modern 530w ATX PSU and I only connected the 12cm built in fan up to the +5v and it works like a charm I didn't need to add a load and loose some efficiency.
Its worth trying it first

[crispin church]

i never put any resistor inline only connect the single wire green i think to a switch and other side to black to turn on and use yellows for 12v to black

[Pete B]

Mine's still going strong but I subsequently modded an Xbox power supply, which looks a bit smarter on the kitchen worktop....

Pete

[John Privett]

Indeed, RichFR, I do believe that more recent PC power supplies don't need a load on the 5v line, though I've never tested that theory myself!

And Pete, I also now have a modded Xbox PSU - good for 16.5A apparently on the 12v line, certainly enough for my Quattro B6 charger to charge 4 x 2600mAh LiPos simultaneously.

My old PC power supply struggled to power the single B6 without triggering the low input voltage warning when I started charging 2600s rather than 2200s. Though replacing the car light bulb loading the 5v line with a higher wattage bulb improved things until I got the Quattro charger - it would never have driven that!

[RichFR]

Mine should be good for 25-30A using two 18A 12v rails.

Its a black large fan PSU, I have binned the wires and use banana plugs on the side it looks smart.

[YakMad]

I did the same as you John and modded an old first gen Xbox 360 PSU early last year which I have to say was fairly easy to do and its been working great ever since, so much so that I have been keeping a look-out for a second one. Not sure about the 16.5 amps, the label on mine says 14 amp although I have never tested it. Just a note to anyone who want to do the same, make sure that its the first generation Xbox 360 PSU as I believe the later models output less amps.

[TigerOC]

I have just built my first foam cutter and looked at this route. I eventually decided it was easier to use a laptop charger. I had an IBM unit from an old laptop lying around. Puts out 18V @ 3A. No fiddling with it. The only hassle was the cord which was a centre core with gauze outer carrying the megative. Just stripped it back and worked it into a wire and soldered both to a 2 core cable. Drives a 80cm bow with problems.

Rob

[John Privett]

Posted by TigerOC on 18/02/2014 22:24:45:

Drives a 80cm bow with problems.

Rob

Rob, I suspect you meant to say, "Drives a 80cm bow with no problems."?

[Phil Green]

These are not worth not having on a homebrew psu (if you catch my drift)

£1.37 post free!

[TigerOC]

Posted by John Privett on 18/02/2014 23:33:15:

Posted by TigerOC on 18/02/2014 22:24:45:

Drives a 80cm bow with problems.

Rob

Rob, I suspect you meant to say, "Drives a 80cm bow with no problems."?

Correct; no edit allowed now

[TigerOC]

As an additional item for this thread; making a bench power supply for either hot wire or lipo charger or other use.

I was also researching this and came upon a ready made and very cheap solution. This is a breakout board;

**LINK**

and you also need the atx connector;

**LINK**

which you solder to the board. I bought some tin plate from the local model train shop and made a little cover.

I cut the all the wires other than the main atx feed in the box. (be very careful inside this box there are some vey big capacitors that are capable of a lethal shock). I drilled holes for 4 m3 bolts to mount the breakout card on top. I then drilled holes for female 4mm banana sockets and put them in the end of the tin cover.

[Ady Hayward]

Hi,

Here is the link to a PSU conversion I did back in May 2012.**LINK**

I'm glad to say it is still working well and the 5v external power line has proven to be very useful indeed when needing to have a system powered up for a long time.

The dual Volts+ Amps displays are useful (Plus add a little bling), especially if using the external 5v as it shows the current draw and highlights a binding servo very nicely.

The only issue I have found is that of charging my 10s 5aH Lipos where only one can be done on its own as it takes all that the 500W charger can give.

I plan to build another using a 1000W supply as these are now getting more popular and are becoming more affordable.

Adrian