Modded PC power supply for charging
 

People have asked how to use a relatively cheap 12v power supply for their battery charger (Hyperion 7i, Multiplex, etc) in the past, so I thought I'd make a little "how-to". Many battery chargers require a 12v DC input to power them up. A car battery works, but it's nice to have something that can be plugged into the wall. The power supply in a computer can be used as a 12v power supply if it has enough current on the 12v line, and is an ATX style (which is in all computers for at least the last 5 years or so). Older power supplies have a 12v line but are not as robust.

You should be able to find a computer power supply that can deliver around 15A @ 12v for about $40. Then it takes a little modding to convert it.

WARNING: The internals of a PC supply contains hazardous voltages in the range of 160-180V DC. Make sure power cords are disconnected and let the unit sit for about 10 minutes to discharge the capacitors before you open the case.

1. First, you have to figure out how much 12v current you'll need. The Hyperion 7i charger requires up to 150 watts. This equates to about 13A @ 12v. The Thunder Power 1010 requires up to 350W, which is about 30A @ 12v. It might be tough to find a generic computer power supply capable of 30A, so then you might want to stick with a heavy duty supply for best results. Note: These figures reflect charger efficiency losses and a safety margin.
   
   
2. Next, you have to make sure the supply will turn on like it should. Plug the supply in and turn on the switch in the back (if there is one). Take a piece of wire and short the green wire to any black wire. These are located in the large motherboard connector. If the internal fan turns on, you are all set. If not, this particular model probably needs one or more of the voltage supplies (either the 12v, 3.3v, or 5v) loaded somewhat for the circuitry to work. You can still use this, but you'll have to use a loading resistor or something, and that is beyond the scope of this post. This is not really common nowadays so you should be fine. Also, many current power supplies actually contain two 12v supplies in one unit and you shouldn't simply tie these together, and you have to make sure at least one of them can supply 15A+ by itself.
   
   
3. A picture is worth 1,000 words, so:
   
   Before
   http://scriptasylum.com/forumpics/ps1.jpg
   
   After
   http://scriptasylum.com/forumpics/ps2.jpg
   
   Actually, these are different supplies, but they looked similar when I started. You get the idea. I got fancy and shortened unecessary wires, re-braided the wires I wanted, and then added a Deans connector for each set of wires. I also left the 5v (red) and 3.3v (orange) supplies usable in case I wanted to use them, but all that's needed are the 12v (yellow) and ground (black) wires. All you really have to do is connect the green wire (in the large motherboard connector) to any black wire to turn the supply on.

Some power supplies need a resistance to start up. I made one in which i used a resistor that took about 4 watts from the 12V

You are correct, but I did make a note of that in step 2. ;) I have also had to add a loading resistor to an older PS as you explained, but I felt that choosing the appropriate resistor size to provide enough load was beyond a "simple mod" for some people. The resistor has to have an appropriate resistance value along with the right power rating and I figured the formulas would be too much. Plus, it might be a pain to figure out which supply rail needs this load. I have seen supplies that need only the 3.3v line for the rest of the supplies (5v and 12v) to work. However, I can go into that if anyone thinks it will be helpful.

I took am aluminum case 15 or 20W resistor, which i mounted inside the case, i don't remember what value i took, it think it was a 22 Ohm, which stayed cool enough in the airflow of the PS. (6.5W wasted energy, small price to pay for a cheap PS.. :) It's not really needed to get the formula, eversince it's about 12 - 13V that's used.

You can determine if you need a resistor by measuring the voltage. if the voltage differs much from unloaded/loaded, a resistor might be needed. In some cases the charger doesn't want to start if the voltage is too low.

Agreed. I just don't want people who really don't know about that kind of thing at all to slap a resistor with a resistance value too high or too low and wonder why it's not working, or think the supply is bad when/if the resistor blows, or the supply shuts down for too much current. You obviously knew what you were doing.

Measuring the unloaded/loaded voltage can work, but if a load is needed, usually only one power rail contains the regulation feedback coil. It depends on the design of the PS for which rail this is.

BTW: I'm not trying to argue with you, I am merely explaining this for the sake of less electrically-minded forum members out there. :)

Could you put a small light bulb on the wire for a resistor?
Then you could have a detection of where or not it is on as well.
Just a thought from a non electrically forum member. ;)

Lol. Yeah, that could work as long as the bulb was not run over or under its rated voltage.

FYI: A bulb's filament has close to 0 ohms of resistance when off. Only as it heats up and glows does the resistance go up - rudimentary current-limiting. This, incidentally, is why a bulb almost always blows when you first turn it on. So if you use a 12v bulb on the 5v line, the bulb may glow weakly or not at all and you'll probably activate the protection circuitry of the PS or blow the bulb.

I have used light bulbs on the 5v output on a couple P.S. before. I used the 1156 or 1157 auto bulbs. I recall having two bulbs on one of them. More load kept the output voltage higher.

OK, if i mail this thing to someone!!! LOL! I'm really showing my electrical stupidity. this is way too confusing!
Ok, so the green wire, there is only one, Got that part, but which ground? My supply is much different! The volt meter, where so i hook that to test how much is going thru? And is this while you are charging? Do i cut the green wire before i plug it in then touch the grounds? It looks like you have yellow and black? Ornage and black? Red and black? what are these for?

You just gave me a headache.... :035:

I'm no electrical wizard either. :005:

For what it's worth, the ones I used a while back I just hooked up the load (1157 bulbs) on the 5v side and it turns on when I plug it in.

Then use the 12v outputs to hook up to the charger...

That's all I did unless I have forgotten everything I once knew....

Here is what i ahve coming out the back=
1yellow
1red
2black=3 of these leads

2yellow
2black=one of these leads

1yellow
1red
1orange
2black=one of these leads
And one lead which looks like a fan lead


And a big harness that has the green wire. I'm assuming yellow is the 12v supply since orange is 3.3v, red is 5v, yellow i can't see.

Neweuser, I just sent you a response PM.

And just for others' benefit, the PM went like this:

Thanks Brian G! I think I got this down a tad bit. i sent you a reply by the way. I'm not getting enough volts thru to the charger, nor to the battery packs....So i'll need a resistor. Now you too much or not enough? Also, There is no way in hexx i can get 7 wires together on one deans! Any solutions? And why do you have deans on the 3.3v and the 5v, what do you use this for? charging lesser v batteries?

You should be able to get 7 wires there. Just strip them individually and then twist the ends together tightly. Then tin the end so it stays together. Cut the very tip so the tinned part ends nice and flat. Now solder it to the Deans.

When you have the whole thing together, charge your biggest pack at the highest safe recommended current. This will put the highest strain on the power supply. If all is well, the charger will simply charge the batteries with no issues. However, if the charger shuts off, or displays some kind of error message along the lines of "input voltage is low", then a little more modding is necessary, but not difficult or expensive. To make sure this is the case, get a voltage measurement of the 12v output of the power supply while the charger is running hard. Chances are that the 12v line will be near or below 11v.

As others have said previously, resistors can be added to the other unused lines to cause the regulator to work better, thereby making the 12v line more stable even under heavy load. As far as what resistor to use: RadioShack carries two resistor values that will work well while drawing the right amount of current. A 10 Ohm, 10W, and a 1 ohm, 10W.

For the 5v line, you have two ideal choices using these RadioShack resistors.

1) You can put two 10 ohm resistors in parallel for a 5 ohm total load. This will draw 1A from the 5v line and dissipate 5W. Using two 10 Watt resistors should be perfectly fine and should get just a tiny bit warm.

2) You can put two 1 ohm resistors in series for a 2 ohm load. This will draw 2.5A from the 5v line and dissipate 12.5 watts. Using two 10 Watt resistors should be ok, but they will most likely get quite warm after a while. However, since this option draws more current, the regulator might be a little more accurate and cause the 12v line to stay nearer to 12v.

For the 3.3v line, you can put two 1 ohm resistors in series for a 2 ohm load. This will draw 1.65A from the 3.3v line and dissipate around 5.5 watts. Using two 10W resistors should be perfectly fine and should get just a tiny bit warm.

These arrangements work well since those resistors come in packs of two. Refer to the diagram for examples of these configurations.

The idea in both cases is to draw between 1A and 3A to properly load each output. 2A to 3A is better, but may be difficult to find cheap resistors that will dissipate the power. While easily done, you have to make sure that you do not exceed the power ratings of the resistors or they will burn up.

Formulas:

resistance = output_voltage / desired_current
resistor_power = (output_voltage * desired current) * 2

Other places to get power resistors: Parts Express, All Electronics, etc.

Another note for drooping power supply voltage for users of the Hyperion 7i: You may have seen this thread already, but it might worth checking out.

Do you recommend doing both the 5v and 3.3? Or just one or the other with the options you gave ?
The other question i have, is the parallel diagram, do you put two resistors one one red(any) wire, and two on one black(any) wire? Just want to make sure. I was thinking of just removing part of the covering instead of the whole end. Also, do you just connect them to the ends and then from the resister, they go to the board? Nothing on the end but the resistor? is that right?

Could somebody help me? Not sure if this thing will work, but I guess it might. Heres a picture of the PS sticker.

http://img216.imageshack.us/img216/8...tureyl8.th.jpg

I think I had a brain fart. Oops..I mean a brian fart :p. Anyway, I don't know what I was thinking...Theres no way in hell that thing would work...duh. Shoulda known that...Anywho, I found alot of PS's on Ebay for $30 that are rated at 24 amps at 12 volts (600 watts).

Holy cow, that's a small PS!! Only 2A @ 12v is only ~24w. I don't think it'll work, unless you don't mind charging at 0.1A or 0.2A. :)

Neweuser: It's hard to tell which output has the regulation feedback circuit. Maybe just one, maybe all three. I think it might vary by manufacturer and model. So, I would just add the load to both the 5v and 3.3. There will be a little wasted power from those resistors, but not a whole lot.

As to your other question; I'm not sure I understand. As the diagram illustrates, the 5v or 3.3v wire hooks to the resistors in the manner shown. I tried to make the actual connections quite clear. Series hooks the resistors end to end with the wires on the very ends (kinda like a typical NiMH battery pack), while parallel hooks the ends together and then to the wires. The colors are exactly the colors as seen in the power supply.

This parallel/series connection is just a way to get the exact resistance value (and power value) needed using the resistors available. If you had an exact 2 ohm resistor, you only need to use one (as long as it is 20w instead), but the choices were limited, especially at RadioShack.

I laid out the resistors physically side-by-side just so the resulting resistor "pack" is neater, easier to wire up, and easier to insulate. You can then add heatshrink over the connections and even over the very end of the resistor(s) as long as the majority of the resistor is exposed to the air to aid in heat dissipation.

I hope I answered your question, but please forgive me if I don't understand.

Brain, would the N-Powe I currently have charge 6S at 8 amps with my E-Station 701? Its 10 amps at 14 volts.

Edit: Brain, you might want to look at the post right above the one that you just posted :p.

LOL, that's OK. Just make sure those other ones are single supplies. Some listings show the total current for a dual 12v power supply. That 24A could be 12A per 12v output. That's why I use Newegg.com, the pictures help eliminate confusion.

I was thinking about getting something like this.

10A @ 14v is 140w. As long as the charger doesn't try to draw more than that, you'll be set. To find out, just multiply the max battery voltage (4.2v per cell for Li) by the charge current. If this value exceeds 140W, then the N-Power won't be enough, unless you simply charge the pack at a lower current.

I would like to take a minute to apologize for any confusion this post might have caused. The intent was to mod a cheap computer PS for use as a generic 12v power supply. As we've seen, it might not be as easy as originally thought. Having an electronics background, I find these projects fun and easy to do, even with the little bumps along the way. However, I don't want people to spend ~$40 on a PS that might be overly difficult to get to work and then become angry if it seems complicated. I'm not trying to be condenscending, I just know how confusing things can be when just starting out.

Yes, that looks like a single 12v output from what I can see on the label. Usually, if there are dual 12v outputs, the label will show the rating for each output seperately. And the current value looks to be adequate as well.

Also, I would like to point out that there is such a thing as a too good of a deal. Most of the units with dual fans and such maybe just a gimmick. After all, the extra "stuff" is taking space away from beefy components. For example, take a look at the difference between the guts of two power supplies here. PC Power and Cooling is the king of computer PS, and they are pricey to boot. Cheaper power supplies can certainly deliver the required current at 12v, but I tend to "derate" the specs. If the spec says 24A, I would estimate that it is really about 18A. Plus, the cheaper units will have a 12v output that will drop as more current is required. This can be overcome, but takes some work, as we've seen.

You got it right, wasn't sure if i was to do both or just one. Thanks brian for all your help. But i am foresurely going to need two resistors right? Not just one? Some people only use one...but I'm assuming i will need two as i probably gong to draw more juice. Thanks Again man, sorry for all the questions, just want to make i don;t fry the whole PS!!!! LOL

A little more in-depth information about this project for those interested. The information below DOES involve opening the case, so please heed the warning in my first post about potentially hazardous voltages!

I made another supply with an extra PS I had in my computer "junk" box (this one) just for some testing. It is an Enlight with a rating of 15A on the 12v line. This brand is generally pretty good and gets good reviews.

When I took it apart, I noticed there were a couple of small trim potentiometers near the output wires. So, with the PS on and voltmeter on the 12v line, I slowly adjusted them and one boosted the 12v from 12.1v to 12.38v. Nice. It actually boosted the 5v and 3.3v lines a little as well, but I don't care about those. The other pot didn't do anything noticable. You have to be careful here because these adjustments may be for something totally different (and potentially harming), like the PWM circuitry. So, if you decide to play with them, make sure they are put back exactly like they were if they don't affect the output. Also note that many power supplies do not have these (the first one I converted did not).

Then, I saw there were two output coils (which is usually where the circuits take a amperage sample), one on the 5v line and the other on the 3.3v line. This is a good hint that one or both are used in the load regulation circuitry. I put a 12 ohm resistor first on the 5v and then the 3.3v lines seperately. Each time, the 12v line came up a bit. However, no gains were noticed if I loaded both at the same time. I'm not saying this is true for all units, but it was for this one.

I got a couple 10 ohm 10w resistors from RadioShack and hooked them in parallel to the 5v line. This creates a 5 ohm load rated for 20W. It's a good thing too, because even though the 5v line, drawing 1A, only generates 5W, the resistors got quite warm. I mounted the resistors on a heatsink using some Arctic Silver Epoxy and that helped tremendously. I chose the 5v line because I wanted to draw at least 1A with the resistors I had. Any less and the regulation circuits may not work as well.

Now, for the test. At no load, the supply delivers 12.7v (thanks to a combination of the trim pot adjustment and the 5v load). When loaded with a 0.75 ohm load (a combo of four 3 ohm resistors in parallel), the 12v line dropped to 11.63v. This equates to 15.5A and 180 watts. Not too bad at all.

If you ever have trouble finding a ground wire in an AC appliance with a 3-prong plug, just use the case of the appliance. They are always grounded, especially power supplies. But again only units with a 3-prong plug which I think all PSU's have anyways. Yellow, Red, and Orange represent the diffrerent voltage outputs. I believe Yellow is 12v, Red is 5v, and Orange is 3.3v, but don't quote me on that.

Yes;

yellow= 12v
orange= 3.3v
red= 5v

However, I would not use the chassis ground as the main output ground. Most of the time, yes, they are the same point electrically, but if the power ground is isolated from chassis ground (for regulator feedback for instance), the supply won't work right and may actually cause problems. Depends on the design of the PS...

Oh no definitely not for the main ground! I meant for grounding the green "turn on" wire if someone had trouble finding a ground inside the chassis of the PS.

lol, oh ok. Just making sure... :)

I have a question what if you want to just use the 5v side do you still need the resistors? I already have a power supply for my chargers and what not but I want to run my lathe,tire warmers without using a battery or my charger.

I just thought I would add this. It might have already been stated. To load the PSU I just plugged in an old CD drive. Worked really well. I already had a 12v power source but wanted a second one and I discovered the PSU worked a lot better then my dedicated 12v power source. I no longer got false peaks when charging.

And it does depend on the PSU as to whether you will need to load the 5v or 3.3v line. from experience I have found I have only had to put load on the 5v line. But that was on 3 very similar psu.

That makes sense. A CD drive uses 5v and 12v, so it loads the 5v. But that also takes some power (albeit little) from the 12v line. To help prevent false peaks, adding a couple 4700uF 16v caps may help. The output of these supplies are fairly quiet from switching noise, but PC motherboards have caps all over the place in key areas to help. A cheapie PS might not be as tight.

@mvalentine6: I guess I missed your question. Even though it was posted a while ago: The 5v line is generally rated for a much higher current (usually at least 20A), so unless you plan to run the 5v line to the max, adding loading won't help.

No the false peaks on the charger were caused by my bought 12v power supply. Expensive little thing and the output was all over the place. As soon as I discovered the psu trick (I should of thought of it before) I sold the piece of **** off. Computer PSU have to provide very constant voltage. They are far better then most of the 12v converters that you can buy.

Just curious, what was your "piece of ****" PSU? I had a Radio Shack 10A PSU that gave me a false peak every once in a while back when I ran NiMH.

It was a ... I can't even remember. It wasn't really for charging batteries. It was for powering car fridges and had a fairly high amp rating. Higher them most computer psu anyway.

Ehh it probably wasn't regulated. Just a transformer, a rectifier, and some capacitors in an expensive black plastic box. :lol: Maybe not even the capacitors. :no:

Unfortunately, with switching supplies, you generally get what you pay for. In the field I use a 17A PS and tweaked it for a little higher voltage (~12.5) to help the chargers switching circuit to operate a little better and to allow for more voltage dips under heavy load without going too low. At home, I use a nice Aston 35A supply tweaked to 14.5v, but it was something like $80-90 on eBay.

Probably a linear regulator at that. You can usually tell by large TO-3 transistor(s) on the back on a largish heatsink. Switchers are MUCH more efficient.

I have a CAE 45A right now. Nice compact unit, built for powering high-end audio systems so you KNOW it's going to be stable.