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-   -   Modded PC power supply for charging (https://www.rc-monster.com/forum/showthread.php?t=3962)

BrianG 09.02.2006 02:06 AM

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:



    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.

Serum 09.02.2006 02:28 AM

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

BrianG 09.02.2006 01:24 PM

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.

Serum 09.02.2006 01:35 PM

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.

BrianG 09.02.2006 01:44 PM

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. :)

coolhandcountry 09.02.2006 02:12 PM

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. ;)

BrianG 09.02.2006 02:35 PM


Originally Posted by coolhandcountry
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.

glassdoctor 09.02.2006 09:36 PM

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.

neweuser 09.08.2006 09:50 PM

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?

glassdoctor 09.08.2006 10:03 PM

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....

neweuser 09.08.2006 10:10 PM

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

2black=one of these leads

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.

BrianG 09.08.2006 10:12 PM

Neweuser, I just sent you a response PM.

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


For your 12v leads, just use the yellow wires found in the harddrive hookup leads and the two yellow wires in the AUX lead (the square plug with just two yellow and two black), and there should be a few yellow largish gauge wires in the motherboard connector. Tie all these yellows together and then tie the same number of black wires together (from the same cables). All the rest of the cables you can cut back and insulate. To make it easier, you can bundle all the orange wires, red wires, and extra black wires together and insulate those as a group. You will be left with several more wires which you should individually insulate. Once it is all done, you'll have something like [I have in the above picture].

neweuser 09.09.2006 09:55 AM

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?

BrianG 09.09.2006 09:16 PM

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.


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

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

BrianG 09.09.2006 09:31 PM

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.

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