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Old 04-26-2007, 03:28 PM   #1
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Default PC ATX power supply conversion

Hi all,
Over a year ago, I thought I saw a article here on RCTECH to convert a PC power supply to drive our chargers and Dischargers? I can not find it.

Do you know of a good link to build your own power supply?

Also, I never relies that we can use the lower voltage levels to power our servo's and receivers. Does anyone know how to limit voltage levels? Meaning take a 5v and 3.3v and output only 6v instead?

Please let me know if I am wrong, but if you put two 3.3v leads together from the power supply you will only get 3.3v still, and not 6.6v? Right?

Thanks for your time,
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Old 04-26-2007, 03:48 PM   #2
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I have used this site with success:

http://web2.murraystate.edu/andy.bat...OWERSUPPLY.HTM

Depending upon the power supply, you will have a variety of voltages to choose from in the power supply.

This is a good exercise for someone with the right abilities, however I would not recommended just anyone trying this. Things can go wrong if you are not careful.

Also, it is not the most cost effective unless you have the parts already.

You are correct in that putting two leads of the same voltage together results in the same voltage not an increased voltage. You can put a potentiometer on a 12V lead and dial down to whatever voltage you are trying to acheive.

Good luck, Mike
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Old 04-26-2007, 03:52 PM   #3
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This is probably the one you saw:

http://rctech.net/forum/showthread.p...t=power+supply
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Old 04-26-2007, 05:39 PM   #4
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Quote:
Originally Posted by mkdut
I have used this site with success:

http://web2.murraystate.edu/andy.bat...OWERSUPPLY.HTM

Depending upon the power supply, you will have a variety of voltages to choose from in the power supply.

This is a good exercise for someone with the right abilities, however I would not recommended just anyone trying this. Things can go wrong if you are not careful.

Also, it is not the most cost effective unless you have the parts already.

You are correct in that putting two leads of the same voltage together results in the same voltage not an increased voltage. You can put a potentiometer on a 12V lead and dial down to whatever voltage you are trying to acheive.

Good luck, Mike
Thanks Mike, I have all the parts includeding the ATX power supply. I work with computers all day. So I have them laying around. I just need the resister it looks like.

Thanks again.
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Old 04-26-2007, 09:52 PM   #5
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[QUOTE=Nickmind
Please let me know if I am wrong, but if you put two 3.3v leads together from the power supply you will only get 3.3v still, and not 6.6v? Right?
[/QUOTE]

There are ways you can do this, if you take the + terminal and connect tit o a - terminal you increase the voltage. + to + increases current draw. (edit: has to be of different original PSU's)

Think about in terms of batteries.

6 cell packs are
+-+-+-
-+-+-+

This gives 7.2v out at 4200mah cap

if however the packs were all
++++++
-------

You would have 6x the capacity of the cells. eg 6x 4200 = 25200mah at 1.2v
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Last edited by ford_racing; 04-27-2007 at 02:09 AM.
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Old 04-27-2007, 12:59 AM   #6
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Quote:
Originally Posted by Nickmind
Hi all,
Over a year ago, I thought I saw a article here on RCTECH to convert a PC power supply to drive our chargers and Dischargers? I can not find it.

Do you know of a good link to build your own power supply?

Also, I never relies that we can use the lower voltage levels to power our servo's and receivers. Does anyone know how to limit voltage levels? Meaning take a 5v and 3.3v and output only 6v instead?

Please let me know if I am wrong, but if you put two 3.3v leads together from the power supply you will only get 3.3v still, and not 6.6v? Right?

Thanks for your time,
I have done the mod ( 3 ATX PSU 400watts ) .............. but I DON'T use it..... I'm not so cheap ( read : dumb ) to screw up my expensive equipments. In my opinions : DON'T do it !!!

However, this is just a snack for your curiosity :

First of all, you need to buy 4 or 3 big resistors ( white cube resistor 20watts rating X 4 or 3, all joint together, and connect them to 5V red cables and black cables ). Why ? The ATX PSU will produce big ampere ONLY when it sees some loads, in this case, you put the loads on 5V red cables so the 12V has clean power or clean ampere. 5V and 12V are generated from 1 power source in PSU, and they are sharing power from that source. So, the loads on 5V cables is necessary, in order to make 12V output has some amperes to power your chargers, etc.

Secondly, you need to connect PS-0N (Green) cable to Ground ( I forget the color, please check again ). This cable is signal cable to tell PSU ON, so you need a ON OFF switch. Another connection is POWER OK or POWER GOOD (Grey) cable to Ground ( or to 5V red ? I forget..... you need to check again )to keep the power supply stay ON.

The 3.3V cables you mentioned are STILL 3.3V regardless how many 3.3V cables you add up. Ditto with 5V and 12V cables. However, If you joint 12V and 5V, they become 7V output ( 12 - 5 = 7 ). Why ? it is DC power, measure from potential difference between Direct Current of 12V and 5V.

I suggest DON'T DO IT
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Old 04-27-2007, 10:13 AM   #7
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Quote:
Thanks Mike, I have all the parts including the ATX power supply. I work with computers all day. So I have them laying around. I just need the resister it looks like.
Just make sure you use a decent power supply. One with a built in switch on the back of the unit and a decent fan. An Aluminum case is nice to have.
Also, make sure you use the appropriate wattage resistor(s). You should be able to pick these up at Fry's or a local electronics store. Make sure to also use some heat sink grease on the resistor and attach it to the power supply case. This is where the aluminum case comes in handy.
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Old 04-27-2007, 10:18 AM   #8
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I was toying with the idea of doing this before I bought a proper power supply. One thing that held me back was that this will produce 12 volts. Typical rc power supplies are 13.8 volts. Does this make a difference? Will this hamper charging a 3 cell lipo that might peak out at more than 12 volts?
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Old 04-27-2007, 10:27 AM   #9
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Yes the downfall of using a PC power supply is that you are limited to 12v.(actually you would be lucky to get 12v as putting the resistor off of the 5v leg causes a voltage drop).
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Old 04-27-2007, 12:14 PM   #10
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But I see and hear from some EE's that I can get the voltage higher with resistor configurations. Put more load will increase voltage levels by .1 to .3 volts to get over or to 12v.

Now My question is this why would you need 13.8 volts, your chargers will never use 13.8 volts, right? And if you think they do, i believe that is a maxium voltage and not a use voltage level.

That goes for amps as well. I see people wanting to get 30+ amp PS, but yet does your charger ever use anything over 10 amps?

Please let me know if i am not seeing this right.

Thanks,
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Old 04-27-2007, 12:19 PM   #11
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Quote:
Originally Posted by asw7576
I have done the mod ( 3 ATX PSU 400watts ) .............. but I DON'T use it..... I'm not so cheap ( read : dumb ) to screw up my expensive equipments. In my opinions : DON'T do it !!!

I suggest DON'T DO IT

Boy, do I love opinions!
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Old 04-27-2007, 12:27 PM   #12
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Quote:
Originally Posted by Nickmind
..., your chargers will never use 13.8 volts, right? And if you think they do, i believe that is a maxium voltage and not a use voltage level.
...
Thanks,
Even a cheap power supply, like the Peak 20 amp, is regulated to 13.8 volts. That means it holds at 13.8 even under load.

My worries were that you can only get out what you put in, unless there is an inverter within the charger. An 11.1 volt lipo may need more than 12 volts input to charge it. Since I run RC planes on 3 cell lipo's, I didn't want to chance it.
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Old 04-27-2007, 12:32 PM   #13
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Quote:
Originally Posted by Iceracer
Even a cheap power supply, like the Peak 20 amp, is regulated to 13.8 volts. That means it holds at 13.8 even under load.

My worries were that you can only get out what you put in, unless there is an inverter within the charger. An 11.1 volt lipo may need more than 12 volts input to charge it. Since I run RC planes on 3 cell lipo's, I didn't want to chance it.
Yes I see that, ok then. I did not know packs got that high in voltage. But I still say if you have a 11.1 volt pack you would even need more then 13.8 volts to get maxium current rate.

Now i dont see many chargers, over 12 volts?
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Old 04-27-2007, 12:54 PM   #14
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I believe lipo's usually peak at 4.2 volts per cell during charge, so a 3 cell would be 12.6 volts.

Perhaps this isn't a problem at all, just didnt want the hassle so I went with a 13.8 volt power supply. Even car batteries, nominal 12 volts, put out over 12 volts when fully charged, and I know some people use "12v" gel cells and the like with field chargers.
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Old 04-27-2007, 04:11 PM   #15
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Quote:
But I see and hear from some EE's that I can get the voltage higher with resistor configurations. Put more load will increase voltage levels by .1 to .3 volts to get over or to 12v.

Now My question is this why would you need 13.8 volts, your chargers will never use 13.8 volts, right? And if you think they do, i believe that is a maxium voltage and not a use voltage level.

That goes for amps as well. I see people wanting to get 30+ amp PS, but yet does your charger ever use anything over 10 amps?

Please let me know if i am not seeing this right.

Thanks,
Yes a different resistor config will get you near 12v, however this will generate more heat in that resistor(s). You just have to keep the resistor cool via a heatsink/fan combo.

You would need over 10 amps if you are running multiple chargers or other items such as lathes, dischargers, motor cooler, etc. Also the longer you run the power supply at or near its max amp draw the shorter its life span.
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