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Old Nov 29, 2005, 12:00 AM
Old Tyme Electric
steve crewdog's Avatar
Joined Mar 2005
667 Posts
Converting PC power supply to power 12v charger?

Stripped a 350W power supply from a new case and replaced it with a 450W. I was wondering what the heck I should do with it when I got to wondering.....



Has anyone tried this? If some, words of wisdom would be welcome.

Thanks


Steve
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Old Nov 29, 2005, 12:17 AM
Use the Force!
LBMiller5's Avatar
San Marcos, CA
Joined Jan 2005
2,179 Posts
Yes, lots of people have done it. I have one sitting on my work bench right now that I use. I took off all the wires and put on a terminal strip to hook my leads to. It is best if you have one that has an on-off switch on the back, because if it does, you do not need to add an on-off switch. You just need to take the green wire from the motherboard connector and connect it to any one of the bleck wires. The green wire is the Power-On wire and needs to be pulled to ground to turn on the supply. You should also put in a couple load resistors to make sure that the power supply pulls itself into regulation. A 10 ohm 25 watt resistor across the 12 volt output, and a 4.7 ohm 10 watt resistor across the 5 volt output is usually enough to make sure the supply will start up and run properly.

Lucien
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Old Nov 29, 2005, 01:07 AM
Old Tyme Electric
steve crewdog's Avatar
Joined Mar 2005
667 Posts
cooolll....


Got a few other projects ahead of this but now I got another one. Thanks!!
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Old Nov 29, 2005, 06:23 AM
Johan Beyers Eflight101
Stellenbosch, South Africa
Joined Feb 2005
118 Posts
and because you can never have too many links

Here's some more:

Power supply conversions - Eflight 101

HTH,

Johan
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Old Nov 29, 2005, 09:33 AM
Registered User
Blackpool, Great Britain (UK)
Joined Sep 2004
63 Posts
Hi There,

I've converted around six or seven PC power supplies and they have all worked OK and have been completely reliable. Every 12 volt battery charger tried on them has operated fine and they have charged all types of flight batteries.

Check out the threads mentioned above.

The only critical bit is the 'load' resistor. 10 ohm 25 watt is about right but the exact value will control the eventual output voltage and you may have to experiment a bit. For instance, you might find that with a 10 ohm resistor your output voltage might be down to under 12 volts and some chargers will give a 'low battery input' fault at this level. However, the resistors are cheap and you can buy a few so that you can experiment. You may have to have two in parallel to get the output voltage you want (ohms law and some knowledge of how to work out the values of dc resistors in parallel and series will help ).

All my units have finished up at around 12.3 volts output and most of them have different value resistors - between 5 and 12 ohms.

The resistors you buy will almost certainly be square section ceramic in these values. They will get HOT in use so pick a suitable place on the inside of the power supply metal case and fix the resistors to it so that the case acts as a heat sink.

All you need to do is drill four small holes in the case and 'tie' the resistor to the inside with two twists of copper wire (taken from mains cable). Some cpu thermal compound is a good idea too.

I've found that the most useful output posts are the standard 4 mm 'banana plug' items. You can get ones that have a screw on cap so you can either trap wires under the cap or have a banana plug on your output wires. Just make sure that you place them in the case so that they miss all the stuff that's inside there.

Many 12 volt chargers have large size aligator clips on their input wires and you can easily solder the guts of a banana plug to the unused 'arm' of the clip and this makes connection very easy.

Chris Bromley
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Old Nov 29, 2005, 10:57 AM
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galloping gimp's Avatar
Ohio, USA
Joined May 2002
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Chris-

With regard to resistor values, you mean to say ohms, not amps, correct?

- Jeff
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Old Nov 29, 2005, 10:15 PM
Registered User
Blackpool, Great Britain (UK)
Joined Sep 2004
63 Posts
Doh!!

Of course it's ohms - serves me right for rushing the post. I'll try and edit.

Chris B
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Old Nov 30, 2005, 03:18 AM
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Purvis, Mississippi
Joined Sep 2005
977 Posts
I use one +12v yellow wire and one black wire to power 3 small chargers at a time without any problems at all with a 400w pc power supply rated at 16amps on the +12v wire. Many say to modify the supply with capacitors and maybe that is a really great idea...on the other hand...my batteries charge perfectly everytime without any modifications...I'm sure it depends on what you are charging.

I only charge 2 7.4v 1000-2100 lipo packs at a time and one 8.4v 600mah nimh.

All together around 1.9amps to 2.9amps at a time using 2 aurora 1-3 cell lipo chargers and 1 parkzone charger (Cub)
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Old Nov 30, 2005, 07:41 AM
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United States, TX, Benbrook
Joined Oct 2005
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I have been using one for some time now with no issues. I got the info from Redís Battery clinic. Here is a direct link. http://web2.murraystate.edu/andy.bat...owersupply.htm
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Old Nov 30, 2005, 11:27 PM
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Arlington Texas
Joined Oct 2005
164 Posts
I went to the local electronics surplus store today and bought this supply and cord. Next I went to radio shack and bought banana jacks, a switch and 2 - 50 ohm 10 watt resistors. Total expendature $28.36. Very cheap for a 12 volt 10 amp switching supply with 5 volts 25 amps as a bonus. It really works great. Here are a few photos.

Ken
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Old Nov 30, 2005, 11:55 PM
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Purvis, Mississippi
Joined Sep 2005
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Somebody explain to me why you need the resistors...I have some lying around but didn't bother using them......I didn't modify it at all...well, if you count removing all accept one yellow and black wire I did. Just curious.

Thanks,
John
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Old Nov 30, 2005, 11:58 PM
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Arlington Texas
Joined Oct 2005
164 Posts
Chris,

I was re-reading the following that you wrote: "The only critical bit is the 'load' resistor. 10 ohm 25 watt is about right but the exact value will control the eventual output voltage and you may have to experiment a bit. For instance, you might find that with a 10 ohm resistor your output voltage might be down to under 12 volts and some chargers will give a 'low battery input' fault at this level. However, the resistors are cheap and you can buy a few so that you can experiment. You may have to have two in parallel to get the output voltage you want (ohms law and some knowledge of how to work out the values of dc resistors in parallel and series will help )."

I don't agree with you that the load resistor is critical. These power supplies have very sophisticated electronic voltage regulators. As an example, your computer is running with one hard drive. You add a second hard drive to the same power supply. The voltage must remain at the same. You add a DVD drive. The voltage must remain the same. Each additional load that you add is like putting an additional load resistor on the power supply. I do like some minimal load resistor, that's why I used a 50 ohm 10 watt resistor. This provides a minimum current flow (0.24 amp) on the 12 volt side when there is no other load on the supply. It probably isn't even necessary but I like it. If you are having trouble with "low battery input" then your charger is pretty intolerant to voltage slightly below 12 volts.

What I've written is in no way meant to be dis-respectful, just another point of view. Too many years in high tech electronics I guess.

Ken
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Old Dec 01, 2005, 12:32 AM
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Ohio, USA
Joined May 2002
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Ken-

The load resistor that many of us are using is on the 5V side. Some (older?) PC power supplies fall short of 12V on the 12V output unless there is a load on the 5V output. Apparently, the 12V regulator does not work unless there is sufficient load on the 5V side.

With no load on the 5V output, my PC power supply produces less than 11V on the 12V output. With an appropriate load on the 5V side, the 12V side produces just over 12V.

If you read through the threads, you'll find many examples of conversions that required a load on the 5V rail.

Have I misunderstood something?

- Jeff
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Old Dec 01, 2005, 11:21 AM
Registered User
Western KY
Joined Sep 2003
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Quote:
Originally Posted by Big_Bird
I don't agree with you that the load resistor is critical. These power supplies have very sophisticated electronic voltage regulators.

<snip>

What I've written is in no way meant to be dis-respectful, just another point of view.
It is true that the PS's are quite sophisticated voltage regulators, which certainly adds to their appeal as desktop supplies. However, although the supplies may meet industry standards WRT operational specifications, the individual designs vary widely, not only between manufacturers, but also within manufacturers. The more recent design guidelines state that the PS should not be damaged if operated without any load, but that they may latch to a shutdown state. This was not always the case and some of the earlier designs could be harmed if run without load. Even today, many will not Latch_ON without some attendant load. While loading either the 12v, 5v or 3.3v rail will maintain Latch_ON, output voltage levels on the 12v may not be sufficient for some usage. The ability to hold 12 plus volts on the rail is more easily controlled by the power consumption on the 5v rail. Although resistive loads on 3.3v will maintain latch, voltage levels generally will fall short of 12v.

When developing a conversion process, I try to meet two goals: (1) What will work in the vast majority of cases and (2) Use easily obtained components.

From my experience, loading the 5v rail with 2 to 10 ohms has succeeded in every situation in maintaining Latch_ON and also seems to be the most effective method in producing stable voltages over 12v on the associated rail across different brands of supplies (I've probably converted 70 or so and pulled voltage readings off another 200+).

Certainly there are several ways to skin a cat, so to speak -- I've simply found that loading the 5v rail is the most universal in achieving my goal. I wholeheartedly support experimentation and changes in mods, but many folks just want a simple easy fix that will be most likely to work in their situation -- that was primarily my intent.

What is your voltage level on your 12v rail? BTW, I've found Antec to be a good solid supply.

Regards,

andrew
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