Tips for using a computer power supply

[Drake600]

Hey guys, I was down in my shed thinking about a power supply to run my lipo charger (imax B6 compact DC) and started wondering how many people use (or have tried to use) a PC power supply for their 12v needs - I've used them myself for a number of purposes over the years and thought it might be a good idea to share some tricks I've discovered.

There's a thread from a while back about this issue - Computer charging supply help? but rather than dig up an old thread I'm starting a new one.

Instability and cutting out at loads well within the specified rating - Place dummy loads on the unused outputs!

I found that while running a 200w car audio amplifier, the 12v line would drop to about 9v. By adding some suitable resistors to load up the 5v and 3.3v line, the 12v output was a lot more stable. I also put a small load on the -5v and -12v just in case but I don't know if it was necessary. Be warned though, they get hot! I used 5W resistors for the 3.3 and 5v lines: 2.2 Ohm for the 3.3v will be just under 5W at about 1.5A, for 5v I think it was 5.6 Ohm which gives 4.4W at just under 1A

My reasoning for this is that a switchmode power supply doesn't like running with no load - the extremely low duty cycle may be difficult to achieve and have undesirable effects such as voltage overshoot. In the case of a PC supply with multiple outputs, trying to run 12v at full load while the rest are open circuit is a condition that they aren't designed for - I found that adding a dummy load significantly improved reliability and stability, even something as simple as a 12v light bulb wired to 5v (between 5v and ground that is) has worked. Doubles as a power light too!

Anyway, I hope this helps. My car amp / twin 6x9" boom box has been running on the same unit for 6 years without an issue, and the sound quality and bass response is brilliant!

 

[Drake600]

Before I forget, if you decide to use a PC power supply and want to cut the unused wires shorter for neatness, watch out for voltage feedback wires which are joined at the motherboard plug - some supplies use these to measure the voltage at the plug and compensate for wire losses, so if you cut it off the supply will refuse to work until you solder it back to the wire it was paired with. (Been there, done that)


Orange is 3.3v, with a brown wire crimped into the same terminal. Sometimes found on 5v and/or 12v, or none at all on older / less powerful supplies.

 

[willsonman]

I suggest server power supplies. They are SUPER cheap for the wattage, have a smaller footprint, and require little to no modding to operate as intended. I do use a 200W ATX power supply for my charger... but it getting long in the tooth. Rather than mod another that I have laying around I'm just going to plunk down $30 for a server power supply.

 

[bitogre]

This is can be a good and cheap option but has some drawbacks. Most power supplies are designed to provide most of their wattage at 3.3V or 5V. They do not expect a high 12V load. As such, you will not get 20A from a 240W power supply (P/V = I). Here is a link to an example of common current rating on typical ATX power supplies used by computers.

On other thing to be careful of. Some computer power supply require a minimum current on the 5V and/or 3.3V lines. If it will not turn on, it may mean you have to put a load resistor on one or both of these lines (This site recommends a 12ohm 3W resistor). The main problem with using a power supply that requites this load is that you are wasting energy (about 3W). This may or may not be acceptable depending on your application.

So, if you can get one cheap enough that has enough current capacity (looking at the 12V capacity and not the wattage) despite the lower current focused on 12V and you do not mind the inefficiency, go for it.

 

[SOOFLY]

The Heli guys have been using server power supplies to charge their high power setups for sometime. They work quite well.

http://www.helifreak.com/showthread.php?t=310694

http://www.helifreak.com/showthread.php?t=331196&highlight=charging+station&page=3

 

[Bleumunkie]

This is what I use -

http://www.ebay.com/itm/12V-30A-360...ing_Parts_and_Accessories&hash=item4add2c3f41

• Cheap - comes in at under $30 bucks, which is the price point I see here often
• Easy to setup - Wire a plug to it, and leads coming off the terminals for your charger and your off
• No dummy loads needed , or case opening / soldering / wire cutting
• Cooling fan auto adjusts to load

I used an old PC power cord and wired it in for my supply, and used small leads with banana plugs at the end for my charger to connect to easily.

Will post pics later if requested.

 

[stay-fun]

One other option is to use an old laptop power supply. Less power, but your B6 doesn't require a more powerful input. Make sure the supply gives the right voltage though. I have a 2002 laptop adapter that does 15V at 4A, that's great! A lot of newer ones do 19V, while my charger could only handle 18... So be careful. But if the voltage is acceptable, it's a lot easier!