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Old Sep 04, 2012, 10:45 AM
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Canada, ON, Ottawa
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Help!
DIY PC power supply shutting off after 5 minutes...

Hi,

I did the DIY power supply of a PC power supply.
It does work super great.

Issue : The power supply just shut off after 5 minutes of use... I use it with my Accucell8 , that push 7amp to a 4S....
The fans works great on the power supply, the power supply does not overHeat i beleive, as the power supply is not hot. (external case)

Any idea of the problem?

Thanks for all the help!!

Max
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Old Sep 04, 2012, 12:10 PM
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Did you put a load on the 5V line? Also what power supply are you using? Are you sure that it's rated to sustain the number of watts you are trying to pull? Often times cheap power supplies will list "peak" amps instead of sustained amps. Also did you tie all the 12V lines together if it has more then one +12V rail?
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Old Sep 04, 2012, 01:11 PM
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Wow, thats a lot of questions that im not sure...
Ill try to answer my best.

5V line are the RED wires if im correct. and the answer is NO, i only tied the Yellow together (total of 3) for the 12v. I did cut the Red wire pretty short close to the board.... about 2 inch long remaining...

What power supply? it is a 350W or a 400W power supply. I will read on the side of it to have more informations. What is the informations you need ?

Are you sure that its rated to sustain the number of watts you trying to pull?
Answer : Yes, because they say that he can pull 18Amp on the 12V.

I dont understand the difference of the Peak amps, instead of the sustained amps... maybe a language barrier, as im french....

All Yellow wire together for sure. and, i beleive they are all on the same rail... again, not sure. how do i verify that?

Thanks a lot for that informations!!

Max
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Old Sep 04, 2012, 01:21 PM
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Quote:
Originally Posted by XstarWake View Post
5V line are the RED wires if im correct. and the answer is NO, i only tied the Yellow together (total of 3) for the 12v. I did cut the Red wire pretty short close to the board.... about 2 inch long remaining...
Every time I have seen a desktop power supply modified you have to put a resistor load on the 5V lines or you will not be able to max out the 12V lines. I'm betting this is your problem. See this thread:
http://www.rcgroups.com/forums/showthread.php?t=1714389
and this:
http://www.rcgroups.com/forums/showthread.php?t=1670090

Quote:
Originally Posted by XstarWake View Post
I dont understand the difference of the Peak amps, instead of the sustained amps... maybe a language barrier, as im french....
Peak means it can only supply that much for a short period. Sustained meaning it can provide that much forever.

Just an example this cheap power supply is listed as 480W which is actually a "peak" rating because when you look at the label in the photo it's only 330W.
http://www.newegg.com/Product/Produc...82E16817170024

Quote:
Originally Posted by XstarWake View Post
All Yellow wire together for sure. and, i beleive they are all on the same rail... again, not sure. how do i verify that?
If it had multiple rails they would be listed separately on the label with individual ratings, +12V1, +12V2, etc.
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Old Sep 04, 2012, 02:04 PM
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Hi sir,

Wow, thanks a lot for that info.

I read both post regarding the load on the 5V line.
Here is what a dude did : I used four 4.7 Ohm, 10 Watt resistors on mine. That's 4 point 7 Ohms each -- wired in parallel.
Q- Were can i find a 4.7ohm, 10 watt resistor? or, can i go 20 ohm, 50 watt resistor? its kinda the same... right?

Thanks for the clarification about the peaks and sustained current.

About the rails, what does it do if there on different rails?

Great help sir,

Thanks a lot again,

Max
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Old Sep 04, 2012, 02:13 PM
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You would have to get the resistor from an electronics supplier such as DigiKey, Mouser, etc. Not sure on Canada sources. The other option as mentioned in one of the threads is just use a few automotive tail light bulbs or flash light bulbs. You just need some kind of load on it.

Think of rails as independent supplies, if it has multiple rails you tie them together to form parallel supplies so you get the power of all of them put together. No good or bad to it, just have to make sure you use all the wires to get all the power.
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Old Sep 04, 2012, 08:41 PM
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Quote:
Originally Posted by XstarWake View Post
Here is what a dude did : I used four 4.7 Ohm, 10 Watt resistors on mine. That's 4 point 7 Ohms each -- wired in parallel.
Q- Were can i find a 4.7ohm, 10 watt resistor? or, can i go 20 ohm, 50 watt resistor? its kinda the same... right?
Four 4.7 ohm resistors in parallel = a 1.175 ohm resistor.
Four 10 watt resistors in parallel = a 40 watt resistor.

Resistors in parallel divide ohms. The wattage does add.

Glen
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Old Sep 05, 2012, 04:20 PM
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Quote:
Originally Posted by XstarWake View Post
I use it with my Accucell8 , that push 7amp to a 4S....
As the voltage on the battery being charged increases, the charger must draw more current from the power supply. For example, charging a 4S Lipo to 16.8V at 7A requires about 130W of charge power (assuming 90% charger efficiency) which is nearly 11A at 12V. If this increased current causes the input voltage to drop then the charger will try to draw even more current (eg. 12A at 11V) until the power supply senses under-voltage and cuts out.

Most pc power supplies have a single transformer with multiple output voltages, but only the 5V line is regulated. Putting a load on the 5V line causes the power supply to increase transformer power to compensate for the lower voltage (on the 5V line), which then raises the voltage on the other outputs.
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Old Sep 05, 2012, 11:32 PM
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Quote:
Originally Posted by Bruce Abbott View Post
Most pc power supplies have a single transformer with multiple output voltages, but only the 5V line is regulated. Putting a load on the 5V line causes the power supply to increase transformer power to compensate for the lower voltage (on the 5V line), which then raises the voltage on the other outputs.
This is incorrect.

A pc PSU is a SMPS (it rectifies the AC to DC then uses a DC-DC converter to get lower voltage). ALL the outputs are regulated (they have to be as ATX spec is only 5% variance on the 12V line).

Older PSUs (and the really cheap crappy designs) need some load on the 5V line or they think something is wrong (which is correct if it is a PC) and shut down. New ones wont care as pretty much all the power on a modern PC is supplied via the 12V lines.

Be aware that none of the cheap no-name PC PSUs are likely to be able to manage more than 200-250W without shutting down (or burning up). The specs are total lies and a 500W and 300W unit is likely to be electrically identical (only difference is the sticker). They dont really make a good choice for a supply, server units from eBay are much better (and just as cheap).
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Old Sep 06, 2012, 07:02 AM
jrb
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YMMV.

Haven't used a load on my recent PC PSUs either -- last 5 years or so; yes prior to that.

Tested the newer supplies w/ & w/o load no difference on 12V performance.

Can you monitor the output of the PSU/input to Charger via a Whattmeter?

The load on the PSU ramps up -- could take 5minutes w/some chargers -- at this point the PSU might be overloaded..
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Old Sep 06, 2012, 07:46 AM
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JRB,
Are you saying that loading the 5V line is not the issue?
I dont have a wattmeter, but can borrow one from my friend.

thanks all for the info... its greatly appreciated. I am learning a lot as i go.

Max
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Old Sep 06, 2012, 07:50 AM
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Instructions from TJinTech say to load it and he's pretty knowledgeable so pretty sure you need it. He says a 1A load:

https://sites.google.com/site/tjingu.../convert-pc-ps

The thing I don't get is he says:
Quote:
I ended up using 2x 1.5ohm 10W in series to create a 3ohm resistor that would draw 5/3A.
Since V = IR wouldn't that be 12v/3ohm which is 4A not 5/3A? But I suppose since it can only dissipate 10W each the math is different and he's doing 20W/12V = 5/3A?
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Old Sep 06, 2012, 08:44 AM
jrb
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As I said YMMV; but, when I tested my latest PSU conversions the 12V performance was the same whether 5V was loaded or not.

I always put my Whattmeter inline between the PSU and charger during the 1st couple of "high power" uses.
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Old Sep 06, 2012, 10:20 AM
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I have converted many of those older computer supplies for our use. All, yes ALL, have required a load on the 5 volt bus to get 12 volts at any decent current out of the 12 volt line. How much load, some required only about 5 ohms but some required as much as 2.7 ohms to get the full voltage out of the 12 volt bus. If you look at the schematics, most of these power supplies depend only on the load on the 5 volt bus to establish proper regulation. Admittedly, all these power supplies have been 5 or more years old. It is easy to find out if that will help if you are converting a power supply, just load down the 12 volt bus to draw the max current you will need from it and then experiment with different values of loads on the 5 volt bus while checking the voltage on the 12 volt bus.
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Old Sep 06, 2012, 10:44 AM
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omg, you guys are smart!

Im reading and learing a lot, but still a lot of info i dont really understand.

Peace,

Max
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