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kyosho501x
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06.27.2008, 01:56 PM

where can i locate the feedback wire? is it the black one? and where cani get a voltage diver? Radioshack?


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kyosho501x
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07.02.2008, 08:20 PM

well i got the power supply to work for about 4 days and now it just shuts down, the fan doesn't work and its plugged in, every connection is good. the LED i put in works for only about 5 seconds.what could be wrong


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tc3_racer_001
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07.03.2008, 12:29 AM

an internal short perhaps? carefully check all the wires u unsoldered/cut and taped/heatshrinked up to make sure they are not touching each other. u also need to put the green and brown wires from the mobo plug onto +5 and ground (PLEASE NOTE: I DONT KNOW WHICH ONE GOES TO WHICH OTHER CABLE SO MAKE SURE U CHECK FIRST AND ONLY USE ONE OR THE OTHER, NOT BOTH!)

so once you figure that out, (its on the net) then try again. if it doesnt work add a reasonably heavy (>1A) load on the 5v rail and it should work better.


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Sammus
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07.03.2008, 09:51 AM

black is usually ground.

Sometimes the psu needs a load to run properly, I got a 10ohm 10W power cap on the 5v rail on mine for this purpose, does the job :P
   
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kyosho501x
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07.03.2008, 06:47 PM

i found the problem, one of the pins on the male connector that connects to the fan slid out and tore the circuit board slightly, but i can try to resolder it bck and hope for the best


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kyosho501x
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07.03.2008, 09:52 PM

well i fixed the fan, but not the power supply. it still shuts down, any ideas to fix it?


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Topspeedtimmy
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07.07.2008, 02:19 PM

I have this old Antec 250w PS, but the voltage is 11.12v with no load and it goes below 11v when theres more than 15w load on the 12v wires. It's rated for 8A on the 12v rail. I don't want to spend any money on this piece of junk (especially since its only 96w), so is there any appropriate resistors I could take out of another circuit board from something else?


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BrianG
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07.07.2008, 02:59 PM

Kyosho: There could be any number of possibilities. If there was a broken PCB at one point, powering it up could have caused something else to go bad. It's really hard to tell. At any rate, fixing these becomes pricey as they are fairly complex and there are so many variations that there is no one schematic to follow.

TopSpeedTimmy: Wow, 8A? It hardly seems worth it. Usually, adding resistors boosts the voltage a little, but it does more to stabilize the voltage under load. Even if you could get 12v @ 8A, it would only be good for an approx 50w charger.

That said, a broken TV may have a few power resistors in it (CRT type), but it's a toss-up if they will be the value you need. If you do decide to go poking inside a TV (which I do NOT recommend), BE VERY CAREFUL AS THERE ARE VERY HAZARDOUS VOLTAGES IN THERE. Depending on the size of the screen, VERY high voltages can still be present even after sitting unplugged for a while.
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BrianG
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08.01.2008, 01:52 PM

I had an old broken Antec TruePower 330w PS laying around in my parts bin. The motherboard connector had melted at the 5v pins, so it was either throw it away, repair it, or do something else. I chose "something else". It is rated 17A @ 12v, so should make a decent 12v supply.

This supply is a bit different than most supplies I've seen. This one has seperate feedback lines for each of the main supply rails. So, on each of the main 3.3v, 5v, and 12v wires, there is a smaller gauge wire of the same color that is connected to the main wire at the connector. Presumably, this feedback wire reads the voltage at the connector. During heavy current draw, there could be voltage drop along the wires, so the PS can compensate for this by boosting it a little.

Loading the 3.3v and 5v lines did nothing to change the 12v line. This PS uses seperate feedback circuits for each line. Nice. This is the way all supplies should be built IMO.

Also, there was no fine-tuning potentiometer to boost the voltage.

So, I got to thinking about those feedback lines; what if I use a voltage divider to trick the 12v circuit into thinking it's only putting out ~10v? The circuit would then boost the output so the feedback wire is at 12v. A quick test confirmed this. After some playing around with different resistor values, I found that anytime I tried to go over 13.5v, some other protection circuit would engage when powered up. I was hoping for ~14.5v, but that's ok. Also, it seems I needed to draw at least 1A for the feedback circuit to act according to my calculations. 1A at 13.5v is too much wasted power IMO. The resistors would heat up for nothing, and I'd have that much less 12v current available.

So, I decided to use a couple series connected diodes with a resistor to draw ~250mA. The combined diode voltage drop of ~1.5v at the sense wire will boost the 12v to 13.5v. I used two 100ohm 10w resistors in parallel to get the 50 ohms I needed. Then, to make sure the diodes wouldn't overheat, I used 3A barrel diodes because they have larger packages to dissipate any heat. Below is a picture of the circuit.



No on to testing. The output of the supply was actually at a default 11.89v with the sense wire attached to the power wire (no v gain). So, after the circuit was added, the actual output voltage rose to 13.35v. What surprised me is that even at over 16A, there was only ~0.1v of voltage drop! Wow, talk about good regulation! I don't think I've ever seen a supply (linear or switching) that doesn't sag more than that when run at so close to the max value. This supply just became my new field PS!

Other mods I did: It comes with two fans. I removed the rear fan and kept the one that blows directly on the heatsinks/circuits. Where the rear fan was, I attached a piece of plastic, and on that is where the banana jacks go. The resistor/diode package was mounted to a piece of channel aluminum (for protection and heat dissipation) on the rear of the PS. All extra wires were completely removed for neatness and I used Deans 12GA wire from the PCB to the banana jacks.

Last edited by BrianG; 08.01.2008 at 01:54 PM.
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BrianG
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08.01.2008, 09:34 PM

And a couple pics of the finished product:

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glassdoctor
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08.02.2008, 04:03 AM

VERY nice Brian!!
That makes a sweet power supply.


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hoober
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08.03.2008, 10:09 AM

That's very nice , a voltage over 12 while running 16 amps is very good. All the ps I've done have sagged a lot when pulling hard on them.

My latest was a brand new $20 ps which I simply pluggd in the digital tester to and left it that way. It reads the output voltage for me on a screen , but when pulling hard it too sags to 10 volts or so.

I will open it up to see if it has the seperate sense wires on it. I don't know how to read electronis digrams. So the two diodes (drop voltage) go in series in between the two yellows? and the resistors get put in parallel between the yellow sense wire and black?

I did not have to load the ps any , but the digital tester I have used probably has the load built into it. It says not to leave it on for a lojng time , and it can do the full rating of 200 watts (with voltage at 10 sagged very bad)
   
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BrianG
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08.03.2008, 02:17 PM

If you open it up, you might see a small PCB mounted adjustment potentiometer. Many don't so don't be suprised by the lack of one. Also, sometimes loading the 5v and/or 3.3v lines makes the 12v rail much more stable. It depends on what rail the circuit uses the sample the current flow. The only way to know for sure is to try it.

You don't need to open the supply to see if it has a sense wire. If it DOES have a sense wire, the 12v point on the main motherboard connector will have two yellow wires going into it; the main current carrying one, and likely a smaller sense wire. The diagram I made above shows how to hook it up just by cutting the mobo connector off and placing the components as shown. The wires have to be in that order or it won't work right (don't put the sense wire "on top").

That digital tester sounds like a wattmeter or something. Voltage meters typically have VERY high input impedance (so they won't load the circuit being tested). Kinda odd that it says to not keep it hooked up for long.
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Sammus
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08.03.2008, 07:19 PM

Cool... my desktop just died and I pulled an Antec Tru430W out of it.. and to think I was just about to go and buy a new power supply :)

The one I have is rated to 28A@3.3V, 36A@5V and 20A@12V.

Do you know if there would there be any harm in connecting those three in series if I didn't pull more than 20A@20.3V? I'd like to have 400W available for my DUO at home.

I'm not sure I'll do your fancy mod, as I don't quote understand it. What are the diodes for? Is that so your only loading up the sense wire? and if so, why are there two?

Cheers

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BrianG
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08.03.2008, 11:42 PM

No you CANNOT put all three rails in series - they all share a common ground.

Hmm, I thought the mod was fairly straight forward. The diodes are just to drop ~1.4v (0.7v each) so the sense wire sees less voltage than there actually is. So, it causes the regulator circuit to boost the output voltage until there is ~12v on the sense wire. This causes the real output to be ~1.4v higher than that, or ~13.4v.

I could have used simple resistors as a voltage divider to do the same thing, but preferred the diodes method.

Think of the sense wire as the feedback pin on a regular linear voltage regulator, or the - pin of an op-amp. Same basic thing.
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