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RE: How to convert an ATX power supply to rc use - 4/9/2012 6:05 PM   
ImpalaPolicePack



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Status: offline 		im charging 5s and 6s 5000mah packs at 7amps. i know its not 1c but i dont care if the packs does not last as long since they are cheap to buy.
i tought that i was only pulling 7amps, why is it more because of a 5s pack ?
anyway, like you said, the ps should handle it...

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RE: How to convert an ATX power supply to rc use - 4/9/2012 9:19 PM   
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quote:

ORIGINAL: ImpalaPolicePack

im charging 5s and 6s 5000mah packs at 7amps. i know its not 1c but i dont care if the packs does not last as long since they are cheap to buy.
i tought that i was only pulling 7amps, why is it more because of a 5s pack ?
anyway, like you said, the ps should handle it...



Ready for a bit of math?

Charging a 5s pack at 7A requires up to 147w of power (4.2v/cell * 5cells * 7amps). Since the charger is not 100% efficient (typically around 80%), add in another 20% of power for 176.4w. That is the power the charger will pull from the power supply. Since the PS outputs 12v, the power supply will need to supply 14.7A (176.4w/12v).

Charging a 6s pack at 7A requires up to 176.4w of power (4.2v/cell * 6cells * 7amps). Again, to account for efficiency losses, add in another 20% for 211.68w. At 12v PS output, it has to supply 17.64A of current.

I say "up to X watts" because the most power a charger will have to output is just before the battery reaches 4.2v per cell and so is still in the CC phase of the charge. Before and after that point, the actual power required is a little less, but we always want to use worst-case to figure things like this.

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RE: How to convert an ATX power supply to rc use - 4/9/2012 9:50 PM   
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ORIGINAL: bgosselin

.............Charging a 6s pack at 7A requires up to 176.4w of power (4.2v/cell * 6cells * 7amps). Again, to account for efficiency losses, add in another 20% for 211.68w. At 12v PS output, it has to supply 17.64A of current.



And it gets worse! Your current demands appear to be loading the supply causing a voltage sag which in turn increases the current load. Using bgosselin's calculations of 211 watts, by the time your voltage has fallen to 10.5v, current requirements are over 20 amps.

Thermaltake claims efficiencies over 80%, even higher if you're on 230v mains (voltage setting on back). What does the spec placard list for 12V1 and 12V2 amperage and wattage - there are several models that are rated at 500W.

Drop your charge rate to 1C and try again.



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RE: How to convert an ATX power supply to rc use - 4/10/2012 2:29 AM   
ImpalaPolicePack



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Status: offline 		bgossellin, wow thanks for the precise explanation. this make sense but the PS should still be enough...
here is a couple pics i took tonight.
the 3 yellow wires, were actually 3 ''bundles'' but to make things easier i de-soldered all 3 and re-soldered 1 single bigger guage wire.
i used the small brown, with one orange for the ''stay on'' but i took any orange, is it supposed to be one in particular ?


andrew, im pluged in 110v main supply. no acces to 220v. i bought this PS to allow me to charge at a faster rate lol, i dont really want to drop to 1c.
look at my pictures to see the info your looking for (not sure what you want)

thanks again for helping me out, this is driving me crazy...

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RE: How to convert an ATX power supply to rc use - 4/10/2012 2:33 AM   
ImpalaPolicePack



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edit : ''all grounds from the same bundle'' are used as negative with the +12v.

should i use ALL grounds from ALL bundles ?

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RE: How to convert an ATX power supply to rc use - 4/10/2012 3:35 AM   
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Status: offline 		Thanks for the pics. Couple points I want to mention though:

1) The two heatsinks doesn't necessarily mean two rails. Actually usually one heatsink is for the AC mains side switching section (FETs, bridge rectifier, etc), the other heatsink (closer to the output wires) is for the output FETs and associated components.

2) To really see how many 12v rails there are, you'd have to pull out the PCB and follow the traces. Even though there are 3 sections, they could simply be tied together via copper traces underneath. However, there are three coils/caps per output wire, but that could just be paralleled/split to help handle the high currents without using larger (and more expensive) components. Again, a peek at the traces (especially before the coils) would tell you more.

3) All the grounds are the same point. Typically, I use the same amount of grounds as the yellow wires. So, if I bundle 6 yellows together, I'd also bundle 6 blacks together. Since all grounds go to the same point, it doesn't matter which ones you use - there are no separate 5v, 3.3v, and 12v grounds.

It is odd that the PCB has three 12v outputs labeled, but specifies only two on the outer case label.

I probably should go look at the ATX spec, but I assume the red/brown connection consists of the 5v line and the 5v sense wire tied together?

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RE: How to convert an ATX power supply to rc use - 4/10/2012 4:05 AM   
ImpalaPolicePack



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Status: offline 		yes for the sense wire with the 5v, all grounds are the same, so i can rule this out.
i will remove the board from the case and try to follow the traces, but even if i find that there is 1,2 or 3 rails, it should not matter to why it can't keep its voltage under load ?

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RE: How to convert an ATX power supply to rc use - 4/10/2012 4:49 AM   
Andrew


 

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ORIGINAL: bgosselin

It is odd that the PCB has three 12v outputs labeled, but specifies only two on the outer case label.

I probably should go look at the ATX spec, but I assume the red/brown connection consists of the 5v line and the 5v sense wire tied together?



I agree WRT to the three 12v outputs.

I think he meant orange/brown for his 3.3v to sense wire connection.

On your motherboard connector, do you have any pins other than orange that have a second wire? Most supplies have only a 3.3v sense wire, but I have seen a few that have sense wires for other rails.

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RE: How to convert an ATX power supply to rc use - 4/10/2012 7:17 PM   
ImpalaPolicePack



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Status: offline 		i only have the small brown wire in the same spot as an orange wire.
all other wires are alone in there spots.

does my brown wire needs to be with the SAME orange wire it was on the connector ? because i didnt labeled them before i cutted the connectors off. so its paired with any orange wire i found...

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RE: How to convert an ATX power supply to rc use - 4/10/2012 8:45 PM   
Andrew


 

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ORIGINAL: ImpalaPolicePack

i only have the small brown wire in the same spot as an orange wire.
all other wires are alone in there spots.

does my brown wire needs to be with the SAME orange wire it was on the connector ? because i didnt labeled them before i cutted the connectors off. so its paired with any orange wire i found...


No, what you have done is fine. With only a 3.3v sense wire, that eliminates one part of the puzzle.

It would really be useful if you would try a charge cycle on your 5s cell starting around .8C . According to the specification placard, your supply certainly has the capacity to carry a higher rate, but obviously it is sagging under the load. If it runs at .8C without a significant voltage drop, then up the rate. If it won't carry a .8C rate, you may have a faulty supply. I must confess that I'm running out of options here; maybe bgosselin has a suggestion that can help out.

andrew

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RE: How to convert an ATX power supply to rc use - 4/11/2012 2:49 AM   
ImpalaPolicePack



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Status: offline 		thanks for your support, i will try tomorow morning a .8c charge rate and see what it will do.
thanks again !

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RE: How to convert an ATX power supply to rc use - 4/11/2012 2:35 PM   
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quote:

ORIGINAL: Andrew

I must confess that I'm running out of options here; maybe bgosselin has a suggestion that can help out.



I'm out of ideas as well...

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RE: How to convert an ATX power supply to rc use - 7/3/2012 8:13 AM   
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Status: offline 		Hi Guys,

I Am converting my old PSU and have 4 1Ohm 10W Resistors and want to know if they should be hooked up in 4s or 2s2p. With 4s i get under 12.1v when i turn it on but that drops to 11.5v when i try to charge. The charger dose not give an error but I have not let it run for more then a min or so. Have not tried 2s2p yet as I don't know how many Watts each resistor will get like this.

Thanks


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RE: How to convert an ATX power supply to rc use - 7/3/2012 1:32 PM   
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Status: offline 		Connect them all in parallel. This should give you a better output voltage. Collectively they will draw about 2 amps which is 10 watts total.

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RE: How to convert an ATX power supply to rc use - 7/3/2012 3:38 PM   
bgosselin


 

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ORIGINAL: Red Scholefield

Connect them all in parallel. This should give you a better output voltage. Collectively they will draw about 2 amps which is 10 watts total.


That was probably a typo (?), but hooking all four 1ohm resistors in parallel will equate to 0.25ohm, which will draw 20A from the 5v line and generate 100w.

Personally, I'd start by putting just two or three of those 1ohm resistors in series (2ohm or 3ohm total). The more current you draw from the 5v line, the more stable the 12v line will be, but you are wasting more power as heat.

[EDIT] Now that I re-read it Red, you probably thought he had four 10 ohm resistors...

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RE: How to convert an ATX power supply to rc use - 7/3/2012 4:09 PM   
Andrew


 

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[EDIT] Now that I re-read it Red, you probably thought he had four 10 ohm resistors...



Even though he stated that his resistors were 1 ohm, it's possible that he intended to write 10 ohms. At 4s, I would think that a 4 ohm load would have been sufficient to hold his 12v rail above 11.5v when attached to his charger; 40 ohms would likely not be enough to prevent sagging out.

Perhaps we can get a clarification on the actual resistance.

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RE: How to convert an ATX power supply to rc use - 7/3/2012 5:04 PM   
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Status: offline 		Yeah, that's possible too. Either way, I wasn't trying to pick on Red or anything, just trying to prevent SirNoob from doing something incorrectly. Hopefully, he reposts the actual value. What a difference a simple 0 can make!

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RE: How to convert an ATX power supply to rc use - 7/3/2012 6:23 PM   
Andrew


 

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Hopefully, he reposts the actual value. What a difference a simple 0 can make!


Amen to that.

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RE: How to convert an ATX power supply to rc use - 7/4/2012 12:58 AM   
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Status: offline 		They are 1ohm 10w just double checked.
 
Could it be that this cheap multi meter is showing the wrong value?
Without any resistors the charger just displays an error saying incorrect input voltage.
I have an extra 4 1ohm resistors if that would help me get a better voltage.


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RE: How to convert an ATX power supply to rc use - 8/1/2012 10:52 AM   
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Status: offline 		Hi All ! I have done my power supply by simple way , and I can get 12.5 w/o load 12.16 w/load , I can charge 4 battery in one time using one quate charger in 5A without any problem , and the resistor I place it on a piece of big alu heatsince , running for 3 to 4 hours and warm only not hot not cool , our country weather now is very hot , about 31-34c , after I test it for over and over charge all my battery from two cell to four cell , no problem at all , but don know if charge up to six cell lipo , so  what I want now is to install the volt and amp meter on it , so that can see all the time , but problem is I don know how to wirie them up , can any body teach me how to do it , tks .

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RE: How to convert an ATX power supply to rc use - 8/1/2012 1:42 PM   
jam398


 

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Status: offline 		Hi Again , forget to tell my power supply spec , mine one is
Enermax 350w
+3.3v 32a   
+5v 32a
+12v 26a
Use 10ohm 10watt resistor on the 5v side and attach on a piece of big alu heatsince , size is 16cm x 10cm x 4.5cm without fan .
Supply power w/o load 12.3v  amp unknown not measure yet , but soon .
Supply power w/load 12.15v  watt unknown not measure yet , do it soon .
The charger that I use is HK quattro 4 x 6 200 watts
50 watts per ch

I charge all my battery from 7.4 to 11.1v without trouble and power keep on steady and normal , no over heating no burning resistor , everything work just find .

But I need someone to teach me how to install the amp meter and volt meter , so that I don have to use the test meter everytime , hope someone can give a helping hand . tks


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RE: How to convert an ATX power supply to rc use - 8/1/2012 6:14 PM   
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Status: offline 		Not sure about your specific charger, but many have a built-in voltmeter that will show you the voltage of the power supply feeding it. If so, you don't really need one on the supply itself unless you want it for other hookups.

But to answer your question:

A voltmeter is wired in parallel with the 12v output wires. So take the positive lead of the meter and hook it to the +12v output line. Then, take the negative lead of the meter and hook it to the ground output. It looks sort of like this: 

POWERSUPPLY(+) ======== OUTPUT_CONNECTOR(+) ===== (+)
                                               VOLT_METER
POWERSUPPLY(-) ======== OUTPUT_CONNECTOR(-) ===== (-)


An amp meter needs to be wired in series with the output. This is very important! This means you have to "break" the positive connection from the power supply wire to the output connector and insert the meter in series. Hard to explain, but it looks like this: 

POWERSUPPLY(+) ===== (+)AMP_METER(-) ===== OUTPUT_CONNECTOR(+)

POWERSUPPLY(-) =========================== OUTPUT_CONNECTOR(-)


To wire both up at the same time, do this: 

POWERSUPPLY(+) ===== (+)AMP_METER(-) ===== OUTPUT_CONNECTOR(+) ===== (+)
                                                                  VOLT_METER
POWERSUPPLY(-) =========================== OUTPUT_CONNECTOR(-) ===== (-)


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RE: How to convert an ATX power supply to rc use - 8/1/2012 10:15 PM   
Andrew


 

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Status: offline 		Nothing to add on wiring in your meters - bgosselin's diagrams are spot on.

Make sure your voltmeter is classified as DC. Some ampmeters are listed as AC or DC; some as DC only. Also ensure your ampmeter has a capacity greater than the amperage you anticipate drawing while charging. Since your 12v rail is rated at 26A, the minimum size I would suggest would be a 30A meter. If you select one with too high a capacity, the scale will be very broad and you will not be able to get a real accurate reading.

If the amp meter requires a shunt, be sure that it's included with the meter or order one when you get the meter itself. Some may indicate that a shunt is included or built in.



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RE: How to convert an ATX power supply to rc use - 8/2/2012 5:44 AM   
jam398


 

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Status: offline 		Hi Guy's ! Thanks for the info , will try it soon when get the volt and amp meter , thank you x 2

jim

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RE: How to convert an ATX power supply to rc use - 10/8/2012 2:28 PM   
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Status: offline 		Looking for discription and wiring diagram for wiring two in series for 24 volt 40 amp for 6S and up. I have two server power supplies at 12.5 volts and 47 amp and would like to wire these in series for the higher voltage.

Dennis

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