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› question for pc power supply users
12-25-2008 04:09 PM  8 years agoPost 1
fenderstrat

rrProfessor

Aston,Pa

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I had read an article where you can up the V on the +12 line by adding some load resistors to the +5 line. they said either use a 10w 10 ohm resistor or 2 1ohm 10W wired in series.so I tried the 2 wired in series and it ups the V nicely to about 12.4 but these things get SMOKING hot as in about 200 to 250 deg F,is this normal???

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12-25-2008 04:20 PM  8 years agoPost 2
dos boss

rrApprentice

Broken Arrow,​Oklahoma - USA

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Yes it is normal, I did the same thing and removed them because of this and I put a 12 volt tail light bulb on the 5v leads for a load. It does the same thing and doesn't put out near the heat.

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12-25-2008 05:12 PM  8 years agoPost 3
fenderstrat

rrProfessor

Aston,Pa

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yeah i just wasn't aware the resistors got that hot,I just measured 272 deg F after 2 charges on a cellpro4s which was only putting out 3a,thats way too hot for my comfort

I may try the 10w 10 ohm resistor and open up the PS to see if there is a V trim pot,if not an auto bulb may be next

ok,update:I tried a charge with the 10W 10ohm res and while it stayed much cooler,the voltage did not hold very well under load(which is rather small)it dropped to about 10.94V with the 4s putting out 3A,so,more experimenting

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12-25-2008 10:00 PM  8 years agoPost 4
Melnic

rrElite Veteran

Columbia, MD,USA

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Lower the ohm value, the more current of course.
2 ohm resistance (2 1 ohm resistors in series)
V = I x R
I = V / R
I = 5 / 2 ohms = 2.5A
P(Wattage) = I x V = 2.5A x 5V = 12.5 Watts (split up between each resistor if you are using 2 so that's 6.5W each)

12.5Watts is a good amount of heat for a small resistor!
A resistor can emit the max amount of wattage that they are specified for but how hot they get depends on the type of resistor. In all cases, the resistor must be allowed to dissapate the heat and not be covered up. Bigger the wattage resistor rating, the cooler the surface will be of course.

If you can put the resistor in the air stream of the P.S. Fan, that would be good.

No, you are unlikely to find a trim pot inside to change the voltage.

Multi output Power supplies have Minimum load specs on the primary voltage typically. That's why they are suggesting loading down the 5V. If there is not enough 5V current being supplied, the 12V won't regulate very well.

Car tail break lamp is a great idea. It also acts like an on Light

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12-26-2008 12:08 AM  8 years agoPost 5
tryan02

rrProfessor

Canton, Missouri

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been running mine unloaded for a year now loading the 5V doesnt really raise the voltage just optimizes it to actually get 12V from it.

No Really I am the club President!!!

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12-26-2008 12:50 AM  8 years agoPost 6
Melnic

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Columbia, MD,USA

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Tryan,
It all depends on the Power supply brand, model.
Some will not regulate the output voltage unless there is a minimum load on the primary output.

If the primary is not loaded, you will get one or more of the following:
1) Secondary voltages will not regulate within spec (usually 5%) and will run high or low. Most common.
2) Secondarys will have ripple voltages exceeding the specs (this can cause a computer charger to say there is something wrong w/ the input voltage. Very common.
3) Power supply will die after a period of time. Sooner than normal.
4) nothing, nothing at all.

I have 2 brands at work and both needed a minimum load on the 5V or they died after a few days of use (but I was using them 24/7).

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12-26-2008 01:24 AM  8 years agoPost 7
fenderstrat

rrProfessor

Aston,Pa

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the one I just got ,here are exact numbers

with NO load on the 5V the +12 is about 11.7 V and holds voltage poorly,at about a 3A draw its below 10V

with 1 10W 10ohm res its about 12.2V

with 2 10W 1ohm res its 12.4V and will hold V up to 10 amps

now, one more question for you experts.are these PS's rated conservatively or at their max?? meaning if its rated for 10A on the +12 line will running short bursts at 12A kill it fast,or can they handle it

I really want to start charging 2 batts at 3c,I think its time I got a real PS,converting the PC Ps's was cool for the 4s(works great) but I think its time i stepped it up a notch for the 10S

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12-26-2008 04:04 AM  8 years agoPost 8
tryan02

rrProfessor

Canton, Missouri

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I think its time I got a real PS
YEP

Melnic I have an old PC supply no logic on and off switch with good regulation. Gotta go back to the 75-133MHZ era. I realize the loading issue but hey thats why I bought a cellpro 4S it will tell me with a alarm that the input voltage is too low.

been using the supply and charger for I would guess 800-1000charges.

I am not disagreeing with you mel dont take this wrong just letting others know its a crapshoot can work fine or not.

But I only charge 3S 210omah packs at 2C.

No Really I am the club President!!!

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12-26-2008 04:35 AM  8 years agoPost 9
Melnic

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Columbia, MD,USA

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Yes, crapshoot. I agree.
But older ones are actually better as you have found. Older PCs pulled more 12V than newer ones do.
The newer ones ATX supplies (where you have to short the green to black to make them work) are more picky.

Generally, the wattage is a total wattage of the P.S. for all voltages (300W for example). Some P.S. manufacturers will show how much max for each voltage, others won't.
They also rate them for room temperature with no additional fans.
The total wattage is derated for a higher than 80F ambient temp.
Airflow is important. 80F ambient is usually where they rate full load (full total Wattage). If you are pulling less of 5V you can pull slightly more 12V then what the P.S. says. The ATX power supplies usually will go into current overload if you pull too much on any one line(which is recoverable by turning the power off for 1 minute or more).

The newest power supplies want to supply a bunch of 3.3V too so they get even worse.

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12-26-2008 04:41 AM  8 years agoPost 10
tryan02

rrProfessor

Canton, Missouri

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I got schematics of supplies I can scan into PDF for audio amps with no ripple and perfect regulation.

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12-26-2008 01:58 PM  8 years agoPost 11
Melnic

rrElite Veteran

Columbia, MD,USA

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Linear supply?

BTW, does anyone use a generic unregulated automotive Battery charger?

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12-26-2008 02:13 PM  8 years agoPost 12
tryan02

rrProfessor

Canton, Missouri

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Yes linear easy to build over switching. XFMR, bridge rectifier, and big honkin caps to kill the ripple. Pretty much all there is too it. Unless you want to get into a high end audio Pwr supply then you gotta go a lot farther than just a few caps to kill the ripple!

I use a number of little sealed gel cells on my 4S so I tried to use my battery charger to my 4S (cut out the middle man) thats a no-go for my charger or most of todays smarter battery chargers. It instantly saw that there was no +v on the load and gave a dead battery fault and refused to turn on. And my charger is just a cheapy $20 one from walmart.

No Really I am the club President!!!

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12-26-2008 02:53 PM  8 years agoPost 13
tryan02

rrProfessor

Canton, Missouri

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XFMR, bridge rectifier, and big honkin caps to kill the ripple. Pretty much all there is too it.
pardon me thats all for an unregulated or poorly regulated like the PC power supplies. You can buy a XSTR regulator and call it good or go the long route and even add some temp comp regulation for the high current stuff.

No Really I am the club President!!!

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12-26-2008 04:51 PM  8 years agoPost 14
Gregor99

rrElite Veteran

Western Wa

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BTW, does anyone use a generic unregulated automotive Battery charger?
I tried it with a 25 year old automotive charger. The charger has 3 current settings, which are actually "voltage settings". As its unregulated, as the current goes up the voltaged goes down. With the 4s, using the charger on the lowest setting, it was fine until the charger started pulling some current. Then it would drop to below 10v. On the middle setting, without any current being drawn, the voltage was above 16v. Since the 4s shuts off if the voltage is not between 10v and 16v, I gave up. I ended up using an old laptop power supply until I got the 10s.

Now I'm using a deep cycle battery and a more current automotive charger. I usually charge from the deep cycle battery and periodically charge it with the automotive charger.

http://www.harborfreight.com/cpi/ct...temnumber=95535

If you have no plans of charging at the field, then a powerful PS will do the trick. The 10s works best (especially for larger packs) at input voltages higher than 12v. Here's a regulated PS I was looking at. I like it because you can adjust the voltage up to 15v and get the most out of the 10s.


http://www.sonicelectronix.com/item...mid+PS26KX.html

Here are a few less expensive options.

18 amp $80
http://www.sonicelectronix.com/item...mid+PS21KX.html

14 amp $45
http://www.sonicelectronix.com/item...wer+Supply.html

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