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RE: How to convert an ATX power supply to rc use - 9/1/2007 6:28 AM   
cjg



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Status: offline 		Instead of a resistor I used a 12v automobile taillight lamp on the 5v for load. Not only does it put out an even 12.01v, but the red taillight inside provides the power on indicator.

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RE: How to convert an ATX power supply to rc use - 9/1/2007 11:01 PM   
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quote:

ORIGINAL: Hack73
I noticed that the sandbar resistors get really hot.
*Question - Is there a way to keep a load without the excessive heat from it?



looks like you did a great job on your mod =)
about the resistors, a couple of things you can do, but a few in series, this will lower the total resistance and also the total heat, or put a few in parallel, that will divide the heat equally between all of the resitstors.

I used 4 1 ohm resistors in series in my PSU to get around 12.2 with about 1A load, or right around 12v with a 10A load.

the formula for figuring out heat on this circuit is 26.5 divided by resistance, so if you use 1 ohm, then you will wind up with 26.5w of power dissapation, which will burn out the 10w resistor, 2 will give you around 13.25w of power which will also burn out the resistor, 4 will give you 6.625v or about half what the resistor is rated for, which if sunk to the side of the case will give you a pretty good temp range of about 100F

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RE: How to convert an ATX power supply to rc use - 9/6/2007 7:36 PM   
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Status: offline 		I have an old AT supply from a computer. It's a 250 W model rated at 10amp on the 12v rail. Being an old AT supply, it is powered on via a switch and does not require a load or any connections to stay on.

I read 11.97 volt with no load, but it jumps to 12.54 with a standard hard drive plugged into it.

I believe that this unit can be used to drive most chargers on the market today by simply providing a way to connect the common and +12volt leads from one of the standard 4 pin molex connectors to my charger (I plan to use bananna plugs for this). Am I correct in thinking that this will work that easily, or will more be required?

I figure there is no need to solder resistors on an AT supply, as the charger running on the 12 volt rail should provide adequate load, and an AT supply does not require a load in order to stay on. Is this correct?

If this will work it will save me a bit of money on buying a charger. I need something that will let me charge both at home and at the field, but I don't really feel like sinking the money into a 989 or buying a separate hobby 12 volt supply.

< Message edited by tommythecat -- 9/7/2007 12:48 AM >

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RE: How to convert an ATX power supply to rc use - 9/10/2007 8:03 AM   
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Status: offline 		sounds like you are good to go to me as long as your charger can run on the power that the PSU puts out. If you are planning on using too many amps you may get an undervolt. Just watch the voltage when you start to put a load on the charger, and make sure that it stays a good amount above the min voltage the charger can operate at =)

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RE: How to convert an ATX power supply to rc use - 9/11/2007 11:26 AM   
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quote:

ORIGINAL: Hack73

I am using (2) 10ohm 10watt resistors in parallel for my PSU. Should I change to (4) 1ohm 10watt resistors in series instead or add another (2) 10ohm 10watt resistors and place them in series?



hrm, the way I figure it two 10 Ω resistors in parallel, is going to give you basically a net resistance of 5 Ω, that's why the voltage went up when you added the second 10 Ω resistor and consequentally so did your amp draw and total heat (but the heat per resistor stayed the same). So four 1 Ω resitors in series is going to give you more current draw than the two 10 Ω resistors in parallel, but the temp of each resistor is going to be less, because it will be split between 4 resistors instead of 2.

Basically the short answer is you can go either way, 2 series 2 parallel 10Ω resistors is going to give you a net resistance of 10 Ω, which will give you the 12.30v with a load and w/o 12.5v

four 1 Ω resistors will give you a net result of 4 Ω, which will give you a little better than 12.65v w/o load & 12.5v, and will be less heat per resistor because it will be split between 4 resistors instead of 2.

You've already got the two 10 Ω resistors though, so it is going to be less expensive to just get another two of those, vs 4 of the 1 Ω resistors, so it really all depends on what voltages you are looking to get, and if you want to buy some extra parts or not =)

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RE: How to convert an ATX power supply to rc use - 9/11/2007 11:33 AM   
mrasmm


 

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Status: offline 		I forgot to include the power dissapation by the resistors

the single 10w 10Ω resistor is going to give you a power dissaption of 2.7w total

the 2p arrangement of the 10w 10Ω resistors is going to give you a net resistance of 5 ohms for a total of 5.3w or 2.7w per resistor

the 2s2p arrangement of the 10w 10Ω resistors is going to give you a net resistance of 10Ω for a total of 2.7w or 0.7w per resistor

the 4s arrangement of the 10w 1Ω resistors is going to give you a net resistance of 4Ω for a total of 6.6w or 1.7w per resistor.

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RE: How to convert an ATX power supply to rc use - 9/12/2007 3:15 AM   
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Status: offline 		My supply is a bit different, but I am going to give it a try.


< Message edited by Stewped -- 9/12/2007 5:00 AM >

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RE: How to convert an ATX power supply to rc use - 9/14/2007 12:12 AM   
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Status: offline 		I converted an Antec SL300S for field use. I only used the 12V and 5V outs since my use will be my charger and for bench testing my receiver and servos.
I did find one glitch. I hooked up the 3.3 sense to the 3.3v out and the 5v sense to the 5v. In this configuration I had voltage but no current. I removed the 5v sense and it latched on perfectly. I also found that if you hook the 3.3v sense to the 5v out the 12v out will put out about 14.6v. However I just removed the 5v sense wire and left it undone. I placed a 12V marker light as a power on indicator on the 5v terminals.
I now have 13.8v - 13.9 at the terminals. The rating for this PS is 15A at 12V. However I have been running it under an 18A load and it performed perfectly with a 13.11V output.
I loaded it up with two beacon rotating lights (6.2A each) in parallel. As you can see in the picture the voltage never fell below 13.18V and it ran for over 3 hours and the fan voltage never went above 6.17. The fan is controlled by the PS circuit and will speed the fan up if the heat sinks start warming up, however it never got warm or sped up.
The lights will peak at 15A when first turned on and settle in about 13.4-13.6A. My field charger (Triton) uses a max of 13A. That should be quite good. I did load it up to over 20A for about 15 minutes with no problems and no overheating. Seems like a good project.
I also converted a newer style AT that works well also. However it was only rated at 8A at 12V but it will put out 15A with no problem.
Pictures are inclosed. The only thing I spent money on were the terminals and the $1.00 marker light. The PS was a pull out of my computer when I upgraded to a larger supply.
Thanks for looking.
Any questions please ask!


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RE: How to convert an ATX power supply to rc use - 9/14/2007 11:15 PM   
mrasmm


 

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quote:

ORIGINAL: Stewped

My supply is a bit different, but I am going to give it a try.



quote:

ORIGINAL: Stewped

I have two 7.5 ohm 10 watt resistors and two 15 ohm 10 watt resistors. Is there a way to make this work with what I have?

I also have 20 and 25 ohm.

I ran all these numbers through my head and its starting to get fuzzy.


Thanks,

Andy



depending on the resistance you want, you could go with the two 7.5 Ω resistors and the 10 Ω resistor in parallel series, what I mean is take a 7.5 Ω and a 10 Ω and put them in series, do this with both sets, then run the two sets in parallel, you'll get about 8.75Ω of net resistance or 0.58A of load, which will give 3w total load/heat dissapation, or about 1.5w per side, and about 0.86w for the 7.5Ω resistor and about 0.63w for the 10Ω resistor

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RE: How to convert an ATX power supply to rc use - 9/30/2007 5:05 AM   
hands without shadows



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quote:

Hook up the batteries BEFORE you plug in the ATX to the wall and when charging is done, unplug ATX from the wall first and then take those batteries out.


That will damage some chargers and could overdraw small batteries.

quote:

CPU fans are 5v


From my experience they come in 12v and 5v variations.

quote:

resitor is not a good idea anyways since it creates lot of heat


They get slightly warm.......and they are cheap, worryless, easy to obtain, and small.

quote:

If you still cannot live without a resistor and need a heat source in cold weather, hook it up to 5v.


Kinda like I described?


I was hoping that since noobs dont visit this forum too often I wouldn't get any BS info like in the quotes above. We know what we are talking about.




quote:

ORIGINAL: edwal07
I also found that if you hook the 3.3v sense to the 5v out the 12v out will put out about 14.6v. However I just removed the 5v sense wire and left it undone.


Interesting. Have you tried running at that voltage for extended periods of time? Or drawn any real load off it at that voltage?

quote:

I noticed that the sandbar resistors get really hot.
*Question - Is there a way to keep a load without the excessive heat from it?


You can use thermal epoxy and zipties to secure the fan to the case in front of the fan. I found that mine never got warm enough to need epoxy but situations differ.


Hack73: I really like the stainless look of yours, and the cutout above the fan looks sick! Great job.

< Message edited by hands without shadows -- 9/30/2007 5:09 AM >

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RE: How to convert an ATX power supply to rc use - 10/7/2007 10:15 PM   
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Status: offline 		What about the light bulb method? Any comments on using those? The one guy said his was ok with it, I assume its a 1157 bulb? Im heading to my electronics shop now and will grab the 4 1 ohm resistors anyway just incase the bulb idea is no good. Thanks.

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RE: How to convert an ATX power supply to rc use - 10/9/2007 3:14 AM   
mrasmm


 

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quote:

ORIGINAL: Savage03

What about the light bulb method? Any comments on using those? The one guy said his was ok with it, I assume its a 1157 bulb? Im heading to my electronics shop now and will grab the 4 1 ohm resistors anyway just incase the bulb idea is no good. Thanks.



yeah, the lightbulb method is fine =) Not sure on the bulb number, I know it's been posted here and there a few times. Alot of people like the bulb because it shows them that the PSU is on. Just get one that is good for a good one amp draw at 5V.

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RE: How to convert an ATX power supply to rc use - 10/9/2007 5:23 AM   
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Status: offline 		Did nt get a chance to get a bulb but made it to the electronics store. Now they didnt have the 1ohm resistors but the old timer knew what I was doing and said this little bugger would do the trick? Its pretty kool looking with the heatsink around it and it matches my gold PS! Now the thing im wondering from you pros is its a 8ohm 25watt, think it will be ok?



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RE: How to convert an ATX power supply to rc use - 10/9/2007 6:25 AM   
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Now the thing im wondering from you pros is its a 8ohm 25watt, think it will be ok?


Eight ohms should be sufficient. If you're not satisfied with the voltage level, you can add a second 8 ohm in parallel to the existing resistor -- that will provide a 4 ohm load.



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RE: How to convert an ATX power supply to rc use - 10/9/2007 7:45 AM   
Savage03


 

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Status: offline 		Can anyone maybe give me some pointers? I hooked everything up like described in here, least I hope but I get nothing. Here are some pics of what I got going so far.




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RE: How to convert an ATX power supply to rc use - 10/9/2007 7:51 AM   
mrasmm


 

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Status: offline 		yeah, what he said =)

8 ohm current will run about 0.64A, and 4 ohm will run 1.29A, and run 3.32W, and 6.63W (3.32W per resistor If you run two 8Ω in parallel) respectively. The heat sink should be real nice, hopefully it didn't set you back too much though.

looks good to me, you've just got to connect the grey power ok to a red 5v line and the green one to a black line.

for more info see here (from red's site)
http://www.marcee.org/Articles/PCPowerSupply.htm

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RE: How to convert an ATX power supply to rc use - 10/9/2007 8:09 AM   
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Status: offline 		Hrm can the grey goto the the red I have on the resistor? I tried touching it to the other reds that I cut off already but nothing happened.
The resistor was cheap, $1.75

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RE: How to convert an ATX power supply to rc use - 10/9/2007 8:15 AM   
Savage03


 

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Status: offline 		Well ok then, took the binding posts off and it worked! The meter is showing 12.28 with no load, fott ago get a charger to see what happens. Is this good so far?

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RE: How to convert an ATX power supply to rc use - 10/9/2007 9:54 AM   
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Status: offline 		sounds good to me, you'll have to see what it comes down to once you put a load on it =)

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RE: How to convert an ATX power supply to rc use - 10/9/2007 10:02 AM   
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Status: offline 		Ok got it running, had to get the posts isolated from the case. Charging a lipo now at 5amps and its showing 12.02v on the meter. Good?

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RE: How to convert an ATX power supply to rc use - 10/9/2007 11:11 AM   
Savage03


 

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Status: offline 		All back together, just waiting to see if I need another resistor but the one I have now is bolted under the top of the case, you can see the 2 screws where its mounted at. It is pretty much cold to the touch, plus the fan is right there blowing on it and in the rear of the PS is another fan sucking air out. I had some thermal grease from my pc upgrade so used some of that as well
Thanks for the help so far


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RE: How to convert an ATX power supply to rc use - 10/10/2007 4:02 AM   
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Status: offline 		Hey Guys, I need some help-I'm making a DC power sourse from a computer power supply. I have read all types of helpfull info and have even done this before but this time I got a Dell power supply ( circa 2000) and I can't figure out which wire I need for on/off function. This model of PS does not follow the industry standards so I don't know what colors to use. Anyone out there have experience with an old Dell?

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RE: How to convert an ATX power supply to rc use - 10/10/2007 4:07 AM   
Savage03


 

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Status: offline 		Just picked up another resistor so will hook that up and see what volts I get then. One last thing, how do you guys measuer what amps your getting from the PS?

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RE: How to convert an ATX power supply to rc use - 10/10/2007 4:27 AM   
Andrew


 

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....... I got a Dell power supply ( circa 2000) and I can't figure out which wire I need for on/off function. This model of PS does not follow the industry standards so I don't know what colors to use. Anyone out there have experience with an old Dell?


I have the Dell pinouts at work -- if the answer doesn't show up tonight, I'll post them tomorrow.

andrew


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RE: How to convert an ATX power supply to rc use - 10/10/2007 6:52 AM   
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Status: offline 		Just put a load on it with 2 resistors and its showing 12.11v, good grief more?




< Message edited by Savage03 -- 10/10/2007 8:48 AM >

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