Question for the gurus

[smurf]

I have a computer power supply I converted for my chargers. It has a 250 watt output.
From that information, can anyone tell me how many amps I should be able to get from it?
I can go up to 4 amps on the charger but anything much above that pulls the voltage down below what the charger will allow.
I read in one of the "how to" threads on bringing the voltage up by installing (2) 1 ohm resistors in series across the +5v and ground. I have a +5 terminal post along with a +12 and ground on top of the power supply.
Is there any other easy way to raise the voltage like hooking a light bulb up across the +5v and ground post?

thanks for you help

[Chippie]

The issue with using a computer psu for R/C work is that the 5v rail is the rail that is used to control the power supply's output.

Even with a 12v lamp attached to the 5v output, this is a fixed load, what you are doing is loading up the 12v output which is not in the regulated loop, it merely 'tracks' the 5v output...


What is required here is the regulation of the 12v rail, even then you will be limted to the amount of power you can draw because of the limitations of the rectifier diodes...probably rated at 8 amps....not close enough to 20 amps to get you to 250 watts....


there was an article in EPE mag some while back that went into the conversion process, but I would deem this as a project for those who have the technical skill...it involves dangerous voltages which can be lethal..

[smurf]

This guy is getting 19 amps at 13.5v with a 350 watt power supply. http://www.rcgroups.com/forums/showthread.php?t=686369

It would be nice to get close to that, if not I'll have to buy one of his.

[Probedude]

As Chippie said, you can only do it by loading the 5V line enough. You can parallel more light bulbs, or resistors - they'll do the same thing.

Regarding how many amps you can pull from a 250W supply - that really depends on the supply itself. The 250W rating is for all the outputs sourcing current. There should be a sticker on the side that says what the max amps that can be sourced from the 12V line alone.


FYI, Don't know if you have a Hobby People, but they're going to sell the Ripmax 20A supply with display for $59.99 over the Memorial day weekend sale.

Or there's this one.
http://www.circuitspecialists.com/prod.itml/icOid/9099

Dave

[rmteo]

Here is a dual 12Vx17A power supply for $60:
http://www.allelectronics.com/cgi-bin/item/PS-411/480/SWITCHING_POWER_SUPPLY,_400W_.html

Input: 100-240 V, 7A, 50/60/400Hz
Outputs:
5.00VDC @ 35.0A
12.00V @ 17.0A
12.00V @ 17.0A
10.75" x 5.00" x 2.52".

[alexcmag]

PC power supplies are designed to be stable at 5V (TTL level), so 12V is a secundary output.

They have an internal PWM regulator that takes the 5V output and uses as a feedback for the PWM.

If you draw too much current on the 12V output of course the PWM ratio will not change because it is based on the 5V output.

You can modify the power supply, isolating the feedback from 5V and wiring it from a voltage divider between 12V output and ground.

This is an excelent tutorial about how to to this, it is in spanish but has a lot of pictures: http://picayzumba.com/contentid-68.html

In my case I used a 10K potentiometer instead of resistors, so I can adjust the output when I want to.

I usually let it set to 13V to most chargers, but it is good to have a variable output on bench, so I can test power of motors at different voltages (from 5V to 17V in my modified power supply).

[smurf]

Thanks for everyones help.

[GlowFly]

Here is a dual 12Vx17A power supply for $60:
http://www.allelectronics.com/cgi-bin/item/PS-411/480/SWITCHING_POWER_SUPPLY,_400W_.html

Input: 100-240 V, 7A, 50/60/400Hz
Outputs:
5.00VDC @ 35.0A
12.00V @ 17.0A
12.00V @ 17.0A
10.75" x 5.00" x 2.52".
I wonder if that's a typo?

2x 12v@17A is 408W without ANY load on the 5v line. Whereas 5v@35A + 12v@17A is 379W or pretty close to the published 400W designation.
--
Steve

[nick_cool]

Try putting a 22ohm from +5v to ground. This should be enough to stabilize the 12v