Clint H

My Feedback: (1)

I have a question I was hoping an EE or other electronics expert could help me with. A recently converted a "new-style" AT power supply (one with a push switch) for RC use. It works fine for charging most of my batteries, but it just doesn't have the power when I try to charge one of my 3S 2100 mah lipos. I've read about applying a simulated load between the +5V and the ground wires to increase output, but there seems to be many opinions about the amount of resistance to use. I've heard everything from 2 ohms to 1000 ohms of total resistance. So, my question is:
What resistance should be used to avoid damage to the power supply?

Thanks,
Clint

EloyM

My Feedback: (194)

I have converted a number of such supplies, all with 10 ohm 10 watt resistor loads on the 5V line. I always mount them on standoffs to the fan frame, so they get the full benefit of the cooling. Luck!

Andrew

When you state that it doesn't have the power, what do you mean? Are you experiencing a voltage drop so your charger shuts down with a low voltage error? BTW, the ATX supplies use a temporary push button to latch the PSU from the DC side - the PS_ON line is held low by the motherboard circuitry. The typical conversion replaces the board with a simple switch. The older style AT supplies used a standard make/break switch on the AC side.

Some of the newer design ATX supplies will maintain latch without a preload, but output voltage on the 12v rail will be lower in many cases and will also tend to sag under load. A preload on the 5v rail helps maintain stability and will reduce voltage sag on the 12v rail as the charger applies a heavier current demand.

Generally, start with a 10 ohm, drop to 5, then to 2 ohms on the 5v rail. If your supply cannot maintain sufficient output to drive your charger at 2 ohms, then you may either have to search for a higher wattage PC PSU or look for a commercial DC supply, usually in the 13.8v range. Resistor ratings higher than 10 ohms may not be effective. There is no real gain in dropping the resistance below 2 ohms as far as voltage stability is concerned and anything lower will simply consume PSU capacity and generate more heat.

Clint H

My Feedback: (1)

Thanks for your replies. I'll start with 10 ohms and drop down from there. The power supply is rated at 200W and states on the label the +12V can deliver 8A; however, when I try to charge a 3S lipo at 2.1A, the charger gives a low voltage error. The error doesn't occur immediately. Normally, it charges the pack to about 12V and then throws the error. Under no load, the power supply registers 11.1V, but this quickly drops to about 10.5V during the charge process. As a test, I tried connecting a 4-ohm light bulb for a few seconds and gained about 0.5V.

Also, as far as I can tell, this is an AT style power supply. I was going by Pat Harvey's description on RCbatteryclinic.com. It has the wiring of an AT but the power switch is a push button on a cord that ran to the front of the computer case. Pat called it the "newer" AT. This particular one doesn’t have a PS_on line.

Thanks again for the help,
Clint

Andrew

The addition of a resistor will likely help with your problem. Use the highest rating that will work for you since any additional load simply wastes power. If you can find a 5 ohm resistor, start with two resistors in series - this will give a 10 ohm load. If you need to drop the rating, use a single 5 ohm. Your next step would be two 5 ohm resistors in parallel - this configuration gives an effective 2.5 ohm load. If your PSU still drops off too much, then you'll have to look elsewhere.

Good luck.