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Brians256 · 11-12-2002, 11:28 AM

It's not that the 12VDC line is tied to the 5VDC line, really. What happens is that the PSU has a protection circuit on the 5VDC line. If the 5VDC line has no load, the PSU assumes that it is not connected to the computer and so it shuts down.

It goes back to switching power supply designs. It may not occur in the newer design styles (e.g. current sensing regulation instead of voltage sensing designs), but a zero or near zero load for a switching power supply is dangerous. The switch attempts to switch on and off so quickly that really high voltages are developed from the inductor, and PSU IC's can be damaged by that. The voltage spikes would not escape the PSU filtering capacitors, but the voltage regulation circuitry in the PSU might become damaged.

To overcome that, simply put a resistor from +5V to ground. Different PSU's require a different threshold load, so you may be able to get away with a 1W 47Ohm resistor or you may need a 5W 10Ohm resistor.

To pick the right resistor, remember Ohm's law (in a couple of variations):
Current = Voltage / Resistance.
Resistance = Voltage / Current
Voltage = Current * Resistance

For calculating the resistor size, choose a wattage above your actual power draw, or you will burn out the resistor. The higher the wattage resistor, the bigger and more expensive it will be. However, it will also be longer lasting.
Power (Watts) = Voltage * Current

For example, a 47Ohm Resistor on the 5VDC line would need to dissipate:
Current = 5VDC / 47Ohm = about 0.106 Amps
Power = 5VDC * 0.106 Amps = about 0.532 Watts
So, pick a resistor of 1W or better. A 1/2W resistor would probably fry itself after a bit.

Back to PSU's. If your PSU may require 100 milliamps (0.1 amps) of current on the 5VDC rail, try this:
Resistance = 5VDC / 0.100Amps = 50 Ohms
Looking at the above example, you can see you would need something like a 1W resistor in this case. If you needed more of a load current to get the PSU running correctly, you would drop the resistance and increase the wattage of the resistor.

Remember, none of this dummy load stuff is needed on a normal computer because the hard drives and fans will load up the 5V line much more than 1A.

11-12-2002, 11:28 AM	#9
Brians256 Pro/Staff Join Date: Oct 2001 Location: Klamath Falls, OR Posts: 1,439	It's not that the 12VDC line is tied to the 5VDC line, really. What happens is that the PSU has a protection circuit on the 5VDC line. If the 5VDC line has no load, the PSU assumes that it is not connected to the computer and so it shuts down. It goes back to switching power supply designs. It may not occur in the newer design styles (e.g. current sensing regulation instead of voltage sensing designs), but a zero or near zero load for a switching power supply is dangerous. The switch attempts to switch on and off so quickly that really high voltages are developed from the inductor, and PSU IC's can be damaged by that. The voltage spikes would not escape the PSU filtering capacitors, but the voltage regulation circuitry in the PSU might become damaged. To overcome that, simply put a resistor from +5V to ground. Different PSU's require a different threshold load, so you may be able to get away with a 1W 47Ohm resistor or you may need a 5W 10Ohm resistor. To pick the right resistor, remember Ohm's law (in a couple of variations): Current = Voltage / Resistance. Resistance = Voltage / Current Voltage = Current * Resistance For calculating the resistor size, choose a wattage above your actual power draw, or you will burn out the resistor. The higher the wattage resistor, the bigger and more expensive it will be. However, it will also be longer lasting. Power (Watts) = Voltage * Current For example, a 47Ohm Resistor on the 5VDC line would need to dissipate: Current = 5VDC / 47Ohm = about 0.106 Amps Power = 5VDC * 0.106 Amps = about 0.532 Watts So, pick a resistor of 1W or better. A 1/2W resistor would probably fry itself after a bit. Back to PSU's. If your PSU may require 100 milliamps (0.1 amps) of current on the 5VDC rail, try this: Resistance = 5VDC / 0.100Amps = 50 Ohms Looking at the above example, you can see you would need something like a 1W resistor in this case. If you needed more of a load current to get the PSU running correctly, you would drop the resistance and increase the wattage of the resistor. Remember, none of this dummy load stuff is needed on a normal computer because the hard drives and fans will load up the 5V line much more than 1A.