Hello All,
My Fezes have a regulator on them which takes between 7 and 12 volts. I have a regulated 5 volt supply (ATX PSU) that I want to use with it. Now I know that I can bypass the regulator and inject power directly to the device, however what does happen when you connect 5 volts to a 5 volt regulator? Does it out put 5 Volts?
It will put out less than 5V and the output will be unregulated.
If you look at something like a LM1117MP it has a Dropout Voltage of 1.4 V at 800 mA so you will need a minimum of 6.4v to get 5v
Also keep in mind that your PC power supply will require a minimum load before it regulates properly (as do most multi output switch type power supplies.) I have an old PC power supply that I made into a bench supply and to get around this fact I added a couple of power resistors bolted inside the case and wired to the 5V part of the supply. It may take 10% or so of the rated load being applied before the power supply regulates properly. You can experiment and see what works for your supply though.
@ Jeff_Brit - really a minimum load? that sucks!
I don’t suppose I can connect Power Bricks in parallel?
Well at least this answers my regulator question.
Can you inject 5V into USB power path?
There are linear regulators with VERY low dropouts that can be used, but there is always a dropout.
As for switching supplies, the minimum load is sometimes required, but not always. Intel specifies in the ATX spec different minimum loads for different rails, but most supplies perform better than the spec requires under smaller than minimum loads.
Some supplies simply won’t start up, some won’t regulate properly, some will work just fine. If you do need a minimum load, just grab yourself a simple power resistor, and you’re good to go.
@ WouterH, I believe that devices like the Fez Domino and Fez Rhino, etc have a pin that allows you to bypass the regulator and inject 5 volts into the device.
and a wire to start the ATX PSU: http://www.bleedinedge.com/forum/showthread.php/29759-Jump-Starting-ATX-Power-Supplies
What about using 12V from the PSU?
If the "regulator kurtnelle is talking about is a LDO, it will get very hot. If it’s a switching one it will be fine.
More specifically, you can calculate the power dissipated as P = (Vin - Vout) * IL. That means if Vin is 12V and Vout is 5V, and you are drawing 500mA, (12-5) * .5 means you’ll be dissipating 3.5W of power. An LM317, for example, should never dissipate more than 1/4 watt without a heatsink. Note that the SMD regulators on your Panda/Domino/etc board are not easy to heatsink.