SparkFun Forums

[Chip Douglas]

Im working on a project where i want to run a fan from a 9v source and a 5v source.When the device is running it will be supplied 9v and when turned off it will automaticly switch to a 5v source thats always active.

What would be the best way to do this?What would be the term for such a device and where could i find schematics?

Ive been searching the past few days with no luck.

thanks

[riden]

Will a pair of diodes (one in series with each supply output) do the trick?

If you need 5v then you must have some type of voltage regulation. What is the voltage requirements of your device and what type of voltage regulation do you have?

[Chip Douglas]

Will a pair of diodes (one in series with each supply output) do the trick?

If you need 5v then you must have some type of voltage regulation. What is the voltage requirements of your device and what type of voltage regulation do you have?

I have two seperate sources. One 12 volt source running to a LM7809 outputting roughly 9 volts and another source outputting 5 volts. When the device is turned on the power supply gives it 12 volts that is reduced to 9 volts with the LM7809, and when the device is turned off the power supply gives it 5 volts. Both sources are on a seperate rail.

Basicly i want to supply the fan the 9 volts when its powered on, and switch to the 5 volt source when its turned off.Im not sure exactly how to accomplish this.I have a few different types of diodes and NPN and PNP transistors laying around and some PIC microcontrollers.Whatever is the safest method would be best.

[riden]

I have a few different types of diodes and NPN and PNP transistors laying around and some PIC microcontrollers.Whatever is the safest method would be best.

I could use a little more information about your device and why you power it with two different voltages. My guess is that you want to run a cooling fan at the higher voltage when the device is on.

One more go around before I offer my suggestions...

Does your device have a 9v input and another separate 5v input, and you want to find a way of selecting one of the two inputs to use to power your device?

Or do you want want to switch between 9v and 5v applied to the same input?

Or is this some type of PC board that takes the 9v and regulates it to 5v? If the 9v is missing, you want to bypass the 5v regulator and use the the external 5v to power your device?

Sorry for all the questions, but I don't have a clear and detailed picture of the device in question.

[Chip Douglas]

I have a few different types of diodes and NPN and PNP transistors laying around and some PIC microcontrollers.Whatever is the safest method would be best.

I could use a little more information about your device and why you power it with two different voltages. My guess is that you want to run a cooling fan at the higher voltage when the device is on.

One more go around before I offer my suggestions...

Does your device have a 9v input and another separate 5v input, and you want to find a way of selecting one of the two inputs to use to power your device?

Or do you want want to switch between 9v and 5v applied to the same input?

Or is this some type of PC board that takes the 9v and regulates it to 5v? If the 9v is missing, you want to bypass the 5v regulator and use the the external 5v to power your device?

Sorry for all the questions, but I don't have a clear and detailed picture of the device in question.

The device has a 12 volt and a 5 volt output.The 5 volt output is always powered.The 12v is only powered when the device is on.I reduced the 12 volt to 9v with a LM7809 9v regulator so it outputs 9 volts.I want to run a fan @ 9v while the device is running, then switch the fan to the 5v output when the device is turned off so the device can still cool itself down for a short amount of time before i unplug the power supply from the wall.

Sorry i didnt want to confuse the situation by asking how to do something on a specific device so i figured i would just say a device with such and such voltage sources as a general idea as to how to accomplish it.

Its a xbox 360 with auto adjusting 5v,7v,12v fan output and a 5v standby output.The system has overheating issues, that im trying to prevent.I bypassed the auto adjusting feature to run @ 12v and I reduced it to 9v with a regulator.I want the fan to still run when the system is turned off, so i want to use the 5v standby source so it can let some air out after i turn it off so i need to figure out how to have it switch to the 5v source.Sorry if you didnt understand what i am trying to do.

[riden]

You can connect a diode in series with each output (anode to the power supply side), connect the cathodes together and hook them up to the fan. The diodes will isolate the supplies from each other. Measure the current drawn by the fan at 9v and you will know what diode to use. Also make sure that the 5v standby output can power the fan. It may only only deliver enough current to power the electronics and you won't want to risk damaging your XBox.

[saipan59]

Just the other day I was talking to one of the 'young' guys in our group, and he was describing how the Xbox360 was designed with inadequate cooling, such that the CPU gets hot enough to sometimes warp the board just a bit, then the chip starts to lose its thermal contact with the board, and then it overheats and shuts down. After this happens enough times, there is enough cumulative damage that it doesn't recover, and then you have a dead Xbox...

Beefed-up cooling seems like a good idea.

Pete

[Chip Douglas]

Just the other day I was talking to one of the 'young' guys in our group, and he was describing how the Xbox360 was designed with inadequate cooling, such that the CPU gets hot enough to sometimes warp the board just a bit, then the chip starts to lose its thermal contact with the board, and then it overheats and shuts down. After this happens enough times, there is enough cumulative damage that it doesn't recover, and then you have a dead Xbox...

Beefed-up cooling seems like a good idea.

Pete

Yeah I water cooled mine.But because the GPU/CPU never gets hot enough to trip the built in thermal sensor, the 360 never gives the fans more then 5v , so for alittle extra cooling i bypassed the transistor so it always runs @ 12 volts, but that is too loud so i reduced it to 9v.But as ive said i want the fans to run for atleast 15 minutes after the 360 is powered down, so i need to have the fans connected to the 5v standby source.I just dont know how to accomplish this without damaging the system.I dont want any currents form either source feeding back into each other.I need some kind of switch that can detect when power from one source is cut off, and then switch to the other source, then when power is detected, switch back to that source.

Maybe its overkill, but Id rather be safe then sorry.I have temp probes on the gpu,cpu and inside the case.I noticed that when i shut the system down the inside case temps start climbing up because the fans arent running anymore.When the fans are running, the inside case temps stay @ ambient temperature, but when the fans are off the temp climbs to the temps of the gpu,and cpu.Then drop equally with the gpu,and cpu.Id like to keep the inside case temps as low as possible.

[saipan59]

As Ralph suggested, you just need a diode in series with each power source that will be running the fan. The diodes prevent current from one supply from back-driving another supply, and it doesn't matter which supplies are powered up or not, etc. Whichever supply is currently providing the *highest* voltage will be the one that drives the fan, so it's like "automatic switching".

If you need the 5V rail to be turned off after a few minutes, then that can be done too, but you'll need more electronics to do it.

Pete

[Chip Douglas]

If you need the 5V rail to be turned off after a few minutes, then that can be done too, but you'll need more electronics to do it.

Pete

Please tell me more.Ive got plenty of components, so more then likely i have enough, if not i'll hit up the local electronics stores.

Sombody gave me a schematic to switch the voltage sources using some diodes,a NPN transistor, and some capacitors, but it doesnt say what kind to use.

[saipan59]

For the diodes, I would suggest using 3-amp types, with a voltage rating of 50V or more.
An example from Radio Shack:
http://www.radioshack.com/product/index ... age=search

As Ralph mentioned, be sure that the 5V standby power in the Xbox is able to supply the current needed to run a fan. It's possible that it can do only very low currents, since (I assume) it only needs to keep the memory alive and such.

As for the delayed turn-off, I don't have a circuit to point you to, but something involving a 555 timer might be the easiest.

Pete

[Chip Douglas]

For the diodes, I would suggest using 3-amp types, with a voltage rating of 50V or more.
An example from Radio Shack:
http://www.radioshack.com/product/index ... age=search

As Ralph mentioned, be sure that the 5V standby power in the Xbox is able to supply the current needed to run a fan. It's possible that it can do only very low currents, since (I assume) it only needs to keep the memory alive and such.

As for the delayed turn-off, I don't have a circuit to point you to, but something involving a 555 timer might be the easiest.
Pete

Any luck in finding a schematic for that timer? Or atleast tell me what its called so i know what to search for.

[saipan59]

Here's a couple of pages to get you started.
In the first link below, focus on the circuit at the beginning of the section called "Monostable Circuits". For this example, you would have to push the "trigger" button to start the timer and turn on the 5V fan. Choose R1 and C to produce the right amount of delay. C will probably want to be something like 470uF, and R1 would be in the 500K to 1M range.
The output of the 555 (pin 3) would drive the Base of a power transistor that is big enough to handle the current required by the fan.

I'm sorry I don't have time to give you a complete design.

Pete

http://www.doctronics.co.uk/555.htm
http://www.uoguelph.ca/~antoon/gadgets/555/555.html
http://ourworld.compuserve.com/homepage ... en/555.htm

[riden]

Figure 8 in Pete's link http://www.uoguelph.ca/~antoon/gadgets/555/555.html looks pretty close to what you want. You need a few additions and changes for your application.

1) Find a way to keep the pins 2 and 4 on the 555 low while the 12 supply is on. A 2N2222 with a 10k base resistor connected to the 12 volt output should suffice. When the 12v supply is turned off, pin 2 and 4 will go high and the timer will begin its cycle.

2) Remove the output LEDs in Figure 8 and replace them with either a bipolar transistor and base resistor or a power MOSFET. This transistor will be the switch that turns the fan on and off.

3) Drive the power transistor with the 12v supply when the XBox is active (instead of the 555 driving it). You need to isolate the 555 output using a small signal diode (anode to 555 pin 3, cathode to transistor) to keep the 555 from interfering with 12v line when it is driving the transistor. When the 555 is "off", pin 3 will be driven to ground.

Hopefully, this is clear enough to enable to take the next steps in your design.

[Chip Douglas]

Thanks guys.I did some serious searching yesterday,and found that someone had already thought of this, and made what he called an "aftercooler" for the 360.But he used a 5v 2A DPDT relay.Unfortunitly I can not find one locally, and my usual online electronic wholesaler doesnt carry them.So i will look into the links you posted.I havent read through them yet, but hopefully the parts are more common.