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Thread: sproggy - Testing voltage outputs

  1. 06-23-2002, 12:15 PM #1
    cheesyhoward
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    sproggy - Testing voltage outputs

    ok i have built me sproggy yay and have been testing it off the 12v rail of an atx psu. I loaded the psu with a small 12v cpu fan. These are the voltages i got:

    +5.0v
    +12.11v
    +3.294v
    -5.0v
    -13.06 << Is this meant to be -12v? Or is this due to their being no load on that line while i was testing it.? Any help would be useful before i plug it in to motherboard
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  2. 06-23-2002, 03:27 PM #2
    jol
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    Re: sproggy - Testing voltage outputs

    Originally posted by cheesyhoward
    ok i have built me sproggy yay and have been testing it off the 12v rail of an atx psu. I loaded the psu with a small 12v cpu fan. These are the voltages i got:

    +5.0v
    +12.11v
    +3.294v
    -5.0v
    -13.06 << Is this meant to be -12v? Or is this due to their being no load on that line while i was testing it.? Any help would be useful before i plug it in to motherboard
    i think that +-0,5volts shouldnt be any problem, but -13 might... try to connect everything except the -12, a part of the mobos wont require it, as it's only some network and graphics that uses it (what i've heard)
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  3. 06-23-2002, 06:40 PM #3
    cheesyhoward
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    ok i tested the supply with a cdrom drive as the load. With the cdrom drive idle without a cd loaded in it the -12v was reading as -12.2v which i guess is ok. However when a cd was loaded and spun up the -12v dropped to -25v and then varied between -18 and -24v. Is this normal? If it is not does anyone have any idea as to what would be causing this?
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  4. 06-24-2002, 04:54 AM #4
    Rob Withey
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    I'd say that was probably expected. Take a look at the sproggy schematic - the voltage sense is connected to the +12v. There is no voltage regulation on the -12v, and with a load on the +12v line, because they are being derived from the same centre tap transformer, you will get different voltages as the regulator chip puts more energy into the transfomer to keep the +12v at the right voltage.

    In a similar way to how some PC power supplies will cut out if you load up the +12v line without loading the +5v line - the overvoltage protection kicks in on the 5v line and the power supply shuts down. There is no overvoltage protection on the -12v line in the sproggy to protect you though.

    As to whether it is desirable, I'd say probably not. If you are not using the -12v line, I wouldn't worry about it. As long as it doesn't exceed the input requirements of the -5v regulator.


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  5. 06-24-2002, 05:54 AM #5
    cheesyhoward
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    what is -12v used for then?
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  6. 06-24-2002, 06:11 AM #6
    Rob Withey
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    Originally posted by cheesyhoward
    what is -12v used for then?
    Mostly serial ports. Potentially sound and graphics too.


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    New system at design/prototype stage on BeagleBoard.
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  7. 06-24-2002, 07:10 AM #7
    cheesyhoward
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    ok firstly since i have the q4438b t6 and not the 39b i had to wire the connections myself and not solder it directly onto the pcb. I only used 22 gauge wire for this so i may rewire it with a larger wire size incase this was causing the problem.
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  8. 06-24-2002, 11:58 AM #8
    cheesyhoward
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    ok i done that and i still get -28v variations when cdrom is spinning up. Is it possible to run a cpu fan (12v 0.080A) off the ground and -12v connections to test the circuit? I guess the fan will just spin the other way. Any guidance would be helpful.
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  9. 06-24-2002, 12:19 PM #9
    Rob Withey
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    Originally posted by cheesyhoward
    ok i done that and i still get -28v variations when cdrom is spinning up. Is it possible to run a cpu fan (12v 0.080A) off the ground and -12v connections to test the circuit? I guess the fan will just spin the other way. Any guidance would be helpful.
    Yep, all voltage is relative. Treat 0v as +12 and -12v as gnd and the fan should be fine.


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  10. 06-24-2002, 12:40 PM #10
    cheesyhoward
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    ok with the fan on the -12v and ground with the cdrom drive acting as load. On the +12v line with no cd in and while opening and closing the tray, the -12v line reads -12.5v which i guess is ok. When a cd spins up the -12v line increases to -18v for about a second then drops back to -12.9v the +12v remains stable during these tests. Could the yellow 100nF Polycap (RS part code 405-8334) be blame for this? Is it likely that my sproggy will fry my motherboard if I test it on that?
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