## Re: [Electronics_101] 12 volts minus 5 volts = 7 volts - This is strange and ...

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• In a message dated 6/30/2006 8:02:55 A.M. Central Daylight Time, lcdpublishing@yahoo.com writes: I always think of current going from ground to positive (even
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In a message dated 6/30/2006 8:02:55 A.M. Central Daylight Time, lcdpublishing@... writes:
I always think of current going from ground to positive (even though I think it is commonly thought to be the opposite). So, in this example, we have the current going from the 5 Volt (instead of ground) into the 12 Volts - both being postive, but at different potentials [is that the correct term here?]. Ground isn't comming into play here at all! Further more, this really messes me up with connections. ...<<
12 VDC is "more positive" than 5 VDC.  So, "to the 12 V., 5 V. "looks like" ground, but only 7 V. less.  So, theoretically, you'd get 7 volts between the two.  Conventional current flows from Positive to Negative(r).  So, "7 V. worth of the 12 V. potential would try to flow "backwards" into the +5 V.  IF this were a battery or a "linear regulator", it would probably work, BUT a PC supply is a SWMPS (switch-mode power supply) and the 5 V. side would almost surely go to live with God, perhaps violently, if you tried to do this!

If you can't discover which components in the switcher to adjust the 5 VDC UP to 7 volts, or the 12 V. DOWN to 7 V. (less likely), quit wasting your time, and spend some money and get a simple linear supply (or a 6.3 V/10 A. transformer, a bridge-rectifier good to 10 A. and a cap, say, 10,000 µF) and be done with the problem!
Okay, at some point I plan to add current sensing and was originally planing to do high-side sensing. Which is the high side - the 12 Volt I would think.<<
Yes, but you don't wanna bother!
The mosfets switch the low side, so is that the 5 volt side of this?<<
Probably BOTH "sides" use MOSFET's!  If you don't know, you shouldn't even be THINKING about trying!
Lastly, the PC power supply I am tinkering with right now has 20 Amps on the 5 volt side and 6 amps on the 12 volt side. So, which one is actually providing the current to the motor? <<
Don't even THINK abowdit!
This last question is critical as the PC power supply will shut down if I try to run the motor on the 12Volt side. However, it runs the motor fine on the 5 volt connection. I am just guessing, but I
suspect the power supply thinks there is a short or something to cause it to shut down.<<
Yes!  Doesn't that TELL you something?  Besides, even if the "voltages" were correct for the motor, the PC supply is designed/expected to feed nice, quiet "DC loads" like IC's and LED's and such, NOT the nasty EMI-Hash from a commutated motor!  If operation on the 5 VDC side alone won't do, BUILD yourself a supply like I suggested above!
To reset the power supply, I have to remove power for about a minute, then it is happy and working again.<<
You are lucky you haven't already started it smoking, a very bad thing for health!

Jan Rowland

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