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Everything posted by OutToLunch

  1. I'd rather have smell-o-vision. That way, when I watch Food Network, I can get extra hungry. I'd have to turn it off for "Dirty Jobs", though.
  2. This is just one perspective - so take it for what it is worth, but I only use my DMM for reading AC voltage, DC voltage, impedance testing, audible continuity testing, diode testing and on rare occasions I will use the ammeter functionality. I also use the LCD backlighting quite a bit as well. There are quite a number of features on my DMM that I never use. I suppose my scope takes up the slack on that bit, tho.
  3. i suppose i'm looking at this from a different perspective. every where i have ever worked has attempted to squeeze as much wattage per square/cubic inch as they possibly could while at the same time adhering to safety and shipping standards. At the last place I worked, if I had placed a tombstome power resistor on a board or a radially leaded power resistor mounted with clearance to the board, it would have meant slopping it with RTV to keep it stationary so the leads don't snap during vibration testing. Also the ambient temp inside an enclosure can many times be expected to be 70C so any power dissipation on parts has to account for that as well. If you have an open circuit with a fan blowing at least a few hundred lfm across anything dissipating power, then, sure, run at 100% rating.
  4. you've gone off into tangential land - a resistor is not a FET. Still, though, depending on the architecture of the SMPS, I would usually recommend starting with a FET that has a max voltage rating at least twice that of the input rail.
  5. Lifetime reliability would also be in question - reliability of the resistor in question and surrounding components that will be absorbing that heat. I suppose I would fall into the category of wasting 50% of the potential of the resistors I use cuz I always derate by that much. You always want a margin of safety - personally, I believe a factor of 2 should always be strived for.
  6. isn't the lower mosfet just acting as the freewheel diode does when the upper switch is off? it's not applying a braking force to the motor, just providing a low impedance path for the current. I would agree that the boot circuit was not implemented properly. Also - where is the decoupling capacitor for the driver IC? There is a very good possibility that even if he did hook everything up right, the turn on of the upper FET would cause the input rail to sag and cause the driver to trip a Power On Reset. I would also suggest decoupling the upper FET with some ceramics.
  7. The use of inductor current sensing is actually becoming more and more the norm with multiphase buck regulators that supply power to a PC processor. It has been around for awhile but it's main use has been to create a droop in the output as the load is increased. There are a number of advantages to this, one of them being the output capacitor bank can be smaller. Another lossless method of current sensing is to use Rds(on) sensing (I say lossless in the sense that you would not be adding a lossy element to the system - the sense element is part of a component that is already passing current). Basically, you know the Rds(on) of the MOSFET and once the voltage across it has exceeded a set value, the system is in overcurrent. The Rds(on) of a FET will change as the junction temp rises, but this can be accounted for since all you're doing is tripping on a fault condition. To detect the DC current passing through the inductor, you need to create a low pass RC filter across it. The voltage across the capacitor will be the DC voltage across the inductor that is created from the DCR drop. All you have to do is to make sure that R*C = L/DCR and you're cool. One point, though: if this is for a one off project, then just measure the DCR of the inductor with an accurate LCR meter. If this is to go into production, then you're going to have to call for a max of 10% tolerance on the DCR of the inductor and design with tolerances in mind.
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