Thank you John, that was exactly what I was looking for... a reasonable explanation as to why my regulator would blow out. I knew all the theoretical characteristics of inductors, but unfortunately have not been exposed to the practical side yet. Veroboard will have to do since I don't have access to board making accessories, however, I will definitely replace that inductor with one capable of handling an appropriate amount of current. This should hopefully help. Thank you all, Jai Quoting John Dammeyer : > Hi Jai, > > Switching supplies can be incredibly simple with the new devices that > are out there and they can easily deliver the expected current with the > suggested circuit from the Maxim data sheet. > > However, there are some fundamental rules that need to be followed. > You're dealing with a high frequency, high power design and so circuit > layout becomes an important issue. That means if you are having > problems, the breadboard or vero board would be the first suspicious > element to remove. > > *** So a proper layout is very important to how well this type of > circuit will function. *** > > I haven't checked the recommended inductor on the data sheet but usually > a number of inductor manufacturers and their part numbers are listed. > If sounds like, from your response on the inductor issue that you aren't > clear on saturation current of an inductor. I suspect you don't have a > scope to look at waveforms either. > > So a bit of theory. When you apply a voltage across an inductor and > measure the voltage at that instant in time, you will measure the > applied voltage. If at that same instant in time you measure the > current it will be 0. After an infinite amount of time the voltage will > be zero and the current will be infinite. That's the theoretical > inductor. > > In reality, the inductor has a certain amount of resistance and the > power supply has limitations on current so one of two things will > happen: the current increases to a point where the I^2R of the inductor > wire is exceeded and it heats up and goes open circuit; the power > supply goes into current limit and holds the current at Xi and the > voltage drops to the I*R value across the inductor. > > So far pretty simple. > > What happens in-between is what is interesting. Simply put, if you were > to watch the current flow on a scope you'd see an exponential increase > in current but when the magnetic field created by this current saturates > the core, there will be a dramatic almost avalanche like increase in > current without any subsequent increase in energy storage. The coil > heats up and could toast the driver transistor. > > So if you are using a 47uH inductor that saturates at 20ma then trying > to run 500ma through the inductor will probably toast the regulator > because the current will increase way beyond the specifications of the > device before it has a chance to shut down the current; unless the > inductor burns out first. > > When specifying an inductor, make sure that the current rating of the > inductor is at least the value of the regulation device. For example: > The Talema inductors listed as being suitable for National Semi Simple > Switchers in the DigiKey catalog has and inductor 54uH at 1A and another > one 53 uH at 3A. The amperage rating has far more to do with core > saturation than wire size and heating since the 1A coil has a DCR of > 0.20 Ohms while the 3A device has a DCR of 0.13 Ohms. > > Hope that helps. > > John Dammeyer > > > Wireless CAN with the CANRF module now available. > http://www.autoartisans.com/products > Automation Artisans Inc. > Ph. 1 250 544 4950 > > > > -----Original Message----- > > From: pic microcontroller discussion list > > [mailto:PICLIST@MITVMA.MIT.EDU] On Behalf Of Jai Dhar > > Sent: Wednesday, March 05, 2003 8:42 AM > > To: PICLIST@MITVMA.MIT.EDU > > Subject: Re: [EE]: Project testing > > > > > > Quoting "Alan B. Pearce" : > > > > > >As for the comment made about breadboards being the worst way to > > > >go... keep in mind I'm just testing it now, I don't plan on boxing > > > >this up and using it with a breadboard :-) > > > > > > I suspect the problem is the breadboard. This is allowing > > all sorts of > > > horrible lead lengths at places they should not be, which > > is allowing the > > > switchmode chip to oscillate in all sorts of uncontrolled > > ways, and this is > > > what has destroyed your chips. > > > > > > The only way you are going to sort this is to go straight > > to a PCB which has > > > been laid out with current paths and suitable sized tracks > > straight off. > > > Then you will get something that will approach the > > stability you require. > > > > > > > Does veroboard have tracks suitable for this application? > > Seems to be my only > > option now, unless I reduce the capabilities of my supply. > > > > As to the type of inductor that I'm using, what is the > > 'correct' type? I > > wasn't aware that a special type of inductor was required for this > > application, so I would appreciate knowing what type of > > inductor I ned to > > obtain. > > > > Regarding Jochen mentioning that I need a bigger capacitor after the > > rectifier... do I really need something as big as 4700? The > > biggest cap I have > > seen used after a rectifier is a 2200 (at least looking at the example > > schematics on psu's). And yes, I did use heatsinks :-) > > > > While all these are great suggestions in improving my design, > > are any of these > > explanations as to why my regulator fried when I simply put > > in an appropriate > > resistor? It seems like all these suggestions could *improve* > > my design, but > > from what I understand, none of them seem to address why > > drawing 500mA would > > fry my regulator. I must be missing something. > > > > Nevertheless, thank you for all of your input thus far, > > > > Jai > > > -- > > > http://www.piclist.com hint: The PICList is archived three different > > > ways. See http://www.piclist.com/#archives for details. > > > > > > > > > > > > > ---------------------------------------- > > This mail sent through www.mywaterloo.ca > > > > -- > > http://www.piclist.com hint: The PICList is archived three different > > ways. See http://www.piclist.com/#archives for details. > > > > > > -- > http://www.piclist.com hint: The PICList is archived three different > ways. See http://www.piclist.com/#archives for details. > ---------------------------------------- This mail sent through www.mywaterloo.ca -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.