Re: [N8VEM-S100:2042] Voltage Regulator

[David Riley <frave...@gmail.com>]

On Nov 12, 2013, at 8:02 PM, Vince Mulhollon wrote:

> On Tuesday, November 12, 2013 6:00:06 PM UTC-6, steve...@gmail.com wrote:
> Perhaps there is a solution to increase output current with short circuit protection with LM7805. Use BD534, PNP Epitaxial Silicon Transistor to handle current ... I do not know if it may need heat sink for BD534 transistor. ... I presume when it run at less than 3A should not product too much heat or possible a small heat sink. What do you think about this method?
> 
> Linear regulators turn (Vin-Vout)*amps into watts of heat, no matter how you build them, so the heatsink would still dump 9 watts regardless of tech.  Also a heatsink of given thermal resistance (degrees C per watt) will always run the same delta T for a given # of watts no matter the tech so that won't help either.

Well, yes.  Linear regulators can only do so much, because they function by burning off excess wattage.  The dropout voltage of the regulator does not matter as far as power dissipation goes; the dropout voltage is the excess bias voltage required for the internal amplifiers and output transistors, but the total wattage dissipated is ALWAYS ((Vin - Vout) * Iout).  No exceptions, at least for linear supplies.

If you really want to decrease the power dissipation of the regulators (8v was presumably chosen because most of the regulators of the time, e.g. the 7805, had a 3v dropout), then you need to go with a switching regulator.  This obviously makes the project somewhat less "vintage" (not that any of the new boards are, but it's the spirit of the thing).  There are several pin-for-pin equivalents to the 7805 that are available.  They're preassembled modules that fit in the space of a TO220 package and have the same pinout.  They're expensive, and there are much more efficient ways of implementing a switching regulator, but they'll work in situations where you absolutely need something to match (for example, to fit an existing PCB).  They'll be much more efficient and run much cooler, though I don't know if they're available in the required current.

For larger designs (like the 68K board), using a "brick" (a prefabricated SMPS) is probably a good idea if you don't mind sacrificing the vintage look of the thing.  Switch-mode power supplies are notorious for being unstable in the hands of novice designers, so the bricks take all the guesswork out for you; they take an input voltage in, provide a fixed/adjustable output voltage, and have a set of capacitors they require on the output.

Otherwise, you can use a handful of parts and an integrated switching regulator (some of which have the switching transistors integrated, some of which don't).  That'll require more design work, though in most cases, the app note has done most of that work for you.  Be careful about layout, though.

Finally, a word of caution: linear regulators (like the 7805) don't really like to be paralleled (i.e. 3 7805s in parallel does not give you 3 times the current of a 7805).  Invariably, one regulator ends up "hogging" the current and then cutting out due to thermal overload, causing the other 2 to trip as well.  Then it oscillates.  Switchers, on the other hand, have the potential to be paralleled as long as they're synchronized; they each take "turns" providing current.  The regulator needs to be designed for that, though.


- Dave