Getting the most out of 3-terminal regulators

It used to be (especially during the tube days), that a regulated power supply was strictly the province of industrial and military gear. It was nearly unheard of in consumer gear. The three terminal regulator changed all of this by providing an easy-to-apply part that delivered the stated output voltage with almost zero extra parts… almost.

simple_psu.bmp

  1. Rough approximation for the transformer: Vsec(RMS) = Vout(dc)
  2. If you raise the secondary voltage, then what happens to the regulator?
  3. Be sure to consider what happens at low line voltage. It isn't always 120VAC! If your transformer is a bit on the shy side, you might be able to make up the difference by making C1 larger.
  4. Look at the way the schematic is drawn. The path for the negative side of C1 is very deliberate. Why?
  5. C1 value (min): 200000/(Rload * ripple), ripple as percent, but not changed to a decimal, C1 in uF, Rload in ohms.
  6. If you make C1 obscenely large, then you may have to beef up the rectifier to accommodate the peak charging current.
  7. The LM317 has several advantages over the 78xx regulators. Yes, you could use an LM337 to do a negative supply (i.e. positive ground). Remember to reverse all of the polarized capacitors and the diodes (d1, d2)!
  8. The 317, like the 78xx has about a 2.5v Vin - Vout minimum. Don't ignore this; these parts make ugly stuff as they drop out of regulation. I learned this the hard way 35 years ago.
  9. The Vin(min) voltage is at the bottom of the ripple waveform, NOT at the peak of the ripple waveform. Don't ignore this either. This is not the voltage your DMM tells you is across C1. (you need a scope to measure this voltage)
  10. If the IC is an appreciable distance from C1, provide a local bypass (like 100nF ceramic) at the IC.
  11. The output voltage is approximately 1.25+ (1.25v (R3/R1)). See the LM317 datasheet  for the exact formula.
  12. R1 is generally somewhere between 100R and 330R. You can fiddle with this value to make the R3/R1 ratio deliver the desired output voltage.
  13. There is a minimum output current required (3.5mA). Usually you can manage this via R1 and R3. If not, then adjust R2 value to ensure that this current is drawn. Remember to include the current through R3. You could use the current through R2 to run an LED. Often the power LED runs from the unregulated side of the supply to minimize any extra current through the regulator.
  14. Don't ignore the power dissipation in R3. It has very nearly the entire output voltage across it.
  15. D1 and D2 protect the regulator from various faults. They are cheap insurance. Don't leave them out.
  16. C2 provides additional filtering as well as stability for the regulator's feedback loop.
  17. Calculate the power dissipation for the regulator IC at high line and elevated temperature (worst case), and provide an adequate heat sink. What if the product is being used in Death Valley? The good news here is these ICs are very well protected, so they won't burn up unless you really push your luck or do something really dumb. The bad news is that going into thermal protect mode means that their output voltage drops below the set point. The power dissipation in the regulator is the voltage across it times the current through it. This is heat, and you have to help the part get rid of it. (read heatsink)
  18. These notes also apply (mostly) to the 78xx and 79xx series parts.
  19. The LM337 is the negative version of the LM317. Everything here applies to it. Reverse all diodes and capacitors on the schematic.
And you thought it was simple. Almost.

Answers to the questions 
LM317 datasheet 


Last modified 06/02/2009. 11:57:28