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.
And you thought it was simple. Almost.
- Rough approximation for the transformer: Vsec(RMS) = Vout(dc)
- The secondary current required for a full-wave bridge rectifier equals the DC output current.
This is not the case for full-wave centertap circuits (uses 2 diodes). (see the quick reference at the end of this article)
- If you raise the secondary voltage, then what happens to the regulator?
- 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.
- Look at the way the schematic is drawn. The path for the negative side of C1 is very deliberate. Why?
- C1 value (min): 200000/(Rload * ripple), ripple as percent, but not changed to a decimal, C1 in uF, Rload in ohms.
- If you make C1 obscenely large, then you may have to beef up the rectifier to accommodate the peak charging current.
- The LM317 has several advantages over the 78xx regulators. Yes, you can 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, d3)!
- Adjustable Vo. You always have the right part on hand. (yes, you have to have the right resistors, or use a pot for R3)
- 40v limit, but it is Vin to Vout, not Vi to ground.
- The 317, like the 78xx has about a 2.5v Vin - Vout minimum. Don't ignore this; these parts make really ugly stuff at their output
as they drop out of regulation. I learned this the hard way several decades ago.
- 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
the voltage your DMM tells you is across C1. (you need a scope to measure this voltage)
- If the LM317 is an appreciable distance from C1, provide a local bypass (like 100nF ceramic) at the IC.
- The output voltage is approximately 1.25+ (1.25v (R3/R1)). See the
for the exact formula.
- 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. Circuit wise, R1 should be
electrically close to the regulator.
- There is a minimum output current required (3.5mA). Usually you manage this via the current thru 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.
- Don't ignore the power dissipation in R3. It has very nearly the entire output voltage across it. (the simplistic way to look at it
is that it HAS the entire output voltage across it).
- The power dissipated in R1 is quite low, because the regulator puts 1.25V across it, and this is independent from the output voltage.
- The diodes are cheap insurance. Don't leave them out.
- D1 protects U1 from C1.
- D2 protects against reverse biasing the output, which can happen in applications using bipolar power supplies.
- D3 protect U1 from C3
- C2 provides additional filtering as well as stability for the regulator's feedback loop.
- C3 reduces noise at the regulator output. It's reactance should be 1% or less of the nominal value of R3 at the ripple frequency.
- Do 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. (translation: heatsink and/or fan. Refer to the datasheet.)
- These notes also apply (mostly) to the 78xx and 79xx series parts.
- The LM337 is the negative version of the LM317. Everything here applies to it. Reverse all diodes and capacitors on the schematic.
Answers to the questions
Also of interest, a handy quick reference for different rectifier/filter circuits:
Last modified 07/27/2018.