Split power supply with 7815 and 7915

[hummusdude]

I’m using a 7815 and 7915 for a split power supply for some TL072 op amps that are part of an audio pre-amp and also a Baxandall tone circuit that also uses a TL072. I know I don't need so much voltage for these circuits, but the reason I'm using the 15V supply is because I'm tapping the DC voltage from the power amp power supply and it puts out 25VDC. Not sure if it makes a difference but I'm hoping that by using a 15V regulator instead of a 5V regulator it will reduce power dissipation and excess heat on the 7815/7915. Does that make sense?

The data sheet for the 7815 has a few different sample application circuits. One is called a “fixed-output regulator” and uses only a couple of caps for the power pins as usual. Another sample circuit is called a “split-power supply” and shows a 7815 and 7915 used together, along with some diodes and extras caps, to form dual output dc power supply for an op amp.

My question is, is there an advantage to using the split-power supply design over using a separate 7815 and 7819 in a fixed-output regulator setup to create the positive and negative power rails for the op amps? The split-power supply design uses more parts, but I’m already using a 7815 and a 7819 in the fixed-output regulator set up on the breadboard to provide power to my op amps and it seems to be working fine.

Appreciate any feedback!

[bob91343]

I don't get it - from your beginning description I thought you were using the dual arrangement.  It should work okay.

And yes, the higher voltage regulator dissipates less heat, even with the same load current.  But you raised the load current by running the amplifiers on a higher voltage.  So I guess it depends.

[hummusdude]

Sorry... I meant that I'm using a 7815 and a 7915 in the fixed output regulator configuration. Each has a couple of stabilizing caps on the power pins as the datasheet recommends. But the data sheet also shows a different configuration called a "split-power supply", also with a 7815 and a 7915, but with a couple of diodes and some extra caps. I'm wondering if there's an advantage to using the split-power supply setup instead of just using a 7815 and a 7915 separately, as it shows in the diagram I attached.

[ledtester]

Except for the diodes, there is no difference between the "split-supply" schematic and the recommended circuit diagrams for the 7815 and 7915.

From the 7915 datasheet, the recommended caps are 2.2uF on the input and 1uf on the output.

From the 7815 datasheet the recommended caps are 0.22uF on the input and 0.1uF in the output (if needed).

The diodes are there to give a form of reverse polarity protection -- e.g. if you use it to charge a battery and you accidently connect the battery the wrong way around. You can omit them if you don't need this protection.

See the following comments about the diodes.

[hummusdude]

Ah...that makes sense now that you explain it. Thanks!!

[Ian.M]

No, the diodes are *NOT* there for incoming reverse polarity protection.  They are there to protect the regulators and load if one of the unregulated input supply rails goes missing, e.g. due to a blown fuse. 

Consider what happens if the -20V input is lost while you've got a load of a board full of OPAMPS drawing say 300mA.  Due to the current through the load, the regulated -15V rail will rapidly rise above ground.  You've now reverse biassed the 7915 regulator's internal circuits between Out and Gnd, and probably damaged it.  Also all the OPAMPs in the load have had any grounded inputs taken well negative with respect to their Vee pin, so the odds are high that they've been damaged as well.   

Re-connect the -20V supply and the 7915 may well let it straight through unregulated due to the damage, and half your OPAMPs are already 'pining for the fjords' and cant wait to 'elect a pope' as soon as they get power to do so.  %DEITY% forbid you've used Tantalum decoupling caps - if so, after a reverse polarity insult, you'd better have a fire extinguisher handy at the next powerup.

The diodes prevent all the drama by limiting rail reversal if one input supply is lost to about 0.6V-0.7V at which voltage, parasitic PN junctions in ICs wont be forward biassed hard enough to pass enough current to damage them especially if the pins in question have  any series resistance.  If you want to be *REALLY* certain, because you've got a particularly electrically delicate and high value load, use Schottky protection diodes across the regulator outputs to limit rail reversal to under half a volt.

Even more fussy circuits may need power sequencing, but that's highly unlikely to be needed in an audio application that doesn't do DSP signal processing.

[hummusdude]

Wow...thanks for the detailed explanation. It makes sense that the manufacturer would present the split power supply with the diodes since op amps are the typical application. Would have been nice if they had included an ounce of prevention along with the circuit. But I guess that is what eevblog forum is for...such an awesome community...thank you!!

[Andreas]

Hello,

there is another reason for the diodes:

On start up when both input voltages are not applied exactly at the same time
one of the regulator outputs may get (temporarily) reverse polarized by the load.

Without the diodes the regulator (usually the negative) remains in this "latchup" state.

with best regards

Andreas

[Ian.M]

We have a winner - Latchup - that's why its in the datasheet, because it won't work reliably in all circumstances without the diodes!

[David Hess]

Not sure if it makes a difference but I'm hoping that by using a 15V regulator instead of a 5V regulator it will reduce power dissipation and excess heat on the 7815/7915. Does that make sense?

Yes, that makes sense, but total power dissipation will be higher because the circuit will likely draw more power at a higher voltage.  I have occasionally done the same thing with a 317 or 337 adjustable regulator to lower power dissipation in the regulator.

My question is, is there an advantage to using the split-power supply design over using a separate 7815 and 7819 in a fixed-output regulator setup to create the positive and negative power rails for the op amps?

The practical difference is that a split design using positive and negative regulators can operate from a split source with a common ground.  If a single center-tapped transformer secondary provides the unregulated positive and negative supply voltages, then a split design using a positive and negative regulator is the simplest.

If two isolated transformer secondaries are used as the source with separate rectifiers, then a pair of positive regulators, a pair of negative regulators, or a positive and negative regulator can be used.

Beware that the pinouts on positive and negative regulators are usually different; the tab on a 7815 is common and the tab on the 7915 is the input so the tabs must be isolated from each other.  This is because the tab connects to the substrate of the IC which the process requires to be at the most negative potential.

[MarkF]

Let's have a look at what the datasheet says (excerpt from ON Semiconductor):