Power & Supplies
The ±15 V, 1 A tracking regulator from the TI LM79xx datasheet: an LM7915 trimmed to exactly −15 V and an LM7815 whose ground pin is driven by an LM307 op-amp, so the positive rail mirrors the negative one.
Circuit overview
Click any component in the schematic above to inspect its electrical ratings, function, and connections.
This is Figure 8 of the Texas Instruments LM79XX datasheet (SNOSBQ7C), a split supply for op-amp and audio circuits that need equal positive and negative rails. Two fixed regulators make the rails: an LM7915 for −15 V and an LM7815 for +15 V (TI draws it as the LM340T-15, which the LM340 datasheet specifies together with the LM7815). On their own the two rails could differ by up to 1.2 V, because each regulator is only guaranteed to within 4% (14.4 V to 15.6 V). The circuit removes that difference in two steps: a trimmer sets the negative output to exactly −15.0 V, and an op-amp moves the ground pin of the positive regulator until a 1% resistor divider between the rails reads 0 V, which forces the positive rail to the same magnitude. Clamp diodes keep either output from being pulled to the wrong polarity. The raw ±VIN inputs come from a separate rectifier and filter (TI measured the ripple with 3000 µF input capacitors); they are not drawn.
| Regulated Output Voltage | +15 V and −15 V, tracking; up to 1 A (TI Figure 8) |
|---|---|
| Load Regulation at ΔIL = 1 A | 40 mV (−15 V), 2 mV (+15 V) (TI Figure 8 table) |
| Output Ripple | 100 µVrms at CIN = 3000 µF, IL = 1 A (TI Figure 8 table) |
| Output Noise | 150 µVrms, 10 Hz ≤ f ≤ 10 kHz (TI Figure 8 table) |
| Temperature Stability | 50 mV (TI Figure 8 table) |
| Input Voltage Range | +VIN 17.7 V to 35 V (LM7815, TI SNOSBT0L); −VIN −17.5 V to −35 V (LM7915, TI SNOSBQ7C) |
| Output Trim Range | ≈ ±0.7 V (calculated from R1, R2 and the ±15 V rails) |
| Rail Matching | Set by the tolerance of R4 and R5 (1% drawn) |
U1 (LM7815) regulates +VIN down to +15 V and U2 (LM7915) regulates −VIN to −15 V. COMMON is the 0 V point between the rails, shared with the load. Each regulator holds its output a fixed 15 V away from its own GND pin, and in this circuit neither GND pin is tied straight to COMMON: that is what allows both outputs to be adjusted.
The GND pin of U2 sits on R1 (50 Ω) to COMMON. R3 is connected across the two outputs, and its wiper feeds current through R2 (1 kΩ) into R1, lifting or lowering the GND pin and with it the −15 V output. With the wiper at either end the shift is about:
ΔVGND ≈ ± VW (R1) / (R1 + R2) = ± 15 V × (50\,Ω) / (1050\,Ω) ≈ ± 0.71 VThe regulator's own ground-pin current, 1.5 mA typical, also flows through R1 and adds about 75 mV; the trim takes that up too. R3 is set for exactly −15.0 V at the output.
R4 and R5, both 10 kΩ 1%, form a divider from +15 V to −15 V. The LM307 compares its midpoint (pin 2) with COMMON (pin 3) and drives the GND pin of U1 from pin 6. If the positive rail is too high, the midpoint rises above 0 V, the op-amp output falls and pulls U1 down by the same amount. The loop settles when the midpoint is at 0 V:
V+ = -V- × (R4) / (R5)With R4 = R5 the positive output equals the magnitude of the negative output, so trimming R3 sets both rails. TI notes that the tolerance of R4 and R5 decides how well the outputs match. The op-amp output only moves by the difference between the LM7815's own output, 14.4 V to 15.6 V, and the target, and it must sink U1's quiescent current, up to 8 mA.
C1 and C2 (25 µF each) sit on the outputs. TI requires an output capacitor on the LM79xx for stability, either 1 µF solid tantalum or 25 µF aluminium electrolytic. D1 and D2 (1N4001) are reverse-biased in normal operation; TI draws them without describing their purpose. If one output is pulled past COMMON toward the other polarity, for example when one regulator starts before the other with a load connected between the rails, the diode conducts and limits the reverse voltage on that regulator to about one diode drop.
C4 and C5 are needed only if the filter capacitors of the raw supply are more than 3 in (about 7.6 cm) away. The LM7815 needs at least 17.7 V for its line regulation specification and tolerates 35 V at most; the LM7915 is specified from −17.5 V to −30 V in its line regulation tests and tolerates −35 V. Each regulator dissipates the voltage it drops times the load current:
PD = (VIN - VOUT) × IOUT = (20 V - 15 V) × 1 A = 5 WAt that level both regulators need heatsinks.
The op-amp holds the R4/R5 midpoint at 0 V, so the positive rail is set by the negative rail and the resistor ratio.
| Symbol | Parameter | Typical value |
|---|---|---|
| R4 | Upper sense resistor | 10kΩ |
| R5 | Lower sense resistor | 10kΩ |
| V − | Negative output | -15V |
| V + | Positive output | 15V |
Shift of the U2 ground pin, and therefore of the −15 V output, with the R3 wiper at either end. The regulator's own ground-pin current is neglected.
| Symbol | Parameter | Typical value |
|---|---|---|
| VW | Wiper voltage at the end of travel | 15V |
| R1 | Trim reference resistor | 50Ω |
| R2 | Trim feed resistor | 1000Ω |
| Δ VGND | Trim range (calculated) | 0.71V |
Heat in each regulator; the 20 V input is an example value.
| Symbol | Parameter | Typical value |
|---|---|---|
| VIN | Input voltage magnitude (example) | 20V |
| VOUT | Output voltage magnitude | 15V |
| IOUT | Load current | 1A |
| PD | Dissipation (calculated) | 5W |
Every part needed to build this circuit. Select a reference designator to inspect it on the schematic.
| RefDes | Component | Value / rating | Package | Role in circuit | Reference |
|---|---|---|---|---|---|
| Positive Voltage Regulator LM340T-15 in TI Figure 8. Same TI datasheet (SNOSBT0L) as the LM7805. | LM7815 | TO-220 | +15 V regulator | — | |
| Negative Voltage Regulator LM7915CT in TI Figure 8. | LM7915 | TO-220 | −15 V regulator | — | |
| Operational Amplifier | LM307 | DIP-8 | Tracking amplifier | — | |
| Resistor | 50Ω | Axial / 0805 | Trim reference | — | |
| Resistor | 1kΩ | Axial / 0805 | Trim feed | — | |
| Trimmer Potentiometer | 5kΩ | Trimmer | Output trim to −15.0 V | — | |
| Precision Resistor | 10kΩ 1% | Axial / 0805 | Tracking divider | — | |
| Precision Resistor | 10kΩ 1% | Axial / 0805 | Tracking divider | — | |
| Aluminium Electrolytic Capacitor 22 µF is the nearest common value. Voltage rating above 15 V. | 25µF | Radial | +15 V output | — | |
| Aluminium Electrolytic Capacitor + to COMMON. Required for LM79xx stability. | 25µF | Radial | −15 V output | — | |
| Aluminium Electrolytic Capacitor Only if the filter capacitors are more than 3 in away. Voltage rating above +VIN. | 25µF | Radial | +VIN bypass | — | |
| Aluminium Electrolytic Capacitor + to COMMON. Only if the filter capacitors are more than 3 in away. | 25µF | Radial | −VIN bypass | — | |
| Rectifier Diode A 1N4007 works the same; only its reverse-voltage rating is higher. | 1N4001 | DO-41 | +15 V clamp | Rectifier Diode pinout & datasheet | |
| Rectifier Diode A 1N4007 works the same; only its reverse-voltage rating is higher. | 1N4001 | DO-41 | −15 V clamp | Rectifier Diode pinout & datasheet |
Traps found in real builds, with the fix for each.
Neither tab is at COMMON. The LM7915 tab is its input, −VIN, and the LM7815 tab is its GND pin, which in this circuit is the op-amp output. Bolting both to one metal heatsink or a grounded chassis without insulation shorts −VIN, the op-amp output and COMMON together.
Insulate each regulator with a mica or silicone pad and an insulating bushing, or give each its own isolated heatsink.
In TO-220 the LM7815 pins are INPUT, GND, OUTPUT, but the LM7915 pins are GND, INPUT, OUTPUT. Copying the positive layout for the negative regulator swaps its input and ground.
Check each part against its own datasheet connection diagram before soldering (TI SNOSBT0L §5, TI SNOSBQ7C Figure 1).
Each regulator dissipates (VIN − VOUT) · IOUT, about 5 W at 1 A from 20 V. Without a heatsink the regulator heats up until its thermal protection reduces the output.
Keep the input headroom small but above the minimum, and size a heatsink for the real load current.
The positive rail copies the negative rail through R4 and R5. A 1% mismatch between them gives about 1% difference between the rail magnitudes.
Use 1% or better resistors for R4 and R5, or matched pairs where the rails must agree closely.
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