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For a small, low-current circuit, the simplest true bipolar supply is two equal batteries in series with their junction used as 0 V. Two nominal 9 V batteries provide approximately +9 V, 0 V and −9 V. If you only have one DC supply, a resistor divider can create a midpoint—but that is a virtual ground, not a robust negative power rail.
Choose the solution by asking three questions: what voltage must each rail provide, how much current will each rail draw, and must the outputs be isolated or independently regulated?
What “bipolar” or “split-rail” power means
A bipolar, dual-rail or split supply provides two voltages relative to a shared reference:
+12 V positive rail
|
| 24 V rail-to-rail
|
0 V circuit reference
|
|
−12 V negative railThe 0 V node is a voltage reference. It is not automatically earth, protective earth, chassis ground or the negative terminal of an external adapter.
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- True dual rails: positive and negative outputs are generated as supply rails and can generally source current independently.
- Virtual ground: a midpoint made from a resistor divider, buffer or rail-splitter circuit. It may be suitable as a signal reference but can move under load.
- Floating supply: an electrically isolated supply whose reference can be assigned to the circuit ground.
This distinction matters. Calling the midpoint of a lightly loaded divider “−9 V” does not make it equivalent to a regulated −9 V output.
Choose the circuit before building
| Requirement | Best starting point | Important limitation |
|---|---|---|
| Fast experiment, very low current | Two batteries in series | Voltage falls with discharge and the rails are not regulated |
| Signal reference only | Resistor divider | Midpoint moves when the load is unbalanced |
| Low-current but stiffer midpoint | Buffered divider or rail splitter | Buffer current, stability and heat limits still apply |
| Adjustable regulated rails | LM317 plus LM337 | Needs suitable raw voltage, filtering and thermal design |
| Fixed ±5 V from a 3.3–5 V source | Split-rail charge-pump converter | Voltage and current are device-specific |
| True isolation or substantial current | Isolated DC/DC module or transformer supply | More expensive and may require filtering or post-regulation |
Do not assume that a 12 V adapter can become ±12 V by adding a midpoint. A passive midpoint produces approximately +6 V and −6 V. To obtain ±12 V from a single 12 V input, you need a boost/inverting or isolated conversion topology.
Build 1: two 9 V batteries
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|
Battery 1
|
0 V -----+----- midpoint
|
Battery 2
|
−9 VParts
- Two equal 9 V batteries, preferably the same chemistry and age.
- Two battery connectors or holders.
- Three labelled wires or a three-pin connector.
- Optional switch and local decoupling capacitors.
Connections
- Connect the positive terminal of Battery 1 to the circuit’s +9 V output.
- Connect the negative terminal of Battery 1 to the positive terminal of Battery 2.
- Use that junction as 0 V.
- Connect the negative terminal of Battery 2 to the circuit’s −9 V output.
Because the battery stack is floating, you may assign its midpoint as the circuit reference. With fresh nominal 9 V batteries, a meter should read approximately +9 V from the midpoint to the upper terminal, −9 V from the midpoint to the lower terminal, and approximately 18 V from the positive rail to the negative rail.
Limitations
- A “9 V” battery is nominal; its voltage varies with chemistry, state of charge, load and internal resistance.
- The two batteries may discharge at different rates, so the rails will not remain perfectly symmetrical.
- Current capability and runtime are limited.
- Rechargeable and non-rechargeable batteries should not be mixed casually.
- Connecting an external grounded device to one terminal can bypass part of the battery stack.
Before connecting an oscilloscope, USB device or another supply, determine whether its ground is already connected to earth or to another circuit. A floating battery supply is forgiving; an accidental connection can still short one battery or defeat the intended midpoint.
Build 2: a resistor-divider virtual ground
A single DC supply can be divided into two nominal halves:
+9 V ── 10 kΩ ──+── 10 kΩ ── 0 V input
|
virtual 0 VWith a 9 V input, the midpoint is approximately 4.5 V. If the midpoint is renamed 0 V, the apparent rails are approximately +4.5 V and −4.5 V. This does not produce ±9 V.
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The divider current in this example is:
I = 9 V / (10 kΩ + 10 kΩ)
I = 0.45 mAA load current comparable with 0.45 mA can move the midpoint substantially. A circuit drawing 10 mA from one side is already demanding far more current than the divider continuously provides.
TI describes the resistor divider as the simplest rail-splitting method, while noting that it becomes unbalanced under load: TI’s split-rail overview.
Good uses
- Biasing an op-amp input.
- Creating a signal reference for a low-current sensor circuit.
- Very low-current analog experiments.
- Circuits whose current is deliberately centred around the midpoint.
Bad uses
- Headphones, speakers or other audio power loads.
- Motors, relays, LEDs or several independent circuits.
- Large or rapidly changing current on only one rail.
- Any design that requires an independently regulated negative supply.
Lower resistor values make the midpoint somewhat stiffer but waste more current. They do not transform the divider into an isolated, independently regulated supply.
Build 3: a buffered virtual ground
A buffered rail splitter uses a divider to establish half the input voltage, then an op-amp or dedicated buffer to hold that midpoint near the desired reference. This is much better than a bare divider for modest signal currents, and it is a common approach in single-supply analog circuits.
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A buffered virtual ground is still not a second isolated supply. It shares the input’s electrical reference and can fail or overheat if an external device forces current into the midpoint. Analog Devices explains the rail-splitter concept and feedback-buffer approach in its virtual-ground article.
Build 4: a regulated LM317/LM337 dual supply
For adjustable, ground-referenced rails with more current capability than a divider, use a genuinely split source followed by separate regulators:
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isolated centre-tapped AC or split DC source
↓
rectifier and filter
↙ ↘
LM317 (+ rail) LM337 (− rail)
↓ ↓
+V −V
midpoint 0 VThe source may be an isolated centre-tapped transformer secondary, a suitable split DC source or another correctly designed isolated supply. A beginner-friendly construction should use an enclosed, certified low-voltage source rather than exposed mains wiring.
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Setting the output voltage
For an LM317-style regulator:
VOUT ≈ 1.25 V × (1 + R2/R1) + IADJ × R2For a first approximation using the common 240 Ω programming resistor:
R2 ≈ R1 × (VOUT / 1.25 − 1)For approximately 12 V:
R2 ≈ 240 Ω × (12 / 1.25 − 1)
R2 ≈ 2.06 kΩA 2.0 kΩ resistor gives approximately 11.7 V before adjustment-current and resistor-tolerance effects. Use fixed precision resistors where possible, or a multiturn trimmer with a safe maximum setting. The LM337 uses the same relationship in magnitude, but its resistor connections and pinout must follow the exact LM337 datasheet and package drawing. Never substitute an LM317 pinout diagram for an LM337 without checking the device.
Input headroom and ripple
A linear regulator needs its input to remain above the output by its dropout requirement plus a margin for ripple. TI gives approximately 2 V as a typical LM317 dropout figure, but the actual value depends on load, temperature and device version.
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With a capacitor-input rectifier, estimate full-wave ripple as:
ΔV ≈ ILOAD / (fRIPPLE × C)On 60 Hz mains, full-wave ripple is 120 Hz. For 100 mA and 2200 µF:
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ΔV ≈ 0.10 / (120 × 0.0022)
ΔV ≈ 0.38 VThe unregulated waveform must remain above the regulator’s required input at the ripple valley. Also allow for transformer regulation, diode drops, mains variation, capacitor tolerance, startup behaviour and the worst-case condition of the negative rail.
Heat calculation
Estimate each regulator separately:
P ≈ (VIN − VOUT) × ILOADFor 18 V into a 12 V regulator at 250 mA:
P ≈ (18 − 12) × 0.25
P ≈ 1.5 WThat is enough to require attention to package temperature and possibly a heatsink. The positive and negative regulators may dissipate different amounts if the rail currents or raw input voltages differ. Internal current limiting and thermal overload protection, listed for the LM317 by TI, do not make continuous overheating safe.
Capacitors and protection
- Use reservoir electrolytics after rectification, with adequate voltage ratings and correct polarity.
- Place 100 nF ceramic bypass capacitors close to regulator input and output pins where the datasheet recommends them.
- Use output capacitors only as permitted by the regulator datasheet and selected for stability.
- Add bleeder resistors across large reservoir capacitors so stored voltage falls after shutdown.
- Use reverse-protection diodes around the regulators where required by the datasheet.
- Fuse the input and use a correctly rated switch.
- Provide an enclosure, strain relief and ventilation appropriate to the source and heat.
Mains-powered transformer construction involves hazardous voltages and requires suitable insulation, fusing, earthing where applicable and safe enclosure practices. For a beginner project, use an isolated low-voltage adapter or a preassembled isolated transformer module instead of working with exposed mains wiring.
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A converter is often a better choice than linear regulators when the input is a battery or when heat must be minimized.
Fixed ±5 V converters
TI’s TPS65133 is specified for ±5 V outputs up to 250 mA per output from a 2.9 V to 5 V input. TI also gives typical efficiency above 90% under stated conditions; this is not a universal efficiency guarantee. It is a surface-mount switching solution, so layout and filtering matter.
The LM27762 provides adjustable positive and negative outputs of approximately ±1.5 V to ±5 V, with up to ±250 mA from a 2.7 V to 5.5 V input. Its low-noise LDO regulation can be useful for analog circuits, but the voltage range does not directly cover ±9 V or ±12 V and its compact package is less convenient for beginners.
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Isolated ±12 V modules
When a single DC input must produce genuinely isolated dual rails, an isolated module is often simpler than designing an inverter. The TRACO TEL 15-2422WIN listing specifies isolated +12 V and −12 V outputs at 625 mA per output from a 9 V to 36 V input. Depending on the application, it may still need filtering, post-regulation or thermal management.
Always verify isolation rating, output regulation, permissible unbalanced loading, ripple, input range and continuous thermal performance. Do not rely on a marketplace module labelled “±12 V” unless its schematic or datasheet confirms how the negative output is produced.
Testing procedure
- Leave the project load disconnected.
- Check regulator orientation and the exact package pinouts.
- With power off, measure resistance between each rail and 0 V to look for an obvious short.
- Power through a current-limited bench supply or protective fuse.
- Measure the positive rail from red meter lead to 0 V.
- Measure the negative rail with the black lead on 0 V and red lead on the negative rail.
- Measure positive rail to negative rail.
- Apply a dummy load to each rail separately.
- Observe midpoint movement, ripple and regulator temperature.
- Connect the intended circuit only after the unloaded and dummy-load tests pass.
For a nominal ±12 V supply, expected readings are approximately +12 V from positive rail to ground, −12 V from negative rail to ground and 24 V from positive rail to negative rail. If the negative rail appears positive, check the meter lead orientation before changing the wiring.
Troubleshooting by symptom
The rails are correct with no load but collapse under load
A divider or virtual-ground buffer may be overloaded. Other causes include regulator dropout, inadequate reservoir capacitance, excessive ripple, insufficient transformer voltage, an incorrect regulator connection or a shorted load.
The midpoint is not exactly zero
This is normal for a passive divider, unequal battery pair or independently adjusted regulators. Measure both rails relative to the midpoint under the actual load rather than assuming symmetry.
One regulator is much hotter
Calculate dissipation independently. The rail currents may differ, or one regulator may receive a higher raw input voltage. Check heatsink contact, airflow and the regulator’s safe operating limits.
Connecting USB causes a short or instability
The USB device may already connect its ground to the computer or charger. That external ground can short or overload a virtual midpoint. Use an isolated supply when the circuit reference must remain separate.
There is excessive ripple or switching noise
Check reservoir capacitance, capacitor ESR and polarity, rectifier wiring, grounding and regulator headroom. Switching converters also require the manufacturer’s recommended layout, inductors or capacitors, short current loops and suitable filtering.
Capacitors heat, leak or fail
Immediately remove power and check electrolytic polarity, voltage ratings, rectifier orientation, centre-tap wiring and possible reverse-voltage conditions. Large capacitors can retain dangerous energy after shutdown; verify discharge with a meter.
Final selection checklist
- Do you need ±9 V, ±12 V, ±5 V or only a midpoint reference?
- Have you calculated positive-rail, negative-rail, peak and transient current?
- Are the rail currents balanced, or does one rail carry most of the load?
- Is a virtual ground acceptable, or do you need true independently usable rails?
- Must the output be isolated from USB, an oscilloscope or another supply?
- For linear regulators, does the ripple valley stay above the required headroom?
- Have you calculated heat for each regulator?
- Are capacitor voltage ratings, polarity, fusing and discharge handled correctly?
- Have you tested both rails relative to 0 V and relative to each other?
For a quick, low-current experiment, use two batteries and their junction as 0 V. For signal biasing from one supply, use a divider or buffered midpoint. For a dependable adjustable supply, use a properly designed LM317/LM337 circuit. For isolation, efficiency or substantial unbalanced current, use an appropriately rated isolated or switching converter.
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