Generating Symmetrical ± Voltages with a Bench Power Supply: Connecting Two Outputs in Series Correctly

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Operational amplifiers, instrumentation amplifiers, analog filters and many other analog circuits require more than a single positive supply voltage. They need a symmetrical power supply.

Typical examples are:

+12 V / 0 V / -12 V

or:

+15 V / 0 V / -15 V

.

This type of supply can be generated using a suitable bench power supply with two separate outputs by connecting the two output voltages in series. The junction between the two sources then forms the common 0 V reference for the analog circuit.

A dedicated tracking function is not strictly required. On power supplies equipped with tracking, this feature merely makes it easier to adjust both output voltages together. To generate a symmetrical supply, the essential requirement is that the power supply used expressly supports series operation of the two outputs.

Errors occur particularly often due to:

• an incorrectly selected 0 V midpoint,
• confusing circuit ground with protective earth,
• incorrect polarity of a supply rail,
• unsuitable power supply outputs or outputs not approved for series operation,
• differently adjusted output voltages,
• current limits set too low,
• one channel switching unnoticed into constant-current mode,
• additional earth connections introduced by oscilloscopes or other measuring instruments.

A symmetrical ±15 V supply does not consist of one “positive” power supply and one special “negative” power supply. Two standard DC voltage sources are connected in series. Only by selecting the midpoint as 0 V do +15 V and -15 V arise relative to this reference point.

Suitable devices can be found under Power Supplies at ICS Schneider and under Multiple-Output Sources / Bench Power Supplies.

Why do analog circuits require positive and negative supply voltages?

Many analog circuits process signals that lie both above and below a defined reference potential.

For example, a typical input signal may range from:

-5 V … +5 V

.

A conventional operational-amplifier circuit can process such positive and negative signal voltages particularly easily when both a positive and a negative supply rail are available.

A typical supply is therefore:

+15 V

0 V

-15 V

.

Typical applications include:

• operational-amplifier circuits,
• instrumentation amplifiers,
• active filters,
• measurement amplifiers,
• strain-gauge signal conditioning,
• analog control systems,
• audio electronics,
• sensor signal conditioning,
• development and laboratory setups.

Alternatively, many modern circuits can operate from a single supply voltage and an artificially generated virtual midpoint.

However, if the circuit is explicitly designed for:

±12 V

or:

±15 V

a genuine symmetrical supply is generally the technically simpler solution.

What does ±12 V or ±15 V mean?

A symmetrical power supply has three electrical potentials.

For ±15 V:

+15 V

0 V

-15 V

.

All voltage values refer to the common midpoint:

0 V

.

Between +15 V and 0 V, the voltage is:

15 V

Between 0 V and -15 V, the voltage is also:

15 V

Between +15 V and -15 V, however, the voltage is:

30 V

.

Accordingly, a supply of:

±12 V

provides the following voltage between the two outer supply rails:

24 V

.

The following applies to the total voltage:

Utotal = U+ + |U-|

How are symmetrical voltages generated from two power supply outputs?

Assume that a suitable dual-channel bench power supply provides:

Output 1 = 15 V

and:

Output 2 = 15 V

.

Each output initially has a positive and a negative terminal.

The two sources are connected in series:

Output 1 (-) → Output 2 (+)

.

This junction then becomes the:

0 V midpoint

.

The remaining outer terminals form:

Output 1 (+) → +15 V

and:

Output 2 (-) → -15 V

.

The complete arrangement is therefore:

Output 1 (+) → +15 V

Output 1 (-) / Output 2 (+) → 0 V

Output 2 (-) → -15 V

.

The negative supply voltage is therefore created solely by the choice of reference potential. Output 2 continues to generate a normal positive voltage of 15 V between its own terminals.

How to wire ±15 V correctly

For an analog module with the terminals:

V+

GND

V-

the following assignment applies:

Bench power supply	Analog circuit	Potential
Output 1 (+)	V+	+15 V
Output 1 (-) connected to Output 2 (+)	GND / 0 V	0 V
Output 2 (-)	V-	-15 V

It is important that the connection between:

Output 1 (-)

and:

Output 2 (+)

does more than connect the power supply outputs in series.

An additional lead must run from this point to the:

0 V or GND terminal of the module

.

The analog circuit to be supplied therefore actually requires three leads:

+15 V

0 V

-15 V

.

How is the common 0 V point created?

The midpoint is created by connecting the two voltage sources in series.

It is not an additional output of the power supply.

With:

15 V + 15 V

the total voltage between the two outer ends is:

30 V

.

If the electrical junction between the two sources is defined as:

0 V

one outer end is at:

+15 V

and the other is at:

-15 V

.

The 0 V point is therefore the circuit’s reference potential

and should, for example, be connected to the relevant module’s:

• GND,
• 0 V,
• signal ground,
• AGND

in accordance with its circuit documentation.

Checking the voltages correctly before connection

Before connecting a sensitive analog circuit, the setup should be checked using a multimeter.

Positive supply:

Black test lead:

0 V

Red test lead:

positive supply rail

Expected reading:

+15.0 V

Negative supply:

Black test lead:

0 V

Red test lead:

negative supply rail

Expected reading:

-15.0 V

Total voltage:

Black test lead:

-15 V

Red test lead:

+15 V

Expected reading:

30.0 V

The module should only be connected once these three measurements are plausible.

What is tracking, and is it required?

Tracking is a convenience function provided by certain multi-channel bench power supplies.

With a tracking power supply

one output can operate as the master channel.

For example, if the user changes its voltage from:

15 V → 12 V

the second output follows accordingly.

With 1:1 tracking, both rails can therefore be changed together, for example from:

±15 V

to:

±12 V

.

However, tracking is not required for a ± supply

Without tracking, the two outputs are simply adjusted separately to the same voltage value.

For:

±15 V

this means:

Output 1 = 15.00 V

and:

Output 2 = 15.00 V

.

The main difference concerns operation

Configuration	Changing the voltage
Two independent outputs	adjust both channels separately
Tracking	the second channel follows the master channel

For a stable ±15 V supply, explicit approval for series connection is more important than a tracking function.

What requirements must the bench power supply meet?

Not every power supply with two output terminals or two channels may automatically be connected in series.

The manufacturer’s approval is decisive

The technical documentation must confirm that:

• series operation is permitted,
• the output potentials are suitable for this purpose,
• the permissible voltages relative to earth are not exceeded,
• no internal connection prevents the intended wiring arrangement.

Connecting arbitrary outputs in series when they are not designed for this can result in:

• short circuits,
• unwanted earth connections,
• malfunctions,
• damage to the power supply.

such consequences.

For the 2228.1 DC Power Supply described below, series and parallel connection of the outputs is expressly specified in the product data.

Understanding current consumption with a ± supply

A symmetrical voltage does not automatically mean that both supply rails must deliver the same current.

For example, an analog module may draw:

+15 V / 120 mA

and:

-15 V / 80 mA

.

This does not necessarily indicate a fault.

Many circuits place asymmetrical loads on the positive and negative supplies.

The decisive factor is:

Each output must be capable of reliably supplying the current required by its respective supply rail.

In addition, the common 0 V point must be connected correctly to the circuit.

Setting the current limits of both channels correctly

When sensitive electronics are commissioned for the first time, the current limit should not automatically be set to the maximum available output current.

The following should be considered:

• normal operating current,
• permissible maximum current of the circuit,
• brief inrush current,
• capacitance on the supply rails,
• current-carrying capacity of cables and connectors.

Example

An analog module requires approximately:

+15 V / 90 mA

and:

-15 V / 75 mA

.

For controlled initial commissioning, the current limit can, for example, be set well above the expected operating current but well below the power supply’s maximum available output.

The precise setting must suit the circuit’s actual switch-on behavior.

What happens if one output switches to CC mode?

During normal operation, a regulated bench power supply attempts to keep the set voltage constant.

This state is known as:

CV – Constant Voltage

.

However, once the output reaches the set current limit, it switches to:

CC – Constant Current

.

The power supply then reduces the output voltage sufficiently to prevent the set maximum current from being exceeded.

With a symmetrical supply, this can cause the symmetry to be lost

Instead of:

+15 V / 0 V / -15 V

the result could, for example, be:

+15 V / 0 V / -7 V

.

Possible consequences include:

• operational amplifiers saturate,
• operating points shift,
• output signals become incorrect,
• the circuit fails to start correctly.

If the supply is unexpectedly asymmetrical, check not only the voltage setting but also the CV/CC state and current consumption of both power supply channels.

Distinguishing between 0 V, ground and protective earth

The common 0 V point of a symmetrical supply is initially an electrical reference potential for the circuit.

It is not automatically identical to:

PE – Protective Earth

or the protective conductor.

With floating outputs

the entire output system can initially be electrically isolated from earth.

Within the permissible technical limits, the user can determine whether and at which point a connection to earth is established.

A connection between 0 V and PE should therefore only be made

if:

• it is required by the circuit,
• it is compatible with the measurement concept,
• the power supply permits this wiring arrangement,
• no other unintended earth connections are present.

Circuit GND and protective earth must not simply be treated as synonymous.

Caution when connecting an oscilloscope

A correctly functioning laboratory setup can suddenly behave completely differently as soon as an oscilloscope is connected.

The reason:

On many benchtop oscilloscopes, the ground of the BNC inputs and probes is connected to protective earth.

When the probe’s ground clip is connected to a point in the analog circuit, that point is simultaneously tied to earth potential.

Example

The midpoint of the ±15 V supply has already been intentionally connected to PE.

The ground clip of an earthed oscilloscope is then inadvertently connected to:

-15 V

.

This effectively short-circuits:

-15 V → PE

.

Before connecting an oscilloscope, it should therefore be established:

• which point in the circuit is connected to earth,
• whether the power supply is being operated with floating outputs,
• which earth connections are introduced by other measuring instruments,
• whether a suitable differential probe may be required.

Why a total voltage of 30 V does not prove that ±15 V is correct

A common mistake when checking a symmetrical supply is to measure only between the two outer supply rails.

The multimeter reads:

30 V

.

However, this does not yet prove that the actual voltages are:

+15 V / 0 V / -15 V

.

The following supply also totals 30 V:

+10 V / 0 V / -20 V

.

For an analog circuit requiring a symmetrical supply, however, this condition would be incorrect.

Therefore, always measure separately:

U(+ relative to 0)

U(- relative to 0)

and additionally:

U(+ relative to -)

.

Asymmetrical loading of the supply rails

Different output currents on the two power supply channels do not automatically indicate a fault.

Example:

Supply	Current consumption
+15 V	130 mA
-15 V	85 mA

This type of unequal loading may be completely normal due to the circuit design.

It becomes problematic, however,

if:

• one channel reaches its current limit,
• one supply voltage drops significantly,
• the current consumption is unusually high compared with the expected condition.

The power supply’s current displays therefore also provide valuable diagnostic information.

Considering switch-on and switch-off behavior

With two independently adjusted power supply outputs, the positive and negative supplies do not necessarily reach their final values at exactly the same microsecond during switch-on or switch-off.

This is not critical for many analog circuits

With sensitive modules, however, briefly asymmetrical supply voltages can cause:

• saturation,
• unexpected output levels,
• current flow through internal protection structures,
• malfunctions during startup.

These are possible consequences.

If the module has specific sequencing requirements

these must also be taken into account.

A simple series connection of two power supply outputs does not replace a defined power-sequencing function.

Practical example: operational-amplifier circuit with ±15 V

An instrumentation-amplifier board requires:

V+ = +15 V

GND = 0 V

V- = -15 V

.

Step 1: Prepare the power supply

Both outputs are initially switched off or disconnected from the device under test.

Step 2: Set the voltages

Both channels are set to:

15.0 V

.

Step 3: Set the current limits

The current limits are selected to suit the board’s expected operating and inrush current.

Step 4: Connect the outputs in series

In accordance with the wiring arrangement specified for the power supply:

Output 1 (-)

is connected to:

Output 2 (+)

.

Step 5: Connect the midpoint to GND

The junction becomes the board’s:

0 V lead

.

Step 6: Connect the positive supply

Output 1 (+) → V+

Step 7: Connect the negative supply

Output 2 (-) → V-

Step 8: Check before connecting

Measurement	Target value
V+ relative to GND	+15 V
V- relative to GND	-15 V
V+ relative to V-	30 V

Step 9: Switch on the module

Now observe the actual currents on both supply rails.

An abnormal indication would be, for example:

+15 V: 80 mA

-15 V: current limit reached

.

In this case, do not simply increase the current limit.

First check, among other things:

• wiring,
• polarity,
• short circuits,
• reverse-polarity electrolytic capacitors,
• defective semiconductor components.

Typical fault patterns

Observation	Possible cause	Recommended check
+15 V is present, but the negative supply is missing	second output is not switched on or is wired incorrectly	check the second output separately
+15 V is correct, but -15 V measures only -7 V	second channel is in current limiting	check current consumption and CC state
30 V is present between the outer rails, but the circuit still does not work	midpoint is missing or individual voltages are asymmetrical	measure both rails separately relative to 0 V
Operational amplifier output is stuck at a supply rail	one supply rail is missing	measure V+ and V- directly at the IC or on the board
Power supply enters CC mode immediately after connection	short circuit, reversed polarity or current limit set too low	check the wiring and expected current demand
Setup works until the oscilloscope is connected	additional earth connection via oscilloscope ground	check the PE and grounding concept
Power supply enters current limiting when the probe is connected	supply rail short-circuited to PE through the oscilloscope	check probe ground and earth reference
Output voltages differ slightly	channels are not set to exactly the same value	adjust and measure both voltages separately
Voltage is correct at the power supply but lower at the board	voltage drop across cables or contacts	measure the voltage directly at the device under test
One supply draws an unusually high current	fault in the circuit	compare the current consumption of both rails

Recommended connection procedure

1. Check the supply requirements: Does the circuit actually require ±12 V, ±15 V or another value?
2. Determine the current demand: Consider the positive and negative supplies separately.
3. Check the power supply: Series operation must be expressly approved by the manufacturer.
4. Do not connect the outputs to the device under test yet: Prepare the setup without the sensitive module first.
5. Set Output 1: For example, 15.0 V.
6. Set Output 2: Also 15.0 V.
7. Set the current limits: Choose values that suit the expected operating and inrush current.
8. Establish the series connection: Connect one output terminal to the opposite-polarity terminal of the second output as specified by the manufacturer.
9. Define the midpoint: Use the series junction as 0 V.
10. Identify the positive supply: Measure relative to the midpoint.
11. Identify the negative supply: Measure relative to the midpoint.
12. Check the total voltage: Measure between the outer terminals.
13. Check PE and ground connections: Do not create any unintended earth connection.
14. Consider additional measuring instruments: Especially earthed oscilloscopes.
15. Connect 0 V to the GND of the device under test.
16. Connect the positive supply to V+.
17. Connect the negative supply to V-.
18. Switch on the device under test: Observe voltages and currents.
19. Check the CV/CC state of both channels.
20. Check the voltages directly at the device under test: Rule out cable losses.
21. Only then carry out the complete functional test.

Suitable bench power supply from ICS Schneider

2228.1 – Dual-channel DC power supply approved for series operation

For the application described here, the 2228.1 DC Power Supply is particularly suitable.

The device has two adjustable DC voltage outputs, each rated at:

0 … 35 V DC / 0 … 4 A

.

ICS specifies the following features for the 2228.1, among others:

• 2 × 0 … 35 V DC output voltage,
• 2 × 0 … 4 A output current,
• 2 × 140 W output power,
• series and parallel connection of the outputs,
• linear regulation,
• automatic CV/CC operating-mode changeover,
• voltage and current regulation,
• short-circuit protection,
• temperature monitoring,
• floating output,
• 4 mm binding posts.

For a ±15 V supply

both outputs are set to:

15 V

and connected in series as intended.

This provides the analog circuit with:

+15 V / 0 V / -15 V

.

Other symmetrical supply voltages are also possible

for example:

• ±5 V,
• ±10 V,
• ±12 V,
• ±15 V,
• ±24 V.

The maximum permissible output ratings and operating conditions of the device must be observed.

No tracking function required

The decisive point for this application is not tracking, but the possibility expressly specified by ICS to connect the two outputs:

in series

.

With the 2228.1, both voltages are set separately to the same value for a symmetrical supply.

Further multiple-output sources can be found under Bench Power Supplies with Multiple Outputs at ICS Schneider.

Conclusion

A symmetrical supply for analog circuits can be generated relatively easily using a suitable dual-channel bench power supply.

±15 V means three potentials

+15 V / 0 V / -15 V

.

Two standard DC voltage sources are sufficient

If two suitable 15 V outputs are connected in series, the total voltage is 30 V.

The junction between the two sources forms 0 V

It is connected to the ground or GND terminal of the analog circuit.

Tracking is only a convenience function

Tracking is not required to generate a symmetrical supply.

Approval for series operation is decisive

Not every dual-channel power supply may be connected in series in any arbitrary manner.

The individual voltages must be checked separately

A reading of 30 V between the outer rails alone does not prove that the ±15 V supply is correct.

Current limiting can affect the symmetry

If one channel reaches its current limit, its output voltage drops and the supply becomes asymmetrical.

0 V is not automatically protective earth

Circuit ground and PE must be consciously distinguished from one another.

Particular caution is required when using an oscilloscope

The earthed probe ground can create an unintended connection between a supply rail and PE.

In practice

Determine the supply requirements → use a suitable power supply explicitly approved for series operation → set both channels to the same voltage magnitude → select appropriate current limits → connect the outputs in series → define the junction as 0 V → measure the positive and negative supplies separately relative to 0 V → check the total voltage → verify the earthing and measurement concept → only then connect V+, GND and V- to the analog circuit → observe the currents and CV/CC state of both channels during switch-on.

FAQ: Symmetrical ± voltage from a bench power supply

How do I generate ±15 V with a bench power supply?

Two suitable 15 V outputs are connected in series. The junction between the two sources is defined as 0 V. The two outer terminals are then at +15 V and -15 V respectively relative to this midpoint.

Do I need a tracking function for this?

No. Tracking merely makes it easier to adjust both voltages together. Two separately adjustable outputs can be used for a fixed symmetrical supply, provided that series operation is expressly permitted.

What does tracking mean on a bench power supply?

With tracking, one output follows the voltage setting of another channel. This allows two coupled voltages to be adjusted together conveniently.

Which is more important: tracking or series operation?

For generating a ± supply, approval for connecting the outputs in series is decisive. Tracking is merely an additional convenience feature.

How is -15 V created if both power supply outputs generate positive voltages?

The negative voltage is created by the choice of reference point. If the junction between two 15 V sources is defined as 0 V, the outer negative terminal of the second source is at -15 V relative to this point.

What is the voltage between +15 V and -15 V?

The voltage between the two outer supply rails is 30 V.

What is the total voltage with ±12 V?

The total voltage between +12 V and -12 V is 24 V.

Why does the circuit require three leads?

A conventional symmetrical supply has a positive supply, a common 0 V reference and a negative supply.

What is connected to the board’s GND terminal?

The midpoint between the two series-connected voltage sources is connected to the board’s GND or 0 V terminal.

Is GND automatically protective earth?

No. Circuit GND or 0 V and PE initially perform different electrical functions.

Does the midpoint have to be earthed?

Not necessarily. Whether an earth connection is required depends on the circuit, power supply and measurement setup.

Can any dual-channel bench power supply be used for ±15 V?

No. The manufacturer must permit series operation of the outputs concerned.

Why must I observe the manufacturer’s specifications for series operation?

Internal ground or earth connections can prevent a series connection or cause short circuits.

What happens when both outputs are set to 15 V?

In a suitable series connection, the total voltage is 30 V. If the junction is defined as 0 V, this results in +15 V and -15 V.

Can I also generate ±12 V?

Yes. Set both outputs to 12 V.

Can I also generate ±5 V?

Yes, provided that both power supply outputs can be adjusted accordingly and the circuit requirements are met.

Must both outputs be set to exactly the same value?

For a symmetrical supply, the magnitudes of the positive and negative supply voltages should match within the required tolerance.

Why is a 30 V measurement between the outer rails not sufficient?

An asymmetrical distribution such as +10 V and -20 V also totals 30 V. Both supply rails must therefore be measured separately relative to the midpoint.

Must both supply rails deliver the same current?

No. Many analog circuits load the positive and negative supplies differently.

What happens when one output reaches its current limit?

The affected channel enters constant-current mode and reduces its output voltage. This can make the symmetrical supply asymmetrical.

What do CV and CC mean?

CV stands for Constant Voltage mode. CC stands for Constant Current mode.

Why is current limiting useful during initial commissioning?

It can limit the energy released in the event of short circuits, wiring errors or defective components.

Why does my analog circuit not work with only one supply rail?

A circuit designed for a symmetrical supply can reach incorrect operating points or saturation states if the positive or negative rail is missing.

Why does the circuit stop working as soon as I connect an oscilloscope?

On many benchtop oscilloscopes, the probe ground is connected to PE. This can introduce an additional earth connection into a previously floating laboratory setup.

Can the oscilloscope ground cause a short circuit?

Yes. For example, if it is connected to the negative supply while the midpoint is already earthed, the negative supply may be short-circuited to earth.

Which power supply from the ICS portfolio is suitable for this application?

The 2228.1 DC Power Supply has two adjustable outputs, each rated at 0 … 35 V DC / 0 … 4 A. ICS expressly states that the outputs can be connected in series or parallel.

Can the 2228.1 generate ±15 V?

Yes. If both designated outputs are set to 15 V and correctly connected in series, the midpoint can be used as 0 V, providing +15 V and -15 V.

What is the maximum output voltage of the 2228.1 per channel?

Each of the two outputs is adjustable from 0 to 35 V DC.

How much current does the 2228.1 deliver?

ICS specifies an adjustable output current of up to 4 A for each channel.

Is the 2228.1 a linear power supply?

Yes. ICS describes the device as a linear power supply with voltage and current regulation.

Does the 2228.1 provide CV and CC modes?

Yes. The device automatically switches between constant-voltage and constant-current modes.

Is the 2228.1 short-circuit-proof?

Yes. This is expressly specified in the technical product data from ICS.

Where can I find more bench power supplies with multiple outputs?

Further devices can be found under Multiple-Output Sources / Bench Power Supplies at ICS Schneider.