Connecting Laboratory Power Supplies in Parallel or Series: Observe Limits and Protection

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A laboratory power supply provides a maximum of:

24 V / 6 A

However, a test setup requires:

48 V

or alternatively:

12 A at 24 V

.

An obvious solution is therefore to combine two power supply outputs. There are basically two different configurations:

• Series connection: The output voltages are added together.
• Parallel connection: The available output currents can be added together.

However, this only works electrically if the specific power supply is suitable for the respective operating mode. Particularly with parallel connection, even small differences in output voltage can lead to highly uneven current sharing or even reverse currents between the power supplies.

Two arbitrary laboratory power supplies must therefore not be connected in series or parallel simply because the voltage and current ratings on their nameplates are similar. Manufacturer approval, galvanic isolation, permissible voltage to earth, reverse-current capability and the behavior of the current limiting function must all be suitable for the intended configuration.

A suitable example from the ICS portfolio is the 2225.2 DC Power Supply. The device has two adjustable outputs, each providing 0 … 24 V DC and 0 … 6 A. The outputs are explicitly designed for series and parallel connection. Further devices can be found under Power Supplies / Laboratory Power Supplies and under Power Supply Technology at ICS Schneider.

Understanding series and parallel connection

The two configurations serve different purposes.

Configuration	What increases?	What remains limited?
Series connection	Output voltage	Current is limited by the weakest output
Parallel connection	Available output current	Output voltage remains the same

Series connection

For two suitable outputs, the following simplified relationship applies:

Utotal = U1 + U2

Example:

24 V + 24 V = 48 V

Parallel connection

For two outputs approved for parallel operation, the following may apply approximately:

Itotal = I1 + I2

Example:

6 A + 6 A = up to 12 A

The key difference is that a series connection is electrically comparatively straightforward, whereas with a parallel connection both voltage sources operate directly on the same output node.

Connecting power supplies in series

For a series connection, the positive terminal of one output is connected to the negative terminal of the second output.

The load is then connected across the two remaining free terminals.

In simplified form:

− Output 1 / + Output 1 → − Output 2 / + Output 2

The connection between:

+ Output 1 ↔ − Output 2

forms the midpoint of the series connection.

Example

Both power supply outputs are set to:

24 V

.

Between the two outer terminals, the measured voltage is then:

48 V

.

Requirement

The outputs must be designed for series operation and must not be internally connected to protective earth or the housing in such a way that the series connection causes a short circuit.

Especially when using several separate power supplies, it must therefore be clarified before connecting them whether the DC outputs are:

galvanically isolated and floating

.

Why current does not add up in series

A common misconception is:

2 × 24 V / 6 A → 48 V / 12 A

This is incorrect for a series connection.

The same current flows through all components in a series circuit.

With two outputs each rated for a maximum of 6 A

the available output is therefore, for example:

48 V / maximum 6 A

and not:

48 V / 12 A

The maximum usable power can nevertheless increase:

24 V · 6 A = 144 W per output

and therefore:

48 V · 6 A = 288 W total

provided that both outputs are approved for this operating mode.

Observe output potential relative to earth and housing

With a series connection, not only the voltage across the load increases.

The electrical potential of individual output terminals relative to:

PE / housing / earth

can also increase.

Example

With two 24 V outputs connected in series, the upper output may, for example, be between:

+24 V and +48 V relative to the lower reference point

.

For the power supply, therefore, not only the maximum output voltage of the individual channel is relevant.

The following must also be checked:

• permissible output voltage relative to earth,
• isolation voltage between the outputs,
• permissible voltage relative to the housing,
• permissible total voltage of the series connection.

An output rated for 30 V is not automatically suitable for operation several hundred volts above earth potential within a larger series arrangement.

Connecting power supplies in parallel

With a parallel connection:

positive is connected to positive

and:

negative is connected to negative

.

The output voltage generally remains the same.

The objective is for both sources to supply the load current together.

Example

Two outputs approved for parallel operation, each rated at:

24 V / 6 A

can, with suitable current sharing, jointly supply a load requiring more than the 6 A available from one channel.

Ideally:

24 V / up to 12 A

However, this only works with controlled current sharing

Parallel connection does not automatically mean:

50% current from power supply 1 + 50% from power supply 2

.

Even small differences in output voltage can cause one power supply to take on a significantly larger share of the load.

Why current sharing is critical

Assume power supply A provides:

24.05 V

and power supply B:

23.95 V

.

If both outputs are connected directly in parallel, the device with the higher output voltage will initially attempt to supply most of the load.

Power supply A may therefore already reach its current limit while power supply B is still providing significantly less current.

Depending on the device, ideal current sharing therefore requires

• matched output voltages,
• a suitable output characteristic,
• manufacturer approval for parallel operation,
• active current sharing where applicable.

Two arbitrary constant-voltage sources should therefore not be connected directly in parallel.

Current limiting and CV/CC operation

A regulated laboratory power supply typically operates in two modes:

CV = Constant Voltage

and:

CC = Constant Current

In CV mode

the power supply maintains the set voltage as long as the load does not require more than the set or maximum available current.

In CC mode

the current is limited and the output voltage decreases accordingly.

With two power supplies connected in parallel

power supply A may initially take on a larger share of the current.

Once it reaches its current limit, it switches to CC mode.

Only then may the second power supply take on a larger share.

The result may function electrically, but it does not necessarily represent clean and stable current sharing.

The decisive factor is therefore which parallel operating method the manufacturer specifies for the particular device.

Master-slave operation and active current sharing

More powerful programmable power supplies often provide dedicated functions for parallel operation.

Master-slave principle

One device assumes the control function:

Master

The other devices follow its setpoint:

Slave

This allows several devices to be adjusted synchronously.

Active Current Sharing

With active current sharing, the devices exchange additional control information.

This allows the total current to be distributed as evenly as possible among the participating sources.

This is particularly advantageous for:

• high power levels,
• continuous parallel operation,
• dynamic loads.

However, such a function may only be used if it is explicitly provided for the particular power supply. Not every power supply that can be connected in parallel has active master-slave control.

Design cable resistances symmetrically

The wiring also influences current sharing.

A conductor has the resistance:

R = ρ · l / A

This results in a voltage drop when current flows:

ΔU = I · R

In a parallel configuration

both power supplies should have electrically similar paths to the load wherever possible.

An unfavorable arrangement would be:

Power supply A	Power supply B
0.5 m cable	3 m cable
4 mm²	1.5 mm²

The different cable resistance changes the voltage at the common load point and therefore affects current distribution.

Better

• same cable length,
• same conductor cross-section,
• common defined distribution point.

For high currents, a star-shaped connection to an adequately rated busbar may be useful.

Using Remote Sense correctly

Some programmable power supplies have separate sense terminals.

These allow the power supply to measure the actual voltage directly at the load and compensate for voltage drop in the power cables.

In standalone operation

this function is very useful.

In parallel operation

incorrectly connected sense lines can, however, cause several control loops to work against each other.

Therefore:

With power supplies connected in parallel, Remote Sense wiring must be implemented exactly according to the manufacturer’s instructions.

If shared parallel operation with Remote Sense is not provided, an improvised control configuration should not be used.

Avoid reverse power flow into power supplies

Many conventional laboratory power supplies are:

sources

They can supply current.

However, they are not automatically:

electronic loads

and therefore cannot absorb arbitrary amounts of current.

This becomes problematic, for example

when a second power supply attempts to drive current into the output of a switched-off or lower-set device.

The device under test itself may also feed energy back.

Possible consequences include:

• impermissible reverse voltage at the output,
• damage to output capacitors,
• damage to the power stage,
• uncontrolled regulation behavior.

Before series or parallel operation, therefore, check

which:

• maximum external output voltage,
• permissible reverse current,
• reverse-voltage or reverse-current protection function

is specified by the manufacturer.

Consider inductive loads and motors

Reverse power flow is particularly relevant with inductive loads.

These include, for example:

• motors,
• solenoid valves,
• relays,
• coils.

Energy is stored in an inductance:

E = 1/2 · L · I²

If the current is interrupted suddenly, the inductance attempts to maintain the current flow.

This can cause

• voltage spikes,
• negative output voltages,
• reverse currents.

Depending on the application, suitable protective circuits may therefore be required, for example freewheeling or overvoltage protection.

These must, however, be suitable for both the load and the power supply.

Pay particular attention to batteries and active loads

A battery behaves fundamentally differently from a purely passive resistor.

It has its own electrical voltage.

If a battery is connected to a group of power supplies, it can therefore supply current even when the power supplies are:

• switched off,
• set too low,
• disconnected from the mains.

The same applies to active devices under test

such as:

• motor drives,
• DC links,
• other power supplies,
• drives operating regeneratively.

For such applications, it must be explicitly checked whether the power supply can absorb energy or whether additional decoupling measures are required.

Observe the switch-on and switch-off sequence

When several power supplies are interconnected, different operating states can occur briefly during switching on and off.

Example

In a series connection, output A is already active while output B is still switched off.

Depending on the load, an external voltage may then appear at the output of the switched-off device.

In parallel operation

one device may already be supplying:

24 V

while the second is still at:

0 V

.

Without a suitable internal protection function, current may then flow into the second device.

Start-up, shutdown and fault states must therefore also be included in the assessment, not just the stable normal operating condition.

Do not unintentionally earth floating outputs

Many laboratory power supplies have a floating output.

If required, it can deliberately be connected to protective earth at one point.

However, when using several power supplies it must be checked

whether a connection to earth already exists via:

• measuring instrument,
• oscilloscope,
• USB connection,
• device under test,
• shielding.

Typical error

Two power supplies are connected in series.

At the same time, an earthed oscilloscope connects an otherwise floating midpoint to PE.

This can:

• alter the series connection,
• short-circuit an output,
• create an unexpected current path.

Before connecting additional measuring equipment, the complete potential configuration should therefore always be considered.

Parallel operation is not redundancy

Two power supplies connected in parallel do not automatically create a redundant power supply.

When increasing power

the following may apply, for example:

Power supply A 6 A + power supply B 6 A → load 10 A

If one of the devices fails, the remaining 6 A power supply can no longer supply the load.

For true redundancy

each remaining power supply system must still be able to supply the required load after the assumed fault.

This typically also requires:

• decoupling,
• separate fuse protection,
• failure indication,
• sufficient power reserve.

Parallel operation for increased power and redundant power supply are therefore two different tasks.

Practical example: 48 V from two 24 V outputs

A laboratory power supply with two outputs approved for this purpose is used:

2 × 0 … 24 V / 0 … 6 A

Objective

A device under test requires:

48 V / 4 A

Procedure

1. Initially set both outputs to 0 V.
2. Disable the outputs or switch off the power supply.
3. Connect the positive terminal of output 1 to the negative terminal of output 2.
4. Connect the device under test to the two remaining outer terminals.
5. Set both outputs to 24 V each.
6. Configure the current limit to suit the device under test.
7. Check the total voltage before connecting or switching on the device under test.

Result

The device under test is supplied with:

48 V

.

The load current is:

4 A

and flows equally through both outputs.

The maximum possible 6 A of each individual output must not be exceeded by the series connection.

Practical example: providing higher current

Two outputs approved for parallel operation each provide:

0 … 24 V / 0 … 6 A

A load requires:

24 V / 9 A

Before connecting the outputs together

both outputs are set to the same voltage value.

They are then connected in accordance with the manufacturer’s instructions:

Positive ↔ Positive

and:

Negative ↔ Negative

.

Both cable paths to the load are designed to be comparable.

Under load

the following are checked:

• total current,
• current from each output,
• CV/CC operating status,
• voltage directly at the load,
• temperature of cables and connections.

The decisive factor is not only that the load receives 9 A, but also that none of the participating outputs is operated continuously outside its permissible load range.

Typical fault patterns

Observation	Possible cause	Recommended check
Two 24 V power supplies in series do not provide 48 V	Output internally earthed or wiring incorrect	Check floating capability and wiring
One power supply provides almost all the current in parallel operation	Output voltages differ	Check voltage matching and manufacturer procedure
One power supply immediately switches to CC mode	Uneven current sharing or current limit set too low	Check current limits and cable resistances
Output voltage does not fall to zero after switching off	Reverse power flow from second power supply or device under test	Check reverse-current path
Series connection works until the oscilloscope is connected	Unexpected earth connection via measuring instrument	Check PE reference of the measuring equipment
Parallel power supplies regulate unstably	Control loops are working against each other	Check parallel-operation approval or current-sharing function
Voltage at the load is lower than at the power supply	Cable loss	Check conductor cross-section and cable length
Current sharing changes as the load increases	Different cable resistances or output characteristics	Use symmetrical wiring
Power supply is damaged when operating a motor	Regenerative feedback or voltage spikes	Check reverse-power capability and protective circuitry
Series arrangement exceeds permissible insulation limits	Voltage relative to earth too high	Check permissible output potential relative to PE

Safe procedure before combining power supplies

1. Check manufacturer approval: Is series or parallel operation explicitly permitted?
2. Check output isolation: Are the required outputs galvanically isolated or floating?
3. Check maximum total voltage: Take into account permissible voltage to earth and between outputs.
4. Assess reverse power flow: Can a power supply or the device under test feed energy back?
5. Set current limiting: Protect the device under test and the cables.
6. Initially disable the outputs: Make the wiring connections while de-energized.
7. Check polarity: Particularly before a parallel connection, clearly verify positive and negative.
8. Measure voltages individually: Check the setpoints before combining the outputs.
9. Dimension the cables: Consider current-carrying capacity and voltage drop.
10. Use symmetrical wiring for parallel operation: Use comparable cable paths.
11. Make the connection: Only according to the intended operating mode.
12. Check total voltage without load: Verify the expected value.
13. Increase the load gradually: Observe current sharing and operating states.
14. Exclude reverse currents: Particularly during switch-off and with active loads.
15. Consider measuring equipment: Avoid unintended earth connections.
16. Check fault conditions: Assess the behavior if one output is switched off.

Suitable laboratory power supplies from ICS Schneider

2225.2 DC Power Supply – two combinable outputs

The 2225.2 DC Power Supply is particularly well suited to the topic of this article.

The key technical specifications are:

• 2 × 0 … 24 V DC,
• 2 × 0 … 6 A,
• 2 × 144 W output power,
• galvanic isolation between input and output,
• floating outputs,
• designed for series and parallel connection,
• CV/CC regulation,
• short-circuit-proof design,
• temperature monitoring.

Series connection

With two appropriately set 24 V outputs, a higher total voltage can be generated.

The maximum current remains limited by the permissible current of the individual outputs.

Parallel connection

The two outputs can also be operated in parallel to provide a higher available load current.

Settings, wiring and loading must be implemented in accordance with the device design.

Further laboratory power supplies

ICS Schneider also offers:

• Single-Output Power Supplies,
• Multi-Output Power Supplies,
• High-Power Bench Power Supplies

for a wide range of testing, laboratory and development applications.

Conclusion

Series or parallel connection can significantly extend the capabilities of suitable laboratory power supplies.

In series, the voltage increases

The output voltages are added together, while the maximum available current remains limited by the weakest participating output.

In parallel, the available current increases

The voltage remains the same. However, the sources must be explicitly designed for parallel operation and must share their load currents in a controlled manner.

Galvanic isolation is particularly important for series operation

In addition, the permissible voltage of the outputs relative to earth or housing must be observed.

Reverse power flow is often underestimated

A laboratory power supply can generally deliver current, but it cannot automatically absorb current. Other power supplies, batteries, motors and active devices under test can therefore generate critical reverse currents.

Master-slave operation and Current Sharing improve parallel operation

However, such functions may only be used if they are provided for the specific device.

For practical applications

Check manufacturer approval → verify galvanic isolation and earth reference → define the required series or parallel configuration → calculate voltage and current limits → consider reverse power flow → set current limits → wire the system while de-energized → check polarity → use symmetrical wiring for parallel operation → verify total voltage → apply the load gradually → monitor current sharing and CV/CC status → check behavior during switching on and off.

FAQ: Connecting Laboratory Power Supplies in Series or Parallel

Can two laboratory power supplies be connected in series?

Yes, provided the outputs are suitable for series operation and are sufficiently galvanically isolated or floating. The permissible voltages relative to earth must also be observed.

What happens to the voltage in a series connection?

The output voltages are added together. Two outputs of 24 V each can, for example, provide a total voltage of 48 V.

Does the current also add up in a series connection?

No. The same current flows through all devices. With two outputs each rated for a maximum of 6 A, the maximum current generally remains 6 A.

Can I connect two 30 V power supplies to obtain 60 V?

Only if both devices are approved for series operation and the permissible insulation or voltage relative to earth is not exceeded.

Why must the outputs be floating for series operation?

An output permanently connected to PE can create an unintended current path or short circuit through the protective earth connection.

Can two laboratory power supplies be connected in parallel?

Only if the manufacturer explicitly permits parallel operation. Two arbitrary voltage sources should not be connected directly in parallel.

What happens to the voltage in a parallel connection?

The output voltage generally remains the same.

What happens to the current?

With suitable devices, the available output current can increase. However, controlled current sharing is essential.

Why must the voltages be set to the same value for parallel operation?

Even small voltage differences can cause the device with the higher output voltage to provide a much larger share of the load current.

Can current flow from one power supply into the other?

Yes. With unsuitable parallel connection or different output voltages, reverse current can occur. Not every power supply is designed for this.

What does reverse power flow mean?

Reverse power flow means that electrical energy flows back into the output of a power supply instead of flowing from the power supply to the load.

Can a switched-off power supply have voltage at its output due to a second power supply?

Yes. This is possible with unsuitable parallel or series arrangements and must be taken into account during system design.

What does CV mean?

CV stands for Constant Voltage. The power supply maintains the set output voltage.

What does CC mean?

CC stands for Constant Current. The power supply limits the output current and reduces the voltage if necessary.

What happens if one power supply in a parallel connection enters CC mode?

That device then limits its current. Depending on the control behavior, the second device may take on a larger share of the load.

What is master-slave operation?

One power supply acts as the master and defines the setpoint, while additional devices operate as slaves and follow that setpoint.

What is Current Sharing?

Current Sharing is active current distribution between power supplies operating in parallel. This allows the load current to be distributed more evenly among several sources.

Do the cables have to be the same length in parallel operation?

Comparable cable lengths and conductor cross-sections improve current sharing because different cable resistances would otherwise cause additional voltage differences.

What is Remote Sense?

Remote Sense measures the voltage directly at the load and allows the power supply to compensate for voltage drop in the connection cables.

Can Remote Sense be used with several power supplies connected in parallel?

Only in accordance with the manufacturer’s instructions. Incorrectly connected sense lines can cause the control loops to work against each other.

Are batteries critical as loads?

Yes. A battery can itself feed current back into the power supply. It must therefore be explicitly checked whether the power supply is suitable for this application.

Why can motors be problematic?

During deceleration, motors can operate regeneratively and feed energy back into the power supply.

Is a parallel connection automatically redundant?

No. In a parallel arrangement used to increase power, failure of one power supply may mean that the remaining unit can no longer supply the load.

Can I use an oscilloscope with a series connection?

Yes, but the earth reference of the oscilloscope connections must be taken into account. An earthed probe can connect an otherwise floating point to PE and thereby alter the circuit.

Which ICS power supply is suitable for series and parallel operation?

The 2225.2 DC Power Supply has two outputs, each providing 0 … 24 V DC and 0 … 6 A, and is explicitly designed for series and parallel operation.

What output power does the 2225.2 provide?

The device has two outputs, each with an output power of 144 W.

Where can I find further laboratory power supplies?

Further devices can be found under Power Supplies / Laboratory Power Supplies at ICS Schneider.