Parallel & Series Circuits


  • Explain the concepts of series and parallel circuits.
  • Establish the path of least resistance.
  • Determine system and branch pressure.

What is a Parallel Circuit?

Most every basic hydraulic system has more than one flow path.

That is to say, once the pump has drawn in fluid from the reservoir, there is more than one potential path through valves and cylinders/motors for fluid flow back to the reservoir.

The flow paths or branches are said to be in parallel with each other.

For Example...

Consider the most basic log splitter.

It has one flow path from pump to reservoir via the flow control valve and cylinder.There is another path to reservoir through the relief valve, if it were to open up.

What is a Series Circuit?

It is also very common for a parallel branch to have more than one point of restriction.

Each restriction (essentially a load) in series with the next, has a cumulative effect on the total pressure at the beginning of the branch.

For Example...

Directional/Flow Control Valve
in Series with Cylinder

A series-parallel example could be a directional control valve in series with the cylinder.

The noticeable loads in series would be the cylinder pushing a log and the directional/flow control valve.

The parallel path is the relief valve connection to the reservoir.

The Path of Least Resistance

In the following examples, spring loaded check valves are used as generic loads on a hydraulic circuit branch.

The check valves can easily be replaced by cylinders or motors that have heavy loads to move.

The spring loaded check valves are only used as simple examples for teaching the concepts.

Consider a system with three parallel flow paths.

100 PSI
200 PSI
300 PSI

Each path has a simple spring loaded check valve.

Each check valve's spring has a different strength expressed in equivalent PSI.

When the system is turned on and oil begins to circulate it will travel through the path of least resistance.

In this case that would be through the 100 PSI check valve.

Which means the system pressure will have to rise to 100 PSI in order to overcome the spring in the check valve.

100 PSI
200 PSI
300 PSI
100 PSI

But what happens when we close off the 100 PSI branch?

100 PSI
200 PSI
300 PSI
200 PSI

Or if we close off both the 100 PSI and 200 PSI branchs?

100 PSI
200 PSI
300 PSI
300 PSI

Critical Concept!

Oil in a pressurized system will always follow the path of least resistance.

In a simple system with more than one flow path (parallel paths), the pressure will only rise to the level needed to take the easiest path.

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