How to Choose a Replacement Amplifier for a Proportional Valve

A Real-World Situation

There is a hydraulic machine. It worked for years. Then the proportional valve amplifier failed. The module says Bosch Rexroth, Parker, HAWE, Wandfluh, Atos, Duplomatic, HydraForce or some other brand. A new original module is expensive, has a long lead time or is no longer supplied.

The question is: can it be replaced with another amplifier?

Often, yes. But not because it "looks similar". It has to be electrically and functionally compatible.

First Identify the Valve Type

The most important question is whether the old module controlled the coil directly or worked with a valve that has a position sensor or built-in electronics.

VL-PVD1-24 and similar current drivers are intended for valves where the coil is driven directly, without a closed spool-position loop inside the amplifier.

Suitable classes include:

• proportional pressure valve without built-in electronics;
• proportional flow valve without built-in electronics;
• proportional cartridge valve with a coil;
• open-loop proportional directional valve;
• directional valve with two A/B coils, if two amplifiers are used.

Unsuitable classes, or classes that require separate analysis, include:

• servo valve;
• valve with LVDT where the amplifier closes the position loop;
• valve with onboard electronics that already accepts a 0-10 V or 4-20 mA command (this is a more complex question; sometimes that module can also be replaced);
• system where the old module outputs special ready/fault signals, participates in safety or is connected to the PLC in a non-standard way.

Check the Coil

You need to know:

• rated coil voltage;
• rated or maximum current;
• coil resistance;
• power;
• connector type;
• whether there is one coil or two.

The best source is the valve or coil datasheet. If there is no datasheet, measure resistance and check the marking.

Important: the "24 V" marking on the coil does not by itself define the required amplifier. For a current driver, the main question is what current is required and whether that current is achievable with the available supply voltage.

Check the Supply Voltage

Most industrial systems use 24 V DC, but the actual supply may be 22 V, 27 V, noisy, sagging under load or connected through long cables. The amplifier has to work in that range.

VL-PVD1-24 is designed for 12-30 V DC supply. At the same time, maximum coil current depends on supply voltage and coil resistance. If the coil resistance is too high, 24 V may not be enough to reach the required current.

Estimate:

Iachievable ≈ (Usupply - Ulosses) / Rcoil

If the required current is higher than the achievable current, you need another operating mode, another supply voltage, another coil or a driver with a boost converter.

Check the Command Signal

The old amplifier may have accepted:

• 0-10 V;
• 4-20 mA;
• 0-20 mA;
• PWM;
• ±10 V;
• digital command;
• potentiometer signal;
• special interface.

VL-PVD1-24 supports 0-10 V, 4-20 mA/0-20 mA, PWM and UART. A bipolar ±10 V input is not supported by default. If the old system really used ±10 V for two directions, the wiring has to be checked carefully: a two-channel version or signal conversion may be needed.

Check the Number of Coils

A single-coil valve is usually replaced with one driver.

A 4/3 directional valve often has two coils. In that case, the options are:

• two single-channel drivers;
• a two-channel version;
• another controller if interlock logic, special inputs or feedback are required.

Do not try to connect two coils to one output unless the circuit specifically allows it.

Check the Old Module Settings

If the old amplifier is still available, it is useful to write down or photograph:

• maximum current;
• minimum current or bias;
• dither frequency/amplitude;
• ramp up/ramp down;
• input type;
• command polarity;
• diagnostic modes;
• zero and span settings.

Even if the values cannot be transferred one-to-one, they provide a good starting point.

Compatibility Checklist

What to Send for Selection

To check compatibility quickly, usually it is enough to send:

1. A photo of the whole valve.
2. A photo of the coil marking.
3. A photo of the old amplifier with marking.
4. A photo of the terminals or wiring diagram.
5. Supply voltage in the cabinet.
6. What comes as the command: 0-10 V, 4-20 mA, PWM or something else.
7. Whether there is one coil or two.
8. What the valve controls: pressure, flow or direction.