Geeplus Pick and Hold Drive Circuits

Pick and Hold Drive Circuit: How it Works

Pick and Hold is a technique for energising solenoids that maximises the force and speed achievable from a given device, and/or reduces heat dissipation and power consumption. The force or torque characteristics for a solenoid device typically show that more force or torque is developed as power applied to the device is increased. The force characteristic also typically shows that force increases as the device moves towards it’s energised position.

A ‘Pick & Hold’ drive circuit applies high power to the device initially, to develop high starting force / fast acceleration. After an initial period, the power is reduced to a lower level sufficient to ‘hold’ the position, but with reduced power consumption and heat dissipation.

The PHu modules can be used to implement control of large solenoids in an end-user application. The user-friendly interface also makes them a superb development tool to explore the maximum performance achievable from a wide range of solenoids during product development.

PHu ‘Pick & Hold’ Modules

Geeplus Pick & Hold drive circuits are efficient PWM current control circuits that facilitate optimisation of solenoid drive conditions. The circuits allow drive conditions to be defined in a PC application for PICK current and duration, and for HOLD current.

While connected to the PC, the application will display the waveform of the excitation current. The current waveform typically exhibits a ‘spike’ when the device impacts an end-stop. This allows the response time to be seen, and the PICK current and duration to be set appropriately for the application.

Capacitive Pick & Hold #1

In the circuit shown, a resistor and capacitor are connected in series with the solenoid, and in parallel with one another. When the solenoid is energised, the capacitor initially has no potential across it, and the full supply voltage is applied across the solenoid coil.

As current flows, the capacitor charges and potential develops across it, reducing the voltage applied across the solenoid coil. The resistor limits the long-term ‘Hold’ current supplied to the solenoid.

This circuit requires the power supply to be able to deliver the initial ‘Pick’ power drawn by the solenoid.

Capacitive Pick & Hold #2

In the circuit shown, a capacitor is connected in parallel with the solenoid and switching device, and a series resistor in line with the supply circuit. When the solenoid. is de-energised, the capacitor charges via the series resistor. It will ultimately charge to the supply voltage

When the solenoid is energised, the capacitor will initially source current to deliver the full supply voltage across the solenoid coil. As current flows, the capacitor discharges, reducing the voltage applied across the solenoid coil. The resistor limits the long-term ‘Hold’ current supplied to the solenoid.

With this circuit the initial ‘Pick’ power is drawn from the capacitor, the current drawn from the supply is limited by the series resistor.

Pick & Hold Implementation

‘Pick & Hold’ drive of solenoids can be implemented in other ways than the examples shown. For example, in systems where multiple supply voltages are available, the device could be energised from different supply rails, or a solenoid could be wound with multiple coils to facilitate excitation with different power from the same supply voltage.

However the drive is finally implemented, the module – along with the associated PC application – offers a simple and user-friendly way to evaluate the impact of different excitation conditions, and to visualise the excitation and the response-time of the device under test.