30-01-2024, 09:38 PM
It's very commonly done. The difference between "pull-in" and "hold-in" is huge, and there are decent power savings to be had.
A capacitor in the drive circuit is the most common, but as it needs to carry a large current into a low impedance, it tends to be quite a large value. More sophisticated schemes exist, such as two voltages, or two transistor, where one maintains the current via a resistor. Also seen a PWM setup, which can be quite neat if you already have a microcontroller in the design.
One of my preferred options is to switch on the relay with a saturated transistor that applies practically the full voltage to the coil. Then after a short delay, the transistor becomes a current source. This has the advantage of being resistant to voltage drops - these circuits are often powered by unregulated supplies.
Assuming you already have a comparator to provide the usual power-on delay, it only takes another comparator, a diode or two, and a couple of passives to do this. As a dual comparator takes up no more space than a single, it's a neat solution.
Also, it's inherently self-resetting, as the same RC is providing both delays. This can be a worry with simple passive systems.
A capacitor in the drive circuit is the most common, but as it needs to carry a large current into a low impedance, it tends to be quite a large value. More sophisticated schemes exist, such as two voltages, or two transistor, where one maintains the current via a resistor. Also seen a PWM setup, which can be quite neat if you already have a microcontroller in the design.
One of my preferred options is to switch on the relay with a saturated transistor that applies practically the full voltage to the coil. Then after a short delay, the transistor becomes a current source. This has the advantage of being resistant to voltage drops - these circuits are often powered by unregulated supplies.
Assuming you already have a comparator to provide the usual power-on delay, it only takes another comparator, a diode or two, and a couple of passives to do this. As a dual comparator takes up no more space than a single, it's a neat solution.
Also, it's inherently self-resetting, as the same RC is providing both delays. This can be a worry with simple passive systems.
