1

Most of the time a relay is powered from a low-voltage DC source like a battery or a permanent stable DC power source. However are there modern relay designs that draw current from the circuit that they are actually switching? Perhaps a smart relay can detect the voltage, type AC or DC and do a conversion and then draw the current from the switching contacts to power the relay controllers. Eg. SIM modem, radio receiver and transistor controlling relay. This will eliminate the provision of a DC source.

A mechanical relay is said to require at the very least 50 mA of current to operate and statistically most practical applications are for actually switching are of higher voltages and currents than what's needed to operate the relays themselves. ls such a design possible where a relay draws the current it needs to operate from it's contacts and does it exist?

Nederealm
  • 157
  • 4
  • 2
    It's possible, and doesn't even require a "modern" relay design. Mains-voltage AC relays have been a thing for pretty much as long as relays have been a thing. – Hearth Oct 08 '22 at 15:27
  • 2
    If you want to switch that relay from a low-power source, you may need a relay driver -- effectively an amplifier that can control just enough load current to switch the bigger relay. This may be as simple as a single transistor, a diode (for back-EMF suppression), and some passive components, if the relay controlling doesn't draw a lot of power, or may be a matter of using a small relay to control the larger one. – keshlam Oct 08 '22 at 15:43

3 Answers3

1

are there modern relay designs that draw current from the circuit that they are actually switching?

Most folk use a relay because they want galvanic isolation hence, the coil circuit and contact circuit sharing the same power rail will be a significant no-no. Isolation will be breached.

However, there's nothing stopping you designing an elegant buck-boost converter that can work from a range of voltages that might be present on the relay-contacts. Then, use an opto-isolator to activate the coil using the power supplied by the buck-boost converter.

I guess the first problem is that a lot of relay-contacts are switching AC so, a bridge rectification scheme will be needed then, if you want to make the relay as general as possible you'd have to diode-OR power from each contact pin to feed the power converter. You would also need to supply a negative rail too and that then means adding an extra wire to the relay-contact circuits.

Then, there's the problem of things like surge protection. Relay contacts are pretty darn robust in this respect. Not so DC-DC converters; you would need TVS diodes and quite possibly fuses too. After all, there are significant regulations about what can and cannot connect between AC live and neutral.

Andy aka
  • 434,556
  • 28
  • 351
  • 777
  • I can think of applications that require same power rail. Of course I could build a seperate circuit that draws and convert the current off the same rail. Only that it would be good if there is a more self-contained unit and ready-made solution that I can buy off the shelf. This modularized approach means I can focus on the controller circuit type. – Nederealm Oct 09 '22 at 03:03
1

Relays are available with many coil voltages for AC or DC operation. You must select the coil rating to suit your application. You can't use a DC rated relay with an AC supply, or an AC rated relay on a DC supply.

It is quite common to use the same supply to control the relay as is applied to the relay contacts.

Peter Bennett
  • 57,014
  • 1
  • 48
  • 127
1

Can a relay be powered from its switching current source?

Here's a typical application.

It's an L & T Type MK1 direct-on-line motor starter that's totally enclosed in an earthed metal housing and intended for use with a 3 x 415 V 50 Hz - 7.5 HP motor.

The schematic is as shown.

enter image description here

The contactor coil voltage is 415 V ~.

It is energised by the line voltage of the three phase supply that drives the motor.

vu2nan
  • 15,929
  • 1
  • 14
  • 42