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switch relay from transistor

Discussion in 'General Electronics Discussion' started by scmojks, Nov 17, 2012.

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  1. scmojks


    Nov 17, 2012
    Attached is a section from a manual from an industrial camera.

    The following is also a snippet from the manual:

    The camera is equipped with one physical output line designated as Output Line 1. The output line
    is accessed via the 6-pin connector on the back of the camera.
    The output line is opto-isolated.
    recommended operating voltages. The absolute maximum voltage is +30.0 VDC. The maximum current allowed through the output circuit is 50 mA.
    A low output signal from the camera results in a non-conducting Q1 transistor in the output circuit.

    Voltage Significance
    < +3.3 VDC The I/O output may operate erratically.
    +3.3 to +24 VDC Recommended I/O output voltage.
    +30.0 VDC Absolute maximum; the camera may be damaged, if the absolute maximum is exceeded.

    A high output signal from the camera results in a conducting Q1 transistor in the output circuit.

    instead of switching the led i would like to switch the solid state relay with details shown below and i would like to use a 24V 1.3A power supply on the circuit. i was wondering how to wire this up and whether i needed to limit the current from 1.3A to 50mA

    solid state relay details
    SSR, 2A, 24VDC
    Control Voltage Range: 16.8V DC to 30V DC
    Operating Voltage Range: 12V AC to 240V AC
    Contact Configuration: SPST-NO
    Load Current: 2A
    Relay Terminals: Screw
    SVHC: No SVHC (19-Dec-2011)
    Blocking Voltage: 275VAC
    Control Current Typ: 10.5mA
    Control Current Voltage: 24V
    Control Voltage DC Max: 30V
    Control Voltage DC Min: 16.8V
    Control Voltage Type: DC
    External Depth: 75.6mm
    External Length / Height: 87.3mm
    External Width: 6.2mm
    Input Voltage Max: 30V
    Input Voltage Min: 16.8V
    Must Release Voltage: 10V
    Output Type: AC
    Power Rating: 300W
    RMS Load Voltage Max: 240VAC
    Series: 38
    Surge Current: 40A
    Terminal Type: Screw

    Attached Files:

  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Hi scmojks and welcome to the forums.

    You can do this easily. Replace the 2.2k resistor and the LED in the diagram you posted, with the "coil" (it's not really a coil) of the solid state relay. You don't need to limit the current; it will be limited by the input resistance of the SSR. The specification states that the input requires ~24VDC and will draw 10.5 mA at that voltage. The transistor inside the camera will have no trouble supplying that much current. The 24V supply that you connect to the top of the relay "coil" input doesn't need to be rated for 1.3A; a rating of 100 mA would be plenty.

    You might want to add a low-value capacitor (e.g. 10 nF) and/or a zener, varistor, or other kind of surge suppressor across the camera's output, to protect the transistor in the camera against possible voltage spikes if there is a breakdown in the relay. I'm not sure if it's worthwhile though; if it happens, the protection may not be enough to save the transistor, and it's very unlikely to happen.

    Thanks for your clear and thorough description.
    Last edited: Nov 17, 2012
  3. scmojks


    Nov 17, 2012

    thanks for the detailed response. i was intending to use a 1.3A 24V supply in the circuit as i want to attach a light at the other end of the solid state relay which draws 0.875A, the light itself is 24V powered with no need for resistors to limit current,

    using the 1.3A supply isnt going to cause an issue is it, i assume the coil of the relay draws little current.

    attached is a cicuit where someone advised me to put a diode across the relay to protect the transistor. Does this ciruit make sense? last thing i want to do is damage the camera as its quite expensive

    Attached Files:

  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Yes, that circuit looks good and should work.

    The diode across the "coil" is only needed if the relay is the electromechanical type, which contains an actual coil, which exhibits inductance and will produce a "back EMF" when the current is interrupted. In this case, the diode bypasses the back EMF and prevents damage to the transistor. If your relay is the solid state type, the diode isn't needed. (Also, it's a 1N4007 not LN.)

    For clarity, you should show the part number and description of the solid state relay on your schematic. Also, the "contacts" (output) of a solid state relay is not a simple transistor as you have shown - this is the type of output you see on an optocoupler. I would represent the contacts of a solid state relay like the contacts of an electromechanical relay. But you really should indicate the part number, and preferably a description, on your schematic.

    Secondly, you've shown the wire from the 24V power supply positive to the top of the "contacts" INSIDE the box that represents the SSR; this connection is external and should be shown as such.

    Finally, it's conventional to draw schematics with all the connections to the 0V rail (called "Your Gnd" on your diagram) going onto a horizontal line at the bottom of the diagram, and running the +24V rail along the top. The power supply (which you've shown as a battery) would then connect across those two rails, at the right hand side of the drawing. This is all just for clarity.

    Apart from those things, and removing the diode, the diagram looks perfect! Nice work!
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