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I want to use a single power supply to operate two resistors alternately

Discussion in 'Sensors and Actuators' started by Corneliu, Mar 14, 2018.

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  1. Corneliu

    Corneliu

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    Mar 14, 2018
    I also need to switch between the two resistors frequently (every minute or so), so I would need a computer-controlled switch. Any ideas on what kind of switch/relay could to the job? I only have a data acquisition card that can output up to 10 V but only 5 mA maximum (and of course USB ports, if they can be of any use). The maximum output of the power supply is 10A / 20V, but during the switching the output voltage and current is brought to zero. The transition time is not critical. Is there anything on the market I cn buy for this? Or, if I need to build a circuit, can you please tell me where I can find the circuit diagram (I have only little experience with electronics). Thank you very much.
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    10 V 5 mA will not drive a relay directly. Use a transistor to drive the relay. We have a resource showing you how to do it. The resource shows a motor. Simply replace the motor by a relay with a 5 V coil and changeover contacts. Use the USB port to power the relay (shown as Vcc in the resource).
    Note that the ground of the data acquisition card and of the USB port need to be connected.

    The choice of transistor is not critical. Any NPN transistor you can get your hands on with a current rating in excess of the relay's current is o.k.
     
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  3. Corneliu

    Corneliu

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    Mar 14, 2018
    Thanks a lot for the advice. Everything is clear, except I am not sure if I picked the right relay, if you have time could you please take a look at the model below?

    https://uk.rs-online.com/web/p/solid-state-relays/8184244/

    Thank you.
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

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    This is a 12 V coil relay. It is not suitable for operation from a 5 V USB port.
    See e.g. here.
     
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  5. Corneliu

    Corneliu

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    Mar 14, 2018
  6. Harald Kapp

    Harald Kapp Moderator Moderator

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    An excellent choice.
     
  7. Corneliu

    Corneliu

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    Mar 14, 2018
    I built in the end the switch (using a relay and a transistor), however I would like to add a safety feature, such that both resistors (which are actually magnets with a large inductance) can discharge in case something goes wrong and the circuit becomes open while the magnet is energized. I made a schematic below (the relay and transistor are not shown, for simplicity). The magnets have a resistance of about 1 Ohm and an inductance of about 1 H. The maximum current when the circuit breaks can be up to 10 A, however the 10 A current can flow in either direction (depending on the experiment), so there must be a discharge path for both current directions. Any ideas if there is a solution for that?
    Also, would it be OK if I leave the circuit as it is and don't add any safety solution (no discharge paths)? If the absolut worst that could happen is a burned-out relay, then I could live with that, since I don't expect a switch malfunctioning to happen often (if at all).


    Schematic_switch.JPG
     
  8. Alec_t

    Alec_t

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    Interrupting a 10A coil current will result in a HUGE voltage spike, unless you include suppression components across the magnet coil.
     
  9. Corneliu

    Corneliu

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    Mar 14, 2018
    Yes, but whar suppression solution would work for both current directions?
     
  10. Alec_t

    Alec_t

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  11. Harald Kapp

    Harald Kapp Moderator Moderator

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    Or use zener diodes in antiserial connection:
     
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  12. Corneliu

    Corneliu

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    Mar 14, 2018
    I just needed one more advice (related to the same circuit shown in post #7 above, except the bipolar supply can reach up to +/-20V): would it still be safe if I used a bidirectional TVS diode (link below, model 5KP36CA) in parallel with each magnet?

    From my estimates, during the discharge the diode would need to handle about 10A for about half a second or so (I could not find in the specs the diode resistance during discharge in order to estimate the power it dissipates). If TVS diodes would work, then they would be more convenient for me (they are smaller and about ten times cheaper). Thank you.

    https://www.distrelec.ch/en/tvs-dio...os=1&origPageSize=25&simi=99.65&no-cache=true

    https://www.distrelec.ch/Web/Downloads/g_/ds/5kp_eng_ds.pdf
     
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