Low-voltage relay help

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    I have some pumps that send a 5VDC signal when running.  Since the PLC digital input requires 24 VDC, we are using relays to make the transition.  The first relays we used would not relay the signal, and upon inspection the relay has listed a minimum activation current of 23 mA.  The pumps only send about 12 mA.


    In my quest to find a 5V relay that will work, I noticed that most don’t list a minimum current that must be applied to the coil side to activate the relay.  I considered using a current-sensing relay but they are over 3 times the cost of the voltage relays.  Current transducers (if that is the right term) would help boost the current, but also add to cost and space.  I also sugessted using the spare analog inputs instead of the relays, but my supervisor isn’t keen on all the rewiring.  >sigh<  If it comes to it, we just won't implement the run bit on the pumps, but I was hoping there was another solution.   Lingering questions:   Is it safe to assume that all voltage relays have a minimum current to activate?  Does anyone happen to know of a relay that will work with such low voltage and current?  Is there another solution I should consider without adding cost?   Thank you in advance for your time and efforts.


    Yes, a polarized miniature DC relay from “Panasonic” for pc mounting of serie DS or TN, or similiar.  It can be driven by a battery. Need impuls current only to switch over, no current after operation …its magnetic locked !     


    Yes there are solutions. You may not want to use a magnetically latched signal as it may require another signal to release.

    Some ideas you can consider.

    The voltage level with which you are dealing is TTL. If you do a web seach on “TTL driven Relay” it will bring up a number of possibilities. One possibility is the Tecnec TCR102  (elunits.com/relays). It requires an external power supply but this can come from your PLC I/O supply (there are a number of voltage options available) . The load on the driving signal is only uA at the TTL 5V level so should suit your application. This is an industial type unit which you can wire straight in.

    Another possibility is to use a reed reed relay. You can get low power coils in these but they are generally designed for printed circuit board applications so are small and fiddly. You would need to devise a mounting and connecting method. Try a websearch on “reed relay” and check out the range that Farnell has.  

    The lowest cost and crudest solution would be to select a suitable transistor, drive the base with your 5V signal and use the collector, powered by the PLC 24V supply, to switch a relay coil. Depending on the configuration of yout PLC input module you may even be able to rig to switch it directly (without the relay). May require a bit of mucking around. A disadvantage of this is that it would common the OV rails of your TTL level signal and the PLC and you may not want to do this.

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