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To layout my situation, I have a 12V DC pump with a built in float switch that I don't want to try to crack open and possibly compromise the waterproof of. The pump draws around 6-7 amps. I would like for an alarm to sound when the pump is running, and was thinking of trying to use a relay to trigger this. However, from what I can tell most relays have very high resistance coils that likely aren't meant to be run in series with another load and I believe will limit the current to the pump too much. Is there any kind of low coil resistance relay that is made for a purpose like this?

Alternatively I was looking at trying to use an inline current sensor to trigger activate a relay but don't have any experience w/ those and aren't quite sure they would work correctly. I was hoping I could find some kind of current sensor / relay combination but haven't have any luck so far.

Otherwise, any other ideas on how I could do this?

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    \$\begingroup\$ Could you not put the relay in parallel? \$\endgroup\$ – Hearth Apr 25 '18 at 18:58
  • \$\begingroup\$ How much voltage drop can you accept from the sensor? How is the pump powered? \$\endgroup\$ – Vladimir Cravero Apr 25 '18 at 19:18
  • \$\begingroup\$ @Felthry but then it would be constantly on. The float switch is built into the pump. \$\endgroup\$ – Simon B Apr 25 '18 at 22:04
  • \$\begingroup\$ @Vladimir The pump is powered by a 12V deep cycle battery; I'd guess I need at least 11 volts for proper functionality. \$\endgroup\$ – farmy Apr 26 '18 at 13:20
  • \$\begingroup\$ I think that the first option from the answer below is good for you, it fit your constraints. Thanks for clarifying that up! \$\endgroup\$ – Vladimir Cravero Apr 26 '18 at 15:41
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If you have a fully-enclosed device with internal "float sensor" and don't have access to internal load, the option is to sense load current when it gets active.

There are many ways to do this.

One simple method is to use a shunt resistor (10-20 mOhms for 6A range) and use a specialized IC like ZXCT1009 or similar,

enter image description here Then use a comparator on Vout to trigger an alarm. The whole circuit could be designed as independent self-contained stand-alone box, Vin - To-load.

Alternatively you can use something like "Current Sensing Relay Switch" if you are willing to spend $70 or something. This is for AC sensing, but anyway:

enter image description here

There are current-sensing switches of industrial control grade, even maybe in a ready-to-go form. Many options are available if searching for right terms.

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  • \$\begingroup\$ Looking at your first option, I am not familiar w/ comparators (I am a novice at this). Doing some research it seems that there are two kinds: Linear/Analog and Logic. I believe I would want a linear in this case right? Also, from what I can tell, the output voltage of these comparators scale w/ the input current - is there a way to stabilize that in case the load (pump) varies? \$\endgroup\$ – farmy Apr 27 '18 at 16:42
  • \$\begingroup\$ @farmy, mentioning "comparator" was an overkill. All you need is a threshold detector. The pump either consumes zero current when off, and 3-4-6-7 A when on, as you say. So you might be good with just a BJT transistor connected to Vout, and put your "alarm" as the transistor load. It might work without any comparator, although a positive feedback with some hysteresis would be desirable. \$\endgroup\$ – Ale..chenski Apr 27 '18 at 18:54
  • \$\begingroup\$ Couldn't a relay also be used in this application instead of the transistor? If so, what would the pros/cons be to using one or the other? I believe relays have some delay, maybe enough to cover the desire for hysteresis you mention above. \$\endgroup\$ – farmy Apr 27 '18 at 21:46
  • \$\begingroup\$ @farmy, no, a relay can't be used without amplification, the circuit is a transconductance differential amplifier with relatively low current output, about 2 mA max. \$\endgroup\$ – Ale..chenski Apr 27 '18 at 22:35

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