Is there a way to put a relay in series with another load?

Question

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?

Answer 1

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,

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:

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.