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Relay Question

Mon Aug 12, 2019 12:54 am

Does anyone having any experience switching inductive loads with relays? The load is from 3a to 5a maximum (water pump) 12v or 24v depending on what I want to buy. The standard Songle SRD-05VDC-SL-C is only rated 3a for inductive loads and the 3a pump doesn't seem like it'll put out the pressure and head that I'll need for a smaller drip irrigation system. Looking at beefier relays like the SLA-05VDC-SL-C but that doesn't have a rating for switching DC voltage. AC water pumps are more expensive for the use, and I prefer to switch/relay DC compared to AC/mains power.

My thought today was a solid state relay in the 20-40a range though I don't see any ratings for inductive loads. Anyone have success with these in this scenario?

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Re: Relay Question

Mon Aug 12, 2019 10:40 am

simple, use your 3A relay to control the larger capacity relay...
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Re: Relay Question

Mon Aug 12, 2019 2:10 pm

Agreed. The relay you have can be used to as an interface between the Pi and your power relay.

For a power relay, stick to name brands like Omron and their data sheets will typically give you complete specs including for switching AC and DC loads as well as sometimes showing resistive and inductive loads. Many relays will be displayed as, for example, a "10A" relay but when you dig into the data sheets the 10A will only be for AC voltages with resistive loads. One I just pulled up randomly on Digi-Key shows that "10A" relay drops to only 4A when switching a 30Vdc inductive load. If the relay doesn't give the specs for what you actually want to do, then move on to another relay.

Automotive relays are a good bet because by definition they are designed to switch DC loads including a lot of inductive motors such as power window motor & power seat motors.

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