Breaking 120V Loads using 24V Relay - Beginner Help Needed

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Discussion Overview

The discussion revolves around using a 24V relay to control 120V loads, with participants seeking guidance on relay selection and circuit configuration. The conversation includes both beginner inquiries and technical considerations related to electrical safety and relay specifications.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Homework-related

Main Points Raised

  • One participant inquires about the appropriate relay to use for breaking 120V loads with a 24V control signal, specifically mentioning the Altronix RAC120 or RAC24.
  • Another participant suggests that it is generally better to break the 'hot' leg of the 120V circuit for safety reasons, although they note that either leg could theoretically be interrupted.
  • It is emphasized that the relay must have a coil rated for 24V and contacts rated for 120V, with a warning about potential arcing if the relay is not suitable for the voltage and current being switched.
  • A later post raises a related question about using 120V to switch a relay for a 230V load, indicating a desire to avoid purchasing a new controller for the 230V pump.
  • Another participant responds by recommending a relay with a coil rated for 110 to 130 Vac that can switch 230 Vac, providing a specific product link as an example.
  • One participant cautions about derating the relay for inductive loads, noting that contact capacities are typically rated for resistive loads only.

Areas of Agreement / Disagreement

Participants generally agree on the need for specific relay ratings and safety considerations when switching high voltage loads, but there are multiple approaches discussed regarding the configuration and relay selection, particularly concerning the use of different voltages for control and load switching.

Contextual Notes

Participants express uncertainty about the specifics of relay ratings and configurations, and there are mentions of potential issues such as arcing and the need for derating under certain conditions, which remain unresolved.

Who May Find This Useful

This discussion may be useful for beginners in electronics and electrical engineering, particularly those interested in relay applications for controlling high voltage loads with low voltage signals.

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I want to use 24v to break 120v loads. I understand the logic for dc circuits but I am not sure how to break 120v loads.

What relay do i use? I was looking at the altronix RAC120 or RAC24.

Do I break just the hot leg of the 120v?

Any help on such a beginner question would be helpful! Thanks!
 
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As in 'digital' circuits, you just need to break the circuit and interrupt the flow of charge. In theory, you could break either leg. In practice, it's better to break the 'hot' leg, just in case someone/thing accidentally grounds out part of the circuit/case that is still hot, or cuts the cable, or some such.

Most power relays can do the trick, as long as the 120V doesn't arc across the contacts (as may be the case when the contact and paddle/reed are particularly close together, as may be the case with low voltage relays). If there's an AC rating for the contacts, it should do the trick. Obviously, the coil must be rated for 24V (AC or DC, you don't specify).
 
You want a relay with a coil rating 24 VDC and 120 V contacts, make sure that it is rated for the current that you are switching.
 
Great. I have relays that will do that. Thanks fellas
 
Anyone know how to do this exact same thing but use 120V (instead of 24V) to switch the relay for 230V (instead of 120V) connection.

I want to turn on and off a 230V pump, but using a controller that already sends 120V on/off. I am just trying to avoid buying another controller that will switch on and off 230V.

Nick
 
nlaham said:
Anyone know how to do this exact same thing but use 120V (instead of 24V) to switch the relay for 230V (instead of 120V) connection.

I want to turn on and off a 230V pump, but using a controller that already sends 120V on/off. I am just trying to avoid buying another controller that will switch on and off 230V.

Nick

Yes: find a relay with a coil rated for between 110 and 130 Vac (or AC/DC, but don't use one that's just for DC) which is capable of switching 230 Vac at the current required. For instance, I found this at Digikey by looking for relays, narrowing it down to those between 110 and 130 Vac and then sorting by carrying current:
http://search.digikey.com/us/en/products/G7L-1A-TUB-J-CB-AC100%2F120/Z814-ND/127305
 
Last edited by a moderator:
Be sure you derate the relay for inductive loads. The stated contact capacities are generally for resistive loads. The manufacturers provide a cut sheet for the derating.
 
Last edited:

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