110v relay to switch a 24volt load

Thread Starter

radllc

Joined Oct 15, 2022
8
I am new to relays. I typically see this the other way around (low voltage switching high voltage). I have the opposite - 110v going in and I need to switch on a 24volt load. If I get a 110v relay, will this work properly? Thanks.
 

Reloadron

Joined Jan 15, 2015
7,080
The relay won't care. Yes, you can use a relay with a 120 VAC coil or about any coil voltage to switch a 24 volt load. There are a few caveats. Make sure the relay contacts are rated for your load. For example switching a DC voltage is not the same as switching an AC voltage. So if switching 24 VDC make sure your relay contacts are rated for your DC voltage and the load current.

Ron
 

BobTPH

Joined Jun 5, 2013
6,080
You didn’t say whether the control voltage and the switched voltage is AC or DC.

The relay coil also must be appropriately rated for AC or DC, whichever you are using. A 120VAC relay will go up in smoke if you give it 120V DC.
 

Thread Starter

radllc

Joined Oct 15, 2022
8
The relay won't care. Yes, you can use a relay with a 120 VAC coil or about any coil voltage to switch a 24 volt load. There are a few caveats. Make sure the relay contacts are rated for your load. For example switching a DC voltage is not the same as switching an AC voltage. So if switching 24 VDC make sure your relay contacts are rated for your DC voltage and the load current.

Ron
Thanks, I think I got it. Basically, 120VAC will work so long as the load it is switching is rated for DC not just AC.
 

Thread Starter

radllc

Joined Oct 15, 2022
8
You didn’t say whether the control voltage and the switched voltage is AC or DC.

The relay coil also must be appropriately rated for AC or DC, whichever you are using. A 120VAC relay will go up in smoke if you give it 120V DC.
The control voltage is 120VAC switching 24vDC. I am trying to find a relay that will work for that. To provide more information, I am using a PID that will output 120VAC. I need to use that to trigger a 24VDC solenoid valve.
 
Last edited:

MaxHeadRoom

Joined Jul 18, 2013
26,286
The control voltage is 120VAC switching 24vDC. I am trying to find a relay that will work for that.
You should not have any problem finding one, any electronic supplier will have them, Digikey, Mouser etc
Printed circuit mount or hardwired, presumably you want the latter!
 

Thread Starter

radllc

Joined Oct 15, 2022
8
You should not have any problem finding one, any electronic supplier will have them, Digikey, Mouser etc
Printed circuit mount or hardwired, presumably you want the latter!
Yes, I just need a hardwired relay. Again, pretty new to this. I am finding plenty of relays that take a DC load in but then the spec says 240VAC for the output or voltage that it is switching. What is throwing me off is say AC and I want to switch a DC load on the outbound side. Will this work even though it is alternating current. If not can you link me to right type of relay that might work for my application. Thanks for all the help.
 

Reloadron

Joined Jan 15, 2015
7,080
OK let's back up a little. You want a relay with a 120 VAC coil 50/60 Hz. We got that. You want to switch a DC load at 24 VDC. The next question is what is the load? How much current does the load draw and if we know exactly what the load is we can look at relay contacts for a resistive load, an inductive load or maybe both. That figures into it. What you are looking for would be classified as a general purpose relay. They are pretty common but yes there are many of them. Some with terminals, some plug into an octal base and the list goes on so what we need to do is fine one which fits your needs and your specifications. You want a 120 VAC coil with contacts DC rated to handle your load current.

Ron
 

MisterBill2

Joined Jan 23, 2018
13,798
What the TS needs to know is that a relay has two specifications that are fairly independent from each other. USUALLY THE CONTACT RATING IS FIRST, because contact rating is important. Contact current and the voltage being switched, and occasionally the type of load, such as tungsten lighting or motor starting. And sometimes the number of operating cycles is specified.

THEN there is the coil rating, usually a voltage and AC or DC. Also duty is specified, because not all relays will survive continuous operation.

There are also special purpose relays that have a lot more details in the specifications, but the folks selecting those relays generally know about those very special applications and we do not need to be concerned about them either.
 

Thread Starter

radllc

Joined Oct 15, 2022
8
I will try to provide more specifics. The input or trigger is from a temperature controller (stc-1000) @ 110vac, which is the input to the relay coil. The relay itself is switching a 24dcvolt solenoid valve that draws about 1.75amps. When the temperature set is met, the STC-1000 energizes and opens the valve.
 

MisterBill2

Joined Jan 23, 2018
13,798
OK, so you need a relay with normally open five amp contacts, and a 120 colt AC coil. The five amp contacts are to handle the solenoid inrush current with an adequate safety margin.
 

MisterBill2

Joined Jan 23, 2018
13,798
There is also an inrush current on DC devices. And a safety margin is ALWAYS a good idea. In addition, there is that tendency to draw an arc when switching off, which the heavier contacts can better handle.
 

wraujr

Joined Jun 28, 2022
40
Depending in how you are getting/using the 24VDC, you could also get a 24VDC 3A Wall-wart supply on Amazon for about $13 and have PID AC output drive the wall wart....with the solenoid wired to wall wart output. Given time of year are we looking at some sort of humidification control?
 

Thread Starter

radllc

Joined Oct 15, 2022
8
Thanks to all your replies I have it figured out. I am designing a glycol chilling system that has three input temperature controls that drive the opening and closure the master chilling unit plus solenoid valves for the various zones. I have decided to go with 12VDC input (I scraped 120VAC input) to trigger 5 relays. Relay 1 - Chilling Unit -120VAC pulling around 5amps max. Relay 2 - Pump (probably 12volts but am still sourcing pumps). Relays 3 - 4 will power the solenoid valves (which will be either 12 or 24 volts). The relay I am currently planning to use is a Songle srd-12vdc-sl-c, which should cover all devices. I will probably drive this with relay boards (they have this relay) as they are common and very low cost - plus pretty much ready to go with all input and output leads. The have a high and low trigger. As such I plan to use the high trigger but will place a 1k resister ahead of the trigger input as is needs much less current to trip the relay. I can also just go with a simple relay (no board), but I would need to find one at a lower cost than the relays boards (and one with that has standard input spades and not pins for a PCB) as I can get 5 boards ready to go for less $10 (and they are easily sourced if I need to replace one). They do make some boards that have multiple relays, but then I am replacing the entire board if have a single relay or zone go bad. That about covers it.
 

Reloadron

Joined Jan 15, 2015
7,080
OK, I get where you are coming from. Personally I would not run with the Songle relays. The reasoning here is most have a habit of not meeting their published specifications and their contacts tend to burn up and weld shut. My project and keeping things simple I would just ru8n with 12 volt coil automotive relays and 12 volt solenoid coils. Automotive relays and their sockets are readily available online and any automotive parts store. Yes, they cost a little more but for your project well worth the additional cost.

Ron
 
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