Relays

Jimmy Gillies · June 2004

Can anyone help me, please? We are doing a job here in Scotland that requires a 240V AC supply to be switched by 12V DC.
I have been told that, if the relay coil is 12V DC then it could pull on 240V AC? Is there such a thing as a dual voltage relay? For a dumb Plumber, I need some help!

Thanks
Jimmy Gillies.

Dave_22 · June 2004

Relay voltage

Well, it could have a 12VDC coil and allow 240VAC to go through it's contacts, but I think it would turn into a cloud of smoke if you wired 240VAC to a 12V relay coil!

chris smith_5 · June 2004

relay

jimmy,
i have had great luke with thease products
http://www.functionaldevices.com/ChartPower.html

chris smith

Ted_9 · June 2004

We just installed a ductless split a/c unit for a customer that had 230 going to the condenser and them it transformed the voltate to 37dc for the evaporator. First time I have seen that.

Dale · June 2004

Coil of relay

The thing to remember on a relay is that it's a load and a switch(s) in one package. The relay will have coil voltage ratings, sometimes in AC and DC and it will have switch voltage and amperage ratings. The thing to remember is that the coil is designed to draw only so many amps and that since it's an inductor the amps at AC will be far lower that what an ohms law I=E/R would figure. So, if the output of the device that's controlling the relay is 12VDC you may only need a 24V AC relay. This is a time to read the small print on the relay box. I would also post this to Bill from Honeywell, they probably make the dc coil rating you want, but I can't find it in my books.

Jimmy Gillies · June 2004

Blue flash relays.

My old Dad was a Linesman that worked on 33,000 Volt lines, most of which he did 'Hot Wire', he always said when your hands are frozen to the steel of a pylon 60Ft up in the air ''it gives you some time to think about your next move''. He's not so well any more, but these big hands are as warm as toast.
Another saying of his, when it came to electrics ''a little knowledge is dangerous''. So thanks to all for your input, I will give it further thought and research, cause a 240v flash is blue and shocking!
Kind regards.
Jimmy Gillies.

Jimmy Gillies · June 2004

Dave.

Thanks for your help. Yes, I know I would be unable to put 240V through 12V coil or contacts. Also I know 12V will not lift a 240V coil. So I guess I'm after a relay with a 12V coil and 240V contacts.
Thanks again.
Jimmy.

Jimmy Gillies · June 2004

Dale.

Thanks, that's very informative. The 240V load will be quite small, all it's doing is a actuator head.

My supplier is looking into getting me a 24V actuator, and that would make things much easier.

Even if my 24V volt actuator will solve my problem, I'm still keen to know if a dual voltage relay is available?
Is it not the case that a higher voltage/load or phase can be switched by a lower voltage, by a contactor?
Thanks again.
Jimmy Gillies

BillW@honeywell · June 2004

The lowest coil voltage we make is 24 VAC...

Hi,Jim. Go to http;//hbctechlit.honeywell.com, and type in DP3040 and that will get you the current sheets on the economy & PowerPro contactors. I only have info on what's available here in the US, not what might be available in Scotland. Good luck.

jerry scharf_2 · June 2004

Staged relays

Jimmie,

It may be that a single relay will not do the job. You didn't state the current at 240V that you need to switch. This is critical, since the size of the coil is usually dependent on the current rather that the voltage.

When I switch AC off a control board, I usually have a CMOS switch which can't handle very much coil current, so I first put a small relay to switch .5-1A of 24VAC, then use that to drive the power relay. Even if I can find a single relay that can "do the job" I'll use a staged pair if I think it's too close to the limits. Marginal coil current will eat up a relay in no time!

Enjoyed reading about your dad as well.

jerry

Mark J Strawcutter · June 2004

relays

Relays, contactors and motor starters are all relays.

Relays are commonly used to isolate/separate control/switching circuit and load circuit with differing voltages or current draw.

The earlier reference to functionaldevices.com is a good one. Their "relay-in-a-box" (aka RIB) product is available in a variety of coil/contact configurations.

Short answer to your question - yes a 24V coil and a 240V switch is a common configuration.

Mark

Matt Undy · June 2004

Woudln't it be better to use a transistor configured as a switch instead of a driver relay, unless the main relay has a particularly high current coil. Of course this is completely off the subject of the original post, whcih asked if a relay with a 24VAC coil and 240VAC contacts existed, which many many do however soemtimes catalogs leave out some parts of the specs, especially in HVAC catalogs.

Matt

Matt Undy · June 2004

Here are some sources. 12VDC and 24VDC coils are very common but not in HVAC work, they usualy use 24VAC control circuits but they are common on the internal packaged control modules and in virtually any electronics.

www.jameco.com
www.mouser.com
www.mpja.com
www.mcminone.com

Matt

Jimmy Gillies · June 2004

Thanks to all.

I guess it's one of these overlaps. When a Plumber/Heating Engineer has a problem like this. You guys, as always, have been a great help.
I'm sorry my information has been a bit thin, but all your posts have been very informative.
When time permits, I will view all the sites you have detailed.
Given this further insight, I can go about my problem knowing there is such a product available.
Thanks again.
Jimmy.

Relays

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