TheBackShed.com

Forum Index : Electronics : high voltage relay

Author

Message

GWatPE

Senior Member

Joined: 01/09/2006
Location: Australia
Posts: 2127

Posted: 12:15pm 11 Sep 2009

Copy link to clipboard

Print this post

I recently needed to switch a relay on a 48V system. I had used 24V relays with caps, and limiting resistors, and 24V supplies, but still ended up with significant coil currents.

I wanted to use 5V low current control signals. I have found that a 240VAC coil on a 30A 240VAC relay will switch on fast with about 55VDC to the coil, with only a few mA. The relay will hold in down to around 10VDC. A mosfet coupled with a diode across the coil, allows full AC side switching from a few micro amps from the micro.

This is a lot easier than making 12 or 24VDC power supplies to drive the relay. Will be looking at other 240VAC contactors.

Gordon.

become more energy aware

Dinges
Senior Member

Joined: 04/01/2008
Location: Albania
Posts: 510

Posted: 01:32pm 11 Sep 2009

Copy link to clipboard

Print this post

Maybe a silly question, but why not use a 48V relay? That's a standard coil voltage, often used in industrial controls (in the past 47V was common too, esp. in British machines). It's also the standard voltage in telephone systems (land lines).

I have a shoebox full of 48V relais; unless you need a very peculiar relay that's somehow not available in 48V configuration, I'd simply get one that has the correct coil value...

To me it sounds as if you've found a complicated solution to a simple problem, but maybe I'm missing something?

Peter.

Don B

Senior Member

Joined: 27/09/2008
Location: Australia
Posts: 190

Posted: 07:50am 14 Sep 2009

Copy link to clipboard

Print this post

The problem with DC relays is that they need a lot more current (or voltage) to pull in than to hold in. Big DC relays will often have an economiser resistance which switches in series with the coil via an auxiliary contact once the relay is closed.

You can also use a capacitor charged by a series resistor to give full supply voltage to close the coil. Once closed, the coil voltage is reduced by the series resistor. It would seem that this is what Gordon had tried initially.

AC relays have substantially lower resistance than their impedance at 50Hz, so will generally operate quite successfully with a lower DC voltage. For minimum power loss (and coil heating), you probably still need some arrangement to drop the coil voltage once the relay is closed.

For many years I used a 24V AC solenoid valve of the kind sold for garden irrigation on 12 V DC, and had no problems.

Another trick that Gordon could have tried was to use two 24V DC relays with their coils in series. The contacts could either have been paralleled for maximum current, or seriesed up for maximum voltage and contact life.

Don B

Print this page

To reply to this topic, you need to log in.