Relay energised indicator?

[bogart219]

Hello. Anyone know of a good "tester" for determining whether the coil of a relay is energized or not? I'm basically talking about 8-11 pin cube relays (ac coils) that do not have clear plastic covers or may be hard to see through. It would also be nice to be able to test small 12 or 24vdc relays on circuit boards where you can't get to the back side of the board. I believe this type of tester senses the magnetic field when the coil is energized. I have only seen one advertised called the Magfinder for around $50. Any one have any experience with one of these or can recommend something else? Thanks!

[shadow]

Re: Relay energised indicator?

To determine if a relay is energised without indicator lamps or the plastic covers, I think the easiest way is to measure the voltage across the coil with a multimeter. If the voltage is roughly equal to the voltage rating of the coil, then the relay is energised.

[fpliuzzi]

Re: Relay energised indicator?

Hello,
The idea of checking to see if relay coils are energized, from the top side of the board only, sounds interesting. You may want to see FIGURE 1 in the following EDN article. It shows a simple to build probe that turns your digital voltmeter (that's set to dc millivolts) into a basic Gaussmeter. The probe consists of only a low cost Hall Effect sensor, two 0.01uF bypass caps, and three AAA batteries (sensor runs on 4.5V to 6Vdc).

http://www.edn.com/design/test-and-m...few-components

NOTE: The sensor in the December 2006 EDN article is no longer available, but a newer Hall Effect sensor is available at digikey for $1.81. The sensitivity of the Hall Effect sensor called out in the article is 2.5mV/gauss and the newer sensor is 1.3mV/gauss.

Digikey part number 620-1019-1-ND

http://www.digikey.com/product-detai...9-1-ND/1006711

I built the probe described in the EDN article years ago and I just tried it out on some of the small relays I had in my parts bin. The probe could pick up the magnetic field of the energized relay coils in every case (my DMM displayed a range of 5 to 100mv dc on the output of the Hall Effect sensor, depending on the size of the relay). The probe even detected the magnetic field of an energized tiny 5V reed relay coil. The probe tip was placed directly onto the relay body during these tests. My probe's tip is covered in heat shrink sleeving for a bit of added safety.

The REL or Delta button on your DMM is used to null the sensor's output reading to zero mV before you start probing magnetic fields for best results. This is explained in the EDN article.

If you want to experiment, you may find that this magnetic field probe allows you to quickly check which of your relays have energized coils when you have no access to the back sides of the PCB to check them in a more standard way. I found this little experiment interesting.

Regards,
Frank

ps; Obviously, if a relay undergoing the "Gaussmeter test" is located close to an AC power transformer in the device, for example, the relatively small magnetic field of the energized relay coil will be swamped by the much larger field emanating from the transformer.

[bogart219]

Re: Relay energised indicator?

Hello,
The idea of checking to see if relay coils are energized, from the top side of the board only, sounds interesting. You may want to see FIGURE 1 in the following EDN article. It shows a simple to build probe that turns your digital voltmeter (that's set to dc millivolts) into a basic Gaussmeter. The probe consists of only a low cost Hall Effect sensor, two 0.01uF bypass caps, and three AAA batteries (sensor runs on 4.5V to 6Vdc).

http://www.edn.com/design/test-and-m...few-components

NOTE: The sensor in the December 2006 EDN article is no longer available, but a newer Hall Effect sensor is available at digikey for $1.81. The sensitivity of the Hall Effect sensor called out in the article is 2.5mV/gauss and the newer sensor is 1.3mV/gauss.

Digikey part number 620-1019-1-ND

http://www.digikey.com/product-detai...9-1-ND/1006711

I built the probe described in the EDN article years ago and I just tried it out on some of the small relays I had in my parts bin. The probe could pick up the magnetic field of the energized relay coils in every case (my DMM displayed a range of 5 to 100mv dc on the output of the Hall Effect sensor, depending on the size of the relay). The probe even detected the magnetic field of an energized tiny 5V reed relay coil. The probe tip was placed directly onto the relay body during these tests. My probe's tip is covered in heat shrink sleeving for a bit of added safety.

The REL or Delta button on your DMM is used to null the sensor's output reading to zero mV before you start probing magnetic fields for best results. This is explained in the EDN article.

If you want to experiment, you may find that this magnetic field probe allows you to quickly check which of your relays have energized coils when you have no access to the back sides of the PCB to check them in a more standard way. I found this little experiment interesting.

Regards,
Frank

ps; Obviously, if a relay undergoing the "Gaussmeter test" is located close to an AC power transformer in the device, for example, the relatively small magnetic field of the energized relay coil will be swamped by the much larger field emanating from the transformer.

Frank, revisiting this post just now for some reason. Thanks so much for the valuable information

[fpliuzzi]

Re: Relay energised indicator?

Bogart219,
Glad you found the info I posted of some interest. The low cost, bare-bones gaussmeter adapter that I built from the EDN article has come in handy on my test bench.

Actually, I'm surprised that the link to the EDN article still works, since I posted it so long ago here. EDN.com has lots of interesting articles, but from my experiences, their site unfortunately has lots of broken links.

Regards,
Frank