Hall effect sensor sensitive enough to detect a relay when on? - Electronic Circuits Projects Diagrams Free

blueroomelectronics · 14th June 2008, 08:22 PM

I have some small PCB mount relays 12V coil 2A contacts. I've got a couple of Hall sensors one is an OHN3120 but neither can pickup the coil when engaged.
I'm not sure this will even work, would a small coil work?

Sceadwian · 14th June 2008, 08:38 PM

Relays are usually pretty well shielded. What kind of current does the coil draw when it's switched? An external coil might work, or trying to find a sweet spot on the relay housing where some of the magnetic flux leaks out, the orientation of the sensor is important too as hall sensors are only sensitive with a specific orientation. If the sensor is small enough you could possible mount it inside the relay housing.

Hero999 · 14th June 2008, 11:41 PM

A more important question is why are you doing this?

Don't you have a spare pair of contacts?

picbits · 15th June 2008, 10:07 AM

Possibly insert a dropping resistor in series with the relay coil and stick a optocoupler across the resistor ?

Hero999 · 15th June 2008, 01:40 PM

Perhaps he wants isolation from the 12VDC.

How about using a hall effect current sensor to detect the coil current?

blueroomelectronics · 16th June 2008, 03:12 AM

Actually it's to monitor a set of relays on a PCB that runs an automation system. I'm working on a daughter board and wanted a solution that doesn't require soldering or modifying the existing board. The hall effect sensors were not sensitive enough so it looks like another solution would be a DIP IC to ribbon cable adapter to tap into the relay signal.

**Krumlink** · 16th June 2008, 03:45 AM

Why not use a magnetic reed switch? They are pretty sensitive actually, as I have used them in a door switch once.

blueroomelectronics · 16th June 2008, 04:18 AM

The flux around the relay coil is too small to activate a reed relay outside the relay casing.

Mikebits · 16th June 2008, 04:52 AM

WHat about an ultra sensitive hall device.
http://www.allegromicro.com/en/Produ.../3212/3212.pdf

blueroomelectronics · 16th June 2008, 07:20 PM

I'm going to try another approach, a simple method to watch the relay coil. The relays are all 12V DC coils and connected directly to +12V and an open collector diode clamped output of an ULN2003. Comments on the following circuit appreciated.

Pommie · 17th June 2008, 01:47 AM

That looks like it should work fine. What is the purpose of the 1k resistor, I can't see why it is needed.

Mike.

blueroomelectronics · 17th June 2008, 02:27 AM

Thanks Mike, I guess your right don't need the 1K

Boncuk · 18th June 2008, 02:38 AM

Hi Bill,

that just isn't it.

You never know if the relay is working at all using that circuit. Mine also has a disadvantage not knowing if the contacts are closed, but at least you'll know that the coil is alright.

Try this. I guess you'll like it more than yours. The relay coil in this example has a resistance of 240Ohm.

Regards

Hans

blueroomelectronics · 18th June 2008, 03:36 AM

Hmm perhaps your right, the ULN2003 will only go to 1.4V when it's on plus the diode drop, another choice would be an opto-isolator like a 4N35. It is a Schmitt trigger input so with a 5V VDD it's 1V low and 4V high. Thanks Hans.

Boncuk · 19th June 2008, 02:02 AM

Quote:

Originally Posted by blueroomelectronics

Hmm perhaps your right, the ULN2003 will only go to 1.4V when it's on plus the diode drop, another choice would be an opto-isolator like a 4N35. It is a Schmitt trigger input so with a 5V VDD it's 1V low and 4V high. Thanks Hans.

Hi Bill,

You probably didn't read the voltage values for 'H' and 'L'. The 'L' is at 458mV and 'H' at 3.298V in my example - good figures for a PIC to "know" the difference between ON and OFF.

Regards again

Hans

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14th June 2008, 08:22 PM	(permalink)
blueroomelectronics	Hall effect sensor sensitive enough to detect a relay when on? I have some small PCB mount relays 12V coil 2A contacts. I've got a couple of Hall sensors one is an OHN3120 but neither can pickup the coil when engaged. I'm not sure this will even work, would a small coil work? __________________ Bill Smart Kits build Smart People http://www.blueroomelectronics.com

14th June 2008, 08:38 PM	(permalink)
Sceadwian	Relays are usually pretty well shielded. What kind of current does the coil draw when it's switched? An external coil might work, or trying to find a sweet spot on the relay housing where some of the magnetic flux leaks out, the orientation of the sensor is important too as hall sensors are only sensitive with a specific orientation. If the sensor is small enough you could possible mount it inside the relay housing. __________________ "Because I be what I be. I would tell you what you want to know if I could, mum, but I be a cat, and no cat anywhere ever gave anyone a straight answer, har har."

14th June 2008, 11:41 PM	(permalink)
Hero999	A more important question is why are you doing this? Don't you have a spare pair of contacts? __________________ I also post at the following sites: http://www.stop-microsoft.org http://www.heated-debates.com Screen name: Aloone_Jonez

15th June 2008, 10:07 AM	(permalink)
picbits	Possibly insert a dropping resistor in series with the relay coil and stick a optocoupler across the resistor ?

15th June 2008, 01:40 PM	(permalink)
Hero999	Perhaps he wants isolation from the 12VDC. How about using a hall effect current sensor to detect the coil current? __________________ I also post at the following sites: http://www.stop-microsoft.org http://www.heated-debates.com Screen name: Aloone_Jonez

16th June 2008, 03:12 AM	(permalink)
blueroomelectronics	Actually it's to monitor a set of relays on a PCB that runs an automation system. I'm working on a daughter board and wanted a solution that doesn't require soldering or modifying the existing board. The hall effect sensors were not sensitive enough so it looks like another solution would be a DIP IC to ribbon cable adapter to tap into the relay signal. __________________ Bill Smart Kits build Smart People http://www.blueroomelectronics.com

16th June 2008, 03:45 AM	(permalink)
Krumlink	Why not use a magnetic reed switch? They are pretty sensitive actually, as I have used them in a door switch once. __________________ MechTronics World of Warcraft is a disease

16th June 2008, 04:18 AM	(permalink)
blueroomelectronics	The flux around the relay coil is too small to activate a reed relay outside the relay casing. __________________ Bill Smart Kits build Smart People http://www.blueroomelectronics.com

16th June 2008, 04:52 AM	(permalink)
Mikebits	WHat about an ultra sensitive hall device. http://www.allegromicro.com/en/Produ.../3212/3212.pdf

17th June 2008, 01:47 AM	(permalink)
Pommie	That looks like it should work fine. What is the purpose of the 1k resistor, I can't see why it is needed. Mike.

17th June 2008, 02:27 AM	(permalink)
blueroomelectronics	Thanks Mike, I guess your right don't need the 1K __________________ Bill Smart Kits build Smart People http://www.blueroomelectronics.com

18th June 2008, 02:38 AM	(permalink)
Boncuk	Relay on/off? Hi Bill, that just isn't it. You never know if the relay is working at all using that circuit. Mine also has a disadvantage not knowing if the contacts are closed, but at least you'll know that the coil is alright. Try this. I guess you'll like it more than yours. The relay coil in this example has a resistance of 240Ohm. Regards Hans Last edited by Boncuk; 7th July 2008 at 11:47 PM.

18th June 2008, 03:36 AM	(permalink)
blueroomelectronics	Hmm perhaps your right, the ULN2003 will only go to 1.4V when it's on plus the diode drop, another choice would be an opto-isolator like a 4N35. It is a Schmitt trigger input so with a 5V VDD it's 1V low and 4V high. Thanks Hans. __________________ Bill Smart Kits build Smart People http://www.blueroomelectronics.com