Hall effect sensor sensitive enough to detect a relay when on?

[blueroomelectronics]

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]

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]

A more important question is why are you doing this?

Don't you have a spare pair of contacts?

 

[picbits]

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

 

[Hero999]

Perhaps he wants isolation from the 12VDC.

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

 

[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.

 

[Krumlink]

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

 

[blueroomelectronics]

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

 

[Mikebits]

WHat about an ultra sensitive hall device.
https://www.electro-tech-online.com/custompdfs/2008/06/3212.pdf

 

[blueroomelectronics]

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]

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]

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

 

[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

 

[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.

 

[Boncuk]

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

 

[blueroomelectronics]

The Schmitt trigger inputs require VDD*0.8 (4V when VDD = 5V).
I've tossed a transistor on the input, although the relay ON should turn off the transistor.

 

[Boncuk]

Hi Bill,

provided the relay is powered by +12V you've plenty to play with. Use 8K2 for R3 and 4K7 for R1 in the voltage divider. This will give you 0.532V for 'L' and 4.292V for 'H' with the same (as mentioned before) coil resistance of 240Ohm.

Using 5K1 for R1 won't change much, but the 'L' condition will also travel upwards.

CMOS Schmitt-Triggers are satisfied with lower input voltages. They just require VDD/2 (in that particular case slightly above 2.5V)

Hans