Magnetic field sensor

Thread Starter

Willy77

Joined Aug 14, 2013
10
Does anyone know how to build a circuit that would sense just the magnetic field of a relay by proximity to see at a glance if it is working or not?
To sense any applied linear DC voltage. Not an oscilating one.
 

Thread Starter

Willy77

Joined Aug 14, 2013
10
Thanks Max.
It would be nice to avoid to uncase them and instead having a device (sensor) that would tell me with an led if the relay is active or not by just putting the sensor near close by the relay.
An instrument just like those neat pen like AC line voltage sensors only this one senses a DC magnetic field of a relay. Avoiding off course having to connect your voltage probes in hard to get places.
I tried some experimenting but could only sense DC oscilating voltage magnetic fields.
 

MaxHeadRoom

Joined Jul 18, 2013
28,768
You never mentioned where you were, but Digikey have the Honeywell, you need to watch the part number as there are unipolar, bipolar and latch.
Max.
 

Thread Starter

Willy77

Joined Aug 14, 2013
10
I'm from Santa Cruz Bolivia.
Not sure if I will order the parts just yet or which I would need. I have a Digikey parts catalog and will look up into it. But first I will investigate for a circuit that would suit my needs.
Anyway I will apreciate any info. Never had an order from Digikey yet, nor I know if Digikey deliver policies reach these places.
Thanks Max.
 

ebeowulf17

Joined Aug 12, 2014
3,307
If you just want the simplest on/off LED indicator, a hall effect switch or even a reed switch would make a simpler circuit than a linear sensor like the 490 series.

Having said that, the switches have pretty wide tolerances on their trip points and that circuit wouldn't be adjustable.

If it were for me, I'd use the 490 series sensor with a 10k pot providing a reference voltage for a comparator so that you can adjust the trip point to suit your needs. You might also need an extra resistor or two to add some hysteresis. I recently made almost exactly this circuit with the Microchip MCP6542-E/SN with great results.
 

ebeowulf17

Joined Aug 12, 2014
3,307
Here's a version of my circuit with the unnecessary bits trimmed out. This one was for external 5VDC power supply, so I'm not entirely sure what it would take to adapt it to battery power.

[EDIT:] U1 isn't properly called out in the drawing, but it's a Honeywell SS495A.

Relay-Field-Sensor.png
 

Thread Starter

Willy77

Joined Aug 14, 2013
10
Here's a version of my circuit with the unnecessary bits trimmed out. This one was for external 5VDC power supply, so I'm not entirely sure what it would take to adapt it to battery power.

[EDIT:] U1 isn't properly called out in the drawing, but it's a Honeywell SS495A.

View attachment 77493
Great! Thanks ebeowulf17 you put me in a good position to start experimenting. I will check this out and comment later on. Thanks again!
 

ebeowulf17

Joined Aug 12, 2014
3,307
Max brought up a good point that I forgot to touch on:
...you need to watch the part number as there are unipolar, bipolar and latch.
Max.
If you use my circuit with the linear, proportional SS495A, the sensor will only do a good job detecting your magnetic field in one orientation, with the face of the sensor perpendicular to the field lines, either face up or face down depending on whether it reads positive or negative gauss.

If you went with the right type of hall switch (omnipolar if I remember correctly,) you'd be able to detect the field regardless of sensor orientation, but then you can't adjust sensitivity/trip point. You'd need to have a reasonable idea of what field strength you're expecting and buy a switch with the appropriate trip point, or buy a variety of them and see which one works best through trial and error.

So I guess the question is which level of flexibility is more important: the ability to sense a field from any orientation, or the ability to adjust your sensitivity as needed.
 

Thread Starter

Willy77

Joined Aug 14, 2013
10
Hi eveowulf17.

Thanks for pointing this up. I wasn't considering the gauss polarity. Actually I'm more concern about sensitivity and being able to adjust it as needed.
This can be very handy if for example there's a situation of a few relays lined up and you're testing a switch to check wich relays are on and which are not.
I will try to get a variety and experiment through trial and error as you said.
Thanks again.
 

ebeowulf17

Joined Aug 12, 2014
3,307
Didn't have any dc coils to test until just a few days ago, but here's some real world data that may be useful for comparison purposes. I tested the field strength on both ends of a 24V, 8W solenoid, using a Honeywell SS495A and an Arduino.

On the more open, exposed end, I found:
500+ Gauss touching the end
100 Gauss @ 1/4"
40 Gauss @ 1/2"

On the business end, which was pulling a small plate tight against itself, I measured:
103 Gauss touching the plate
54 Gauss @ 1/4"
28 Gauss @ 1/2"

Depending on what size relays you're testing, their coils may be weaker or stronger than this.
 
Top