Op-Amp help

[Adco]

Hello

I need a solution please.
I have a Hall Effect sensor that I want to use in a magnetic lock to sense when the armature( strike plate) has mated with the stator. There is a change in the magnetic field when the armature couples with the stator. I want to make use of that property to show that the lock is secure.

The Hall Effect sensor, A1302, from Allegro is what I have in mind. It is a linear ratiometric device. The quiescent output sits at half supply voltage, in this case 2.5V with a 5V supply. The output varies up and down as the magnetic fields change. This change is very small, typically 1.35mV/G. I can see the variations on my scope but I need to make the changes bigger. This is where I need an op-amp expert.

I want to make the 2.5V offset move down to 0V and only amplify the positive movement so that I have a range that goes from 0V to about 5V for the small change from the sensor. I cannot use an AC amplifier as this is a changing DC voltage only.

I am stumped by this and would appreciate some help if you are prepared to do that.

Thank you.
Adrian Carboni
Adco Electronics
South Africa

 

[ericgibbs]

hi Adrian,
What you are asking is a typical problem.

It is possible to apply an offset to remove the +2.5V and just amplify the 'change' from +2.5V [0V] to give a much higher output.

Which type of OPa's do you have and what DC supply levels do you have available.?

 

[Adco]

Hello Eric

I would like to use an LM358 as it works well with a single supply voltage. The incoming voltage is 12VDC which I would like to regulate down to 5VDC. I will be embedding a PIC12f675 into the system to read the voltages that gets "fixed" from the hall effect sensor.

Thanks
Adrian

 

[ericgibbs]

Hello Eric

I would like to use an LM358 as it works well with a single supply voltage. The incoming voltage is 12VDC which I would like to regulate down to 5VDC. I will be embedding a PIC12f675 into the system to read the voltages that gets "fixed" from the hall effect sensor.

Thanks
Adrian

hi,
The problem with the LM358 when powered from +5V, is that the maximum output swing is approx 5V-1.5V, ie 3.5V max, is that OK.

If not, you could use +12V and use a resistor/zener clamp on the LM358 to hold it at say 5.1V.

As I understand all you are asking for is: is the LM358 output High or Low, showing the seating of the bolt in the plate.?

E

 

[Adco]

I can live with the output only being 3.5V full scale. It will be easier to keep the supply voltage at 5VDC so that the PIC voltage is sorted. I am going to read the output of the LM358 into the A/D of the PIC12F675. From there I will calculate the change in the output of the hall effect sensor to determine the status of the maglock.

The magnetic field is different between just a voltage (12V) applied to the maglock coil and when the armature is attracted to it. There is also a dip as the coupling occurs. This can be read by the micro and a decision can then be made in the software.

The LM358 is there to amplify the smaller hall effect output and not to act as a level detector.

 

[ericgibbs]

OK,
Can you give me a 'meter measured' value in the change of the Hall output from the in and out positions of the lock.??

 

[Adco]

The hall effect has a sensitivity of 1.35mV/G. I guess that the real working range will be about 100mV full scale. i.e. 2.5V offset +/-100mV of signal. So, with no magnetic field, the output is at 2.5V and goes more positive by about 100mV as power is applied to the coil. Then there is a change as the plate is attracted to the body of the maglock. That is what I am really interested in. The subtle change between no armature being there to the armauture being held in place. So my small signal change has to be amplified.

 

[ericgibbs]

The hall effect has a sensitivity of 1.35mV/G. I guess that the real working range will be about 100mV full scale. i.e. 2.5V offset +/-100mV of signal. So, with no magnetic field, the output is at 2.5V and goes more positive by about 100mV as power is applied to the coil. Then there is a change as the plate is attracted to the body of the maglock. That is what I am really interested in. The subtle change between no armature being there to the armauture being held in place. So my small signal change has to be amplified.

Its possible to detect and amplify the 100mV change when the coil is energised, but you say you are interested in the subtle change as the magnet/metal components are held in place,, what is the magnitude of that subtle change.??

Using LM358 OPA's with a single supply will possibly not work in the way you want with such a small change, also considering any changes in the LM358 and Vref over a temperature range.

 

[Adco]

The temperature range should not be an issue. The main device is an electromagnetic lock used to hold doors closed. They always have power on them, 24hours. Therefore, the coil warms then unit up and sits at about 30deg C day and night.

When the coil gets power but the armature hasn't pulled yet, the output is about 100mV. When the armatur pulls, there is a reasonably big dip in the hall effect output down to about 30mV and it then rises up to about 80mV. That should be enough to detect using the PIC micro. The micro has a 10 bit A/D.

 

[ericgibbs]

hi,
This is the 'best' so far, using LM358's and a +5V supply.

Shown in LTSpice simulation.

EDIT:
Image2 gives a better output, it also shows its OK over 0C to 50C
Note the pots near the output OPA.

 

[Adco]

That circuit looks workable. I will breadboard it and let you know.
Thanks for your effort.