Using a Hall Sensor to trigger a transistor

Hi there,

I am using an A3144E Hall Sensor to detect the pole of a rotating magnet such that it switches on the base of a transistor. The circuit is shown in the attached pic. Firstly, I note that the sensor is pole dependent in that its pin 3 drops to near zero volts when a south pole approaches but not a North pole but the problem I have found is that the voltage at pin 3 is not stable when no magnetic field is present. It should be high at the same value of the supply voltage, in my case a nominal 12V lead acid battery.

My questions are: has any one else noted that the ‘high’ voltage at pin 3 wobbles around between about 3 and 12V seemingly affected be the position of the leg and how the body of the sensor is angled.

Secondly, so I can check voltages and currents, am I correct in thinking that when Pin 3 goes low (with a detected mag field), that current will flow into the base of the tranny, that the voltage at its collector will then drop as current starts to flow from collector to emitter, and which will cause the input to the MOSFET Gate to go low and which, in ‘Depletion Mode’ will cause it to start to conduct between its Drain and Source?

Thanks in advance,

Jules

the problem I have found is that the voltage at pin 3 is not stable when no magnetic field is present.

Please define “no magnetic field”. How far away is the magnet?

It should be high at the same value of the supply voltage, in my case a nominal 12V lead acid battery.

Incorrect. Approximately 6-7 volts is the highest voltage that can appear on pin 3 using the circuit attached to your post.

You say you have a depletion mode mosfet. What is the part number?

Is this for a car application?

Can you use a LM7812 voltage regulator as your 12v source to your hall sensor? You could also put a electrolytic cap between your output and GND to buffer the signal if you are getting voltage fluctuations (like in the case of an alternator in a car).

What exactly is the problem you are seeing with the hall sensor? Are you getting false triggering?

The magnet is over a meter away. I can see the output go from 3-11.8V to zero as the magnet approaches a few centimetres from the sensor.

I get the 11.8V when I hold the sensor in a particular way (at the moment its not connected to the MOSFET) so maybe that's why I can get a higher voltage?

The MOSFET number is IRF840.

Its not for a car application but a generator that creates back emf pulses so the sensor is causing a coil magnetise and then demagnetise rapidly so that a large back EMF is generated.

I don't think its false triggering which works fine but rather the High output on Pin 3 (with no mag field) varies depending on how the sensor is positioned and whether Im touching it or not. I need the full 12V so that there is no current into the base of the transistor. I imagine anything less will start to trickle current into the base and hence start to reduce the collector voltage etc.

J

Several observations:

The pin 3 output of the A3144 is low, sinking current when the sensor detects a magnetic field above sensors operating point.

When pin 3 is low, the collector of the npn transistor will be high since there is no base current flowing. Why the base of npn transistor is connected via a voltage divider to 12 volts is puzzling, it adds nothing to the circuit.

With the gate of the IRF840 (an enhancement mode n-channel device) effectively connected to 12 volts via the 500 ohm resistor, the drain of the mosfet would be low if wired correctly for low side switching.

Given the driver circuitry, the mosfet will most likely self destruct due to excessive heat due to the slow gate dv/dt. This ultimately depends upon switching speeds and the load - both of which you have provided no information.

I don't seem to be able to add a new circuit diagram here to show the full circuit. So I will create a new post so you may be able to give me some helpful advice. Thanks, Jules

Kerrowman:
Hi there,

I am using an A3144E Hall Sensor to detect the pole of a rotating magnet such that it switches on the base of a transistor. The circuit is shown in the attached pic. Firstly, I note that the sensor is pole dependent in that its pin 3 drops to near zero volts when a south pole approaches but not a North pole but the problem I have found is that the voltage at pin 3 is not stable when no magnetic field is present. It should be high at the same value of the supply voltage, in my case a nominal 12V lead acid battery.

Its an open-collector output, its already got a transistor. Unless you provide an external pull-up resistor
for the open-collector output it will not work.

Your circuit appears to provide this, but there's a 1K resistor on the base of your external transistor which should removed, and the other 1k perhaps changed to 10k so its not too strong for the hall
switch. Remove the redudant diode.