----- Original Message ----- 
From: "Spehro Pefhany" <speff@interlog.com>
To: "Microcontroller discussion list - Public." <piclist@mit.edu>
Sent: Thursday, September 10, 2009 7:49 AM
Subject: Re: [EE] two wire inductive magnetic gear tooth sensor circuitry


> At 11:12 PM 9/9/2009, you wrote:
>> > The OP has not provided enough information to allow anyone to do more 
>> > than
>> > make guesses at what he has and what he needs.
>>
>>I would strongly recommend that the original poster see what happens if he
>>treats the sensor as a current loop device:
>>
>>Connect +24V to +ve lead of sensor
>>Connect -ve lead of sensor to load resistor (250 ohms or so)
>>Connect other lead of load resistor to ground
>>Scope signal across load resistor
>>
>>If the device is indeed 4-20ma current loop, then the signal should swing
>>somewhere between 1 and 5 volts.
>>
>>-- Bob Ammerman
>
> If the signal goes up to a large DC value, slowly changes over the next
> few minutes, then makes an abrupt change, then you know it is
> (was, actually) a VRS. ;-)

The OP has *already* applied 24V to the sensor (without a series resistor) 
and it seems to generate a low-level sinusoid(?) superimposed on the supply 
voltage. This test apparently didn't fry the sensor, so it probably isn't a 
(bare) VRS. I am assuming he has a high enough impedance power supply that 
the device is able to force it up and down the 50mv he is seeing. If the 
sensor is really a 4-20ma device the 50mv signal would indicate a source 
impedance of the power supply on the order of  2.5 ohms, which seems rather 
high, however.

-- Bob Ammerman
RAm Systems

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