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Guru
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Location: AM-51, Deen Dayal Nagar, Gwalior, Madhya Pradesh, MP 474001, India
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Transmitting Frequency Signal and Power on 2-Wires

06/14/2013 3:21 PM

Hello Friends,

In the series of new sensor interface designs, while we opto come across DC level changes in 4-20mA current signals, here I am proposing 4-20mA signal switching for frequency signals as well for limited frequency range from 1Hz (or perhaps 20Hz) to 10kHz (or perhaps 20kHz). Such frequency opto can be modulated over the 24DC power line with full switching of 4-20mA current. Signal then can be extracted by an audio transformer for limited frequency range and by using opto-isolator for the entire frequency range.

To start with I am suggesting that5 any relaxation RC oscillator can be used for frequency generation and then let us examine how this generated frequency can be transmitted over the DC power itself.

I will also add some circuit here and if anyone want to put up an idea then we can discuss that also.

Circuit shown above is for simple timer to generate frequency and to transmit on DC power line of limited voltage range from 5V to 15V. Do not try 24V for this circuit as 555 is designed for maximum 15V. R3, R4 and Cs in different combinations can generate a lot of different frequency sets. Higher frequency will get lost in the power line capacitance. Hence, working with < 20kHz is recommended. Try this circuit yourself and let all know the results.

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Prof. (Dr.) Shyam, Managing Director for Sensors Technology Private Limited. Gwalior, MP474001, India.
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Guru

Join Date: Jan 2008
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#1

Re: Transmitting Frequency Signal and Power on 2-Wires

06/14/2013 11:28 PM

The existence of the low/high current modulation of a current source is sufficient to prove the continouos existence of the loop current. F1/F2 modulation of frequency shift keying is sufficient in a similar way, alone.

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Guru
India - Member - Sensors Technology Popular Science - Cosmology - Dream, Think and Act United Kingdom - Member - New Member United States - Member - New Member Canada - Member - New Member

Join Date: Aug 2006
Location: AM-51, Deen Dayal Nagar, Gwalior, Madhya Pradesh, MP 474001, India
Posts: 3408
Good Answers: 32
#2
In reply to #1

Re: Transmitting Frequency Signal and Power on 2-Wires

06/15/2013 2:38 AM

If you are referring to HART communication which has standard well defined two FSK frequency bands, then these can easily be added and sensed by transformer isolation. However, for wider range sensor frequency signals, frequency can be anything in time from 1Hz to 20kHz dynamic range that we have considered here. This requires new design methods to be implemented.

In the figure above, I have used loop current modulation using NPN transistor which limits the amount of maximum current to be switched to high level when signal is also high level. When signal is at low level, the loop current remains at the lowest power consumed by the oscillator circuit.

I have also used a diode to separate the power during modulation of the current as that will drop the voltage on the power / signal line and during high current switching oscillator is prevented to look at the power line voltages. However, when power line current goes to minimum and power line voltage recovers, the diode permits the current flow to charge the capacitor to power the oscillator circuit to the extent its voltage is dropped out before. This circuit also permits high dynamic range for the power supply in use from 5V to 15V but not above 15V. In industries, dynamic range of the voltage should be 10V to 30V. Perhaps a low dropout voltage regulator can be added which can take input as high as 40V and its output can be fixed to 2V to 5V range and output current of 200uA to <1mA will be ideal.

HART communication requires an intelligent circuit to be added at the node point to handle communication properly.

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Prof. (Dr.) Shyam, Managing Director for Sensors Technology Private Limited. Gwalior, MP474001, India.
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