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Participant

Join Date: Feb 2017
Posts: 1

# DC Bias for Loop Powered Devices

02/03/2017 6:11 PM

This question relates to instrumentation and devices. Can a two wire HART Transmitter with analog 4-20mA be called as as Device requiring DC Bias? Hart Protocol link: https://en.wikipedia.org/wiki/Highway_Addressable_Remote_Transducer_Protocol.

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Guru

Join Date: Dec 2016
Posts: 1607
#1

### Re: DC Bias for Loop Powered Devices

02/03/2017 6:24 PM

Depends on the device. Perhaps a review in is order:

Part 1

Part 2

Part 3

Guru

Join Date: Jul 2010
Posts: 644
#2

### Re: DC Bias for Loop Powered Devices

02/03/2017 7:53 PM

Although the 4mA minimum, which is probably what you term a 'DC bias', is frequently called a "live zero" the real purpose of a minimum 4mA is to provide power for the operation of the 2 wire transmitter. The transmitter uses up to about 3.6mA to power its own operation. That means a 0-20mA signal is generated from a 4 wire device, because no 2 wire, loop powered device will run at 0.0mA, 1.0mA, 2.0mA, or 3.0mA.

The current levels between 3.6mA and the 4.0mA zero are underrange or calibration trim below the live zero and/or a fail-safe fault level.

This sounds like some lazy teacher's trick question, revealed by the use of the long form of HART, which only ever appears in the student world. In 35 years of instrumentation, I've NEVER heard Highway Addressable whatever. It is always HART.

Guru

Join Date: Dec 2016
Posts: 1607
#4

### Re: DC Bias for Loop Powered Devices

02/04/2017 3:56 PM

I know, right? I went for years before realising ACRONYM stood for "Alphabetically Coded Reminder Of Names You Misremember."

Guru

Join Date: Mar 2007
Location: at the beach in Florida
Posts: 16884
#3

### Re: DC Bias for Loop Powered Devices

02/04/2017 12:48 PM

"The AC signal applied to them is superposed on this DC bias current or voltage. The operating point of a device, also known as bias point, quiescent point, or Q-point, is the steady-state (DC) voltage or current at a specified terminal of an active device with no input signal applied."

https://en.wikipedia.org/wiki/DC_bias

https://www.acromag.com/sites/default/files/Acromag_Intro_TwoWire_Transmitters_4_20mA_Current_Loop_904A.pdf

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