4-20mA loop powered indicator

[n ranjan]

i am into design of an indiactor for a 4-20mA current from a pressure transmitter. the indiacator has to take power from the 4-20mA current loop.can any direct me to appropriate resource or can give ideas to design loop powered indicator

 

[Russlk]

What kind of indication do you want? You could put an LED across the line to indicate that there was current.

 

[n ranjan]

thanks for the reply. let me clarify what i am looking for. the indicator should be a LCD 7 segment 3 1/2 indiacator to indicate 0-100 % for 4-20mA input. the indicator should work with the input ( 4-20mA) only which means indicator will not have either battery or a seperate power source. the 4-20mA output deviec is powered by 24V. this technic is used in process industry instrument where the power line carries the information which is 4-20mA.

normally any indicator or for that matter any instrument needs a power source and the signal is applied at the input for indication or any processing. the indicator i intend to design shall work only with input which means its electronis hardware shall take power form the signal source without loosing efficiency. in short it willbe analogus to a moving coil meter where only input is given and the indication is by deflection whereas it will be by digital indication here.

i think i hv given a very long brief but i hope there is no uncertainity here about what i am lookin for.

 

[instruite]

Hi
what I can understand from your post is that your pressure transmitter is a "two wire transmitter" type. it will be better if you can post the datasheet of the same

and you can use the power supply on 4-20 mA line but you will have to make sure that at 24VDC supply you should consume current less than 4 mA generally better to keep it less than 3 mA
and regarding interface then you can search on google for "two wire transmitters" "4 20 mA" and you will find lots of block diagrams

 

[Russlk]

Here is one idea: The DC to DC converter uses 4mA. When the current increases, the voltage regulator uses the excess current. If you want to show 1.999 for full scale with a 200mV meter, the resistor is 12.5 ohms.

 

[n ranjan]

thanks for the idea. i shall give the feed back in a week as i am away on holidays.

 

[panic mode]

it's nice and simple idea but i'm not sure it is going to be good enough. 4mA consumption of the switching power supply is not precise and it can (and will) change during operation. it is better to measure total current and then compensate for 4mA offset. also does indicator have to be digital?

 

[Russlk]

The load on the switching supply will be constant, so the input current will be constant. If it varies with temperature, you can compensate with a thermistor. You said you wanted a 7 segment display? that is digital. If you can use an analog meter, just change the meter scale, no electronics needed.