A device to measure 4-20mA and re-transmit w/o loading

[Mrtryhard]

I'm trying to measure 4-20mA output signals from a pyrometer and manometer to a controller and measure the 4-20mA output signal of the controller so as to graph them via interface to a PC.By the time I get the signals through the interface to the PC I've lost much of the signals and can't graph properly.Is there a device to measure or compare the signal and re-transmit exactly?

 

[nbucska]

You can't measure current without load -- if the circuit is open, there is no current.

<nbucska@pcperipherals.com>

 

[buzzp]

There are many units available that accept a 4-20 signal, display, and options for retransmitting. Try Omega Engineering, and other temp control vendors. Temp controllers are used all over, so the price is fairly low. You can program them to display the units you want. Jumo makes one for sure, sells for around $100 in quantities of 100/yr.

 

[elfgriper]

mrtrhard,
As stated above you must load a current source to develop a signal. With out a load even a very small current will charge up a high impedance input line to it's compliance voltage, that is the max voltage that can be sourced. what most people do is to normalize the current for the desired signal level. ex: if your looking for a 4 to 20v signal then you must place a 1k resistor across the current to be measured, a 500 ohm res will give you 2 to 10 volts and so on. most pcb input interfaces have a very high input impedance so you don't normally need to worry about that.
elf

 

[hacksaw]

hopefully you are not using the controller output as the it will modify your signals, unless you have configured it to simply repeat your analog signals.

additionally most pc input cards are voltage input and single ended,and cannot be connected to a current output without a current sensing resistor, what type of card do you have?

 

[OperaHouse]

How about a resistor! The beauty of 4-20 ma (about the only) is that you can have resistance (within reason) in series and it doesn't effect the operation. A 10 ohm resistor would only have a max burden of 200mv. Even a LED opto isolator would only have a burder of 2V max. Unless your equipment is very poorly designed, you shouldn't have a problem with these solutions.