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#129091 07/12/04 10:26 AM
Joined: Apr 2004
Posts: 156
R
rad74ss Offline OP
Member
I need help trying to figure out how to convert a 0-5vdc signal to 4-20mA. This is to convert the signal on a 0-500psi transducer.

The company wants to use resistors instead of buying a converter.

I would appreciate any help from the electronically inclined electricians.

Thanks.

Robert

Joined: May 2004
Posts: 3
M
Junior Member
Without going into a long-winded dissertation, forget about using resistors. The differences in the "zero" point base (0 volts vs. 4 mA) and in how instrument designs handle voltages versus currents do not lend themselves to a simple conversion using only resistors.

A 0-5V/4-20mA signal conditioner can be bought for around $250. You will burn up that money in a heartbeat trying to configure and calibrate a homebrew resistor setup.

Joined: Apr 2002
Posts: 2,527
B
Moderator
5V/0.020A=250Ω, but it’s not usually that simple.

There is an offset problem for transducer output—below 1 volt, the current channel will treat the reading as zero, or as a signal error when under 4mA. A resistor may work with the right 4-20mA transducer output and 1-5V input channel.

Joined: Sep 2001
Posts: 806
N
Member
If you are handy with a soldering iron, you could build your own V/I converter using a chip like the XTR110 from Burr-Brown:
http://kolter.de/xtr110.pdf

Because you need to source 4 mA of current with zero voltage input, you will NEVER get ther with a purely passive network of resistors.

Joined: Dec 2004
Posts: 329
I
Member
if you're can read a schematic and use a soldering iron try here: http://www.discovercircuits.com/C/co-vtoc.htm

also a good source for many other circuits

Joined: Nov 2000
Posts: 2,148
R
Member
Robert,
Are you sure that you need a conveter? Many 4-20ma inputs just use a 250 ohm resistot internal to convert the current to a voltage, and many have a setting that will change the input from 4-20 to 0-20. If both of these conditions are present, all you have to do is to remove the internal resistor and change the setting to 0-20ma.
Don


Don(resqcapt19)

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