Re: Need advice on voltage controlled current source
- From: Nemo <zzz@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>
- Date: Mon, 5 Oct 2009 21:06:34 +0100
In article <02d4637a-5f2c-4fa9-9bef-a3db3510718d@xxxxxxxxxxxxxxxxxxxxxxxxxxx>, Andrew <andrewkgentile@xxxxxxxxx> writes
Hello,
I'm working on a tachometer which will output a current (4-20 mA)
If you are new to 4-20mA loops, be aware:
Current sourcing, like this, is fine; but current sinking is simpler to implement. You could get by with one op amp that way. Of course, current sinking requires that the external thing you interface to provides the power. I can post more details on this if you want.
Some people use more than the pure 4-20mA range of a 4-20mA loop. Typically, they use 2mA or 24mA as "fault" or "overrange" signals. If you are not sure how the receiving equipment interprets signals outside the 4-20mA range, your software should limit the output from the DAC to 1 - 5V.
If your tachometer could be hooked to a variety of equipment, you can never be sure exactly how the far end has implemented the 4-20mA "standard". That's why many 4-20mA loop interfaces have jumpers or switches to configure them to be source / sink and set their fault, etc outputs. It's handy if you know that the far end will ALWAYS be item X.
The other "gotcha" with 4-20mA interfaces is voltage drops along long loops, or through zener barriers. (A zener barrier is a combination of R's and zener diodes to limit the energy that can travel through it. Often used in certain industries.) For long cable runs, say a kilometre or two, you need to take into account the wire resistance. Then double it, because the voltage drop is *both ways* (2 wires). Let's say the total wire resistance is 15 ohms in one direction, which is 30 ohms over the entire loop. Now the stupid bit: some people sense the voltage at the far end by simply running it through a 250 ohm resistor. This gives a convenient 1 to 5 volt signal and saves them an amplifier. I have actually seen this in very old / legacy equipment; amplifiers like the LM324 now only cost 10 cents*, but used to be a significant cost.
For a maximum loop current of 24mA, that will drop 0.024 x (30 + 250) = 6.72 volts - which doesn't sound like much, but if there are other voltage drops in the system like zener barriers (which if memory serves can be 300 ohms), and you take into account the tolerance of your "24V" supply (often +/-10% in industrial applications) you can actually run out of volts, i.e. the equipment at the far end can't quite see 5V when you try to output 20mA.
--
Nemo
* Cue howls of laughter from some members of the group - "You throw away 10 cents? I just draw the symbol
for an LM324 on the back of my PCB in pencil, it works just as well" etc, etc
.
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