Looking for 4-20ma circuits

Now tell us the science as to why it begins at 4mA.

Current is current. It is VERY precise, and properly linearized, very accurate at any point in the loop.

A vacuum tube circuit (including filaments) powered by, say, 3mA? Do tell...

Best regards, Spehro Pefhany

It is no science, just pure practical engineering.

a) It is much easier to make a current source/sink which is good down to 4 mA than one down to zero.

b) It is easier to distinguish between a bad loop and lowest signal value when the low signal value is not 0 mA.

Wrong answer.

Wrong answer.

Teletype machines!

It also allows a break to be distinguished from zero reading.

well I guess it would be accurate at any point in the loop. It would have to be the same wouldn't it? It is a loop.

Jamie

The reason why they don't spec down to 0 is many devices need a threshold value to operate with. The loop not only serves as a way of maintaining a constant current due to long runs and devices being in series, it also allows for devices to use the loop current to power themselves.

Many receiver units act on currents that drop below the 4 ma spec and will assume a broken loop or sensor. These types of units are normally self powered and only the sensor is deriving its power to operate in the loop ( 2 wire types).

jamie

In a 4-wire current loop, there's no reason that current couldn't go right down to zero, or negative for that matter. The input Z is always the load resistor- nothing really changes.

It's every bit as important for receivers connected to the '4-wire' type of transmitter. No difference.

Best regards, Spehro Pefhany

Jamie,

Out of curiosity, what's the maximum compliance voltage one typically finds in a 4-20mA system?

Thanks,

---Joel

Wrong. That is/was ONE of the reasons it was chosen, but it is for instrumentation signals, not just industrial process control(s). There were many considerations. In Europe, it is 0 - 20mA. Always was, and still is.

So, yes, it will power the transmitter which it is meant to "read" from.

But that is still not the actual reason.

It is because below 4mA, transistors (nearly all) are not linear (were not then, even if some are (more linear) now).

That is the reason. Also, the 10 - 50 definitely uses more juice for the same job.

Also, the 16 unit span can easily be digitized at 4 bits. But that bit of luck came later, since they were not considering digitization modes when they arrived at 4 to 20mA. Some say that 5 to 25 should have been chosen as that makes the head math easier for translation to percentages, etc.. It is a 20mA span.

Wrong answer.

The span is 16mA, so mid-scale is 12mA, 25% is 8mA, 75% is 16mA.

No, because there were RF links in a lot of '20 mA loops' and locations where the 'loop was regenerated. Otherwise, the open loop voltage would be high enough to break down the insulation in the machines. Some '20 mA loops' were even delivered as 'RO', via a small earth station.

Even a low grade ham should know about RTTY.

The reason it doesn't go to 0 is for open loop fault detection. Don't you ever get anything right?

So why do europeans use 0-20 mA?

And why would anybody depend on transistor linearity in a transducer signal conditioner?

Who would want to digitize anything to 4 bits?

But that bit

Whose head?

ok. yeah I forgot about quantisation.

I don't think they do. Remember who suggested 0-20