4-20ma using 780x
4-20ma using 780x
(OP)
I've seen a circuit using a 7805 to generate a constant current depending on the value of a resistor(R1) connected from the output of the 7805 to the common lead. The common lead to ground then is supposed to supply a constant current depending on the value of R1 and NOT affected by the load from the common to ground.
When I built it, with a specific value of R1 I get the expected current if I measure from the common to ground.
However, if I add some series resistance from the common to ground, my current changes.
I'm using a 9V battery to power the circuit. Do I need a higher input voltage?
When I built it, with a specific value of R1 I get the expected current if I measure from the common to ground.
However, if I add some series resistance from the common to ground, my current changes.
I'm using a 9V battery to power the circuit. Do I need a higher input voltage?





RE: 4-20ma using 780x
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: 4-20ma using 780x
Anyway, that setup won't fly. You have 5 V drop over the resistor, and the 7805 takes 2...3 V by itself. You will maximum have an output voltage swing of 1...2 V.
Benta.
RE: 4-20ma using 780x
Battery + connects to 7805 input. 7805 out connects to resistor. Other side of resistor connects to 7805 sense lead and load. Your current will depend upon the resistor used and not your load.
This is a student-style posting (not allowed here), but I'll let it go as it doesn't seem to be directly classroom related...
Dan - Owner
http://www.Hi-TecDesigns.com
RE: 4-20ma using 780x
For gooberpat, these three pin regulators have a pin that is denoted Common in the symbol. And it is usually connected to ground. In the circuit you use, it turns into a "sense pin" (or rather the zero reference for the controller, the actual sensing is tied to the output of the 7805) and the controller tries to keep voltage between sense and zero reference equal to nominal output, five volts in your case.
I think that you should use something more modern. There are voltage regulators that have an internal 1.25 V band-gap reference and they will perform a lot better. Then you only drop 1.25 V over the resistor and, with the low current you have, the regulator needs less than 3 V to operate. So your 9 V battery will be OK - provided you don't need many volts across your load. Then add another battery.
If you like to work with ancient technology, there are the LM317 and the uA723. But there are modern ones as well. Just google "three pin regulator"
Gunnar Englund
www.gke.org
RE: 4-20ma using 780x
But the LM317 as current regulator will only go down to 10 mA (worst case, 5 mA typ.), as it needs to be able to dump its operating current somehow.
Regards,
Benta.
RE: 4-20ma using 780x
Gunnar Englund
www.gke.org
RE: 4-20ma using 780x
You are correct, I am not a student, but a 55 year old embedded firmware engineer. (70%SW, 30%HW)
The circuit I asked about was one I found here on an earlier post.
No-one was amazed at the ancient technology suggested by one of the members on that post. I am aware the '317 has a lower voltage drop, but the other post referenced a 7805.
Yes, I knew I could get a Burr-Brown (TI) part designed to do the job, but all I needed was a box with a switch to source either 5ma (yes I meant 5 not 4) or 20ma for our factory people to do a go/nogo test.
To the rest, thanks for your constructive(?) replies.
RE: 4-20ma using 780x
RE: 4-20ma using 780x
Dan - Owner
http://www.Hi-TecDesigns.com
RE: 4-20ma using 780x
RE: 4-20ma using 780x
get the data sheet and set up the temp compensated
circuit. (one extra resistor and diode).
RE: 4-20ma using 780x
RE: 4-20ma using 780x
Stuck in analogue? I don't see a digital solution here. What would it look like?
Gunnar Englund
www.gke.org
RE: 4-20ma using 780x
What happens at the load when 20 mA is sent to it? Will the voltage over the load be 2 V? 200 V? 1000 V?
And at 5 mA?
Until this is addressed, there is no sane answer.
Regards,
Benta.
RE: 4-20ma using 780x
http://www.calright.com/pd_408.aspx
http://www.scan-data.com/loopcal.htm
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Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: 4-20ma using 780x
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If you compensate for the 40 uA, you will have a 5/20 mA (or 4/20) current source that can work from one 9 V battery and still have about 6 V compliance.
Gunnar Englund
www.gke.org