4 to 20 mA generator help

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- 29th October 2008, 20:01
Snap

4 to 20 mA generator help

Hi, I would like to make a 4 to 20 ma generator to test the analog input on a plc. As always I have been working on this for a while before I come running for help to the forum and am still at a loss. I can generate a 0 to 5 volt dc signal with a simple pot but can not get the mA to co-relate. I also have researched and know that a 250 resistor will convert the 0 to 5 to 4 to 20.but I can't make it work and to top it off I blew the fuses in my fluke and it cost 80 bucks for new ones. I would like to display the settings on an lcd so I would like to use a pic and pic basic. I can handle the lcd stuff but I just need a direction. As always I will post the results when I get it working.
Thanks again great forum,
:D Snap
- 29th October 2008, 20:09
mackrackit

Here is a start
http://www.picbasic.co.uk/forum/showthread.php?p=60872

Quote:

a 250 resistor will convert the 0 to 5 to 4 to 20.

I think that is wackbards?
- 29th October 2008, 20:16
Snap

Sorry I ment to say A 250 ohm resistor should convert 0 to 5 volts to 4 to 20 mA.
- 29th October 2008, 20:22
mackrackit

Quote:

Originally Posted by Snap

Sorry I ment to say A 250 ohm resistor should convert 0 to 5 volts to 4 to 20 mA.

Thought so, just checking :)
- 29th October 2008, 20:27
Snap

Cool Halloween picture.
- 29th October 2008, 20:30
mackrackit

Quote:

Originally Posted by Snap

Cool Halloween picture.

Thanks!
What I look like now and what I will someday look like :D
- 29th October 2008, 21:40
ronjodu

What I used.

In my old days as a service tech I used this.

http://www.kele.com/olcat/TE7/CLC-100.PDF

Unfortunately I don't have a schematic for it. I think it sells for around $600 without the PWM option. There's got to be a cheaper way to roll your own.
- 29th October 2008, 21:50
Snap

Thanks, but I don't need all the bells and whistles and would like to build it myself (to learn). There has to be an easy way to do this I am just missing something. I just need a start. I see the problem might be where to put the extra amps when I throttle them down from 20 to 4. It I pulsed the pic like controlling a dc motor speed it might work but what does that do to the amps? I think there is a IC that might work has anyone done this before?
- 29th October 2008, 21:59
mackrackit

Quote:

Originally Posted by Snap

I think there is a IC that might work has anyone done this before?

Did you read the thread from the first post?
This was in there
http://www.maxim-ic.com/appnotes.cfm...te_number/722/
has a list of chips.
- 29th October 2008, 22:45
Snap

Duh:eek:, I cant believe I missed that. I must have been too caught up in your picture.
I will order one and see if I can modify it. I will post results thanks.
- 29th October 2008, 23:16
aratti

1 Attachment(s)

Try this simple circuit using a LM350 and 120 ohms pot, it should resolve your need.

See the attached diagram.

Al.
- 30th October 2008, 00:10
Andy Wood

1 Attachment(s)

Quote:

Originally Posted by Snap

Hi, I would like to make a 4 to 20 ma generator to test the analog input on a plc. As always I have been working on this for a while before I come running for help to the forum and am still at a loss. I can generate a 0 to 5 volt dc signal with a simple pot but can not get the mA to co-relate. I also have researched and know that a 250 resistor will convert the 0 to 5 to 4 to 20.but I can't make it work and to top it off I blew the fuses in my fluke and it cost 80 bucks for new ones. I would like to display the settings on an lcd so I would like to use a pic and pic basic. I can handle the lcd stuff but I just need a direction. As always I will post the results when I get it working.
Thanks again great forum,
:D Snap

I came across this diagram some time back. Maybe this gives you what you want?
- 30th October 2008, 12:55
Snap

:)Thanks for all the help I ll get working on it and post the results.
Snap