# 4-20ma source build



## sseivard (Apr 25, 2012)

Does anyone have a simple build for a 4-20ma source using two 9v batteries as a power supply?


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## MechanicalDVR (Dec 29, 2007)

sseivard said:


> Does anyone have a simple build for a 4-20ma source using two 9v batteries as a power supply?


 
Buy one at Grainger it's easier. I have had the Extech model for a while.


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## wildleg (Apr 12, 2009)

a battery, a resistor, a meter

you could get fancier (use a couple transistors and save battery power, add a switch), but that's about all you need


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## JRaef (Mar 23, 2009)

I've used the lantern batteries, the ones with the spring connections, because they are easy to connect to with alligator clips. Then put a trim pot in series and calibrate it with your Multimeter. Just use simple Ohm's law to determine the range on your trim pot.

The big drawback to this is stability. As you use it, you are draining the battery and thereby changing the voltage, which because the resistance is fixed, means your current is constantly changing. Not much, but remember you are dealing with a small range of just 16ma, so even small changes affect your results. So basically if you are only going to do a quick initial test of a field device this is often OK, that's what I use it for. But if you are going to run a time based test, i.e. a calibration setup, it isn't. What the pre-made testers do for you is add voltage regulation so that the current remains constant. They also usually give you a nice little display on the front that shows you what your output is, that comes in really handy.

You can still make it yourself (minus the display), here is a simple diagram if you are handy with electronics assembly. If you are not registered for PLCTalk.net you might get redirected and have to do so.

www.plctalk.net/qanda/attachment.php?attachmentid=6425&d=1173530806


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## Big John (May 23, 2010)

wildleg said:


> a battery, a resistor, a meter...


 Exactly. A 1kΩ half-watt potentiometer and a little milliamp meter. 

You could probably put it in a nice little box for about $35 total.

-John


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## MechanicalDVR (Dec 29, 2007)

Big John said:


> Exactly. A 1kΩ half-watt potentiometer and a little milliamp meter.
> 
> You could probably put it in a nice little box for about $35 total.
> 
> -John


 
Or pay @$45.00 and get one with a regulated output.


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## Big John (May 23, 2010)

Does someone make one that cheap? I don't think I've ever seen one less than $100.

-John


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## sparky970 (Mar 19, 2008)

What are you planning on sourcing?


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## JRaef (Mar 23, 2009)

Big John said:


> Does someone make one that cheap? I don't think I've ever seen one less than $100.
> 
> -John


I've got one of these, I really like it:


The problem is, it was given to me on a job that I refused payment for (small job for a friend), so I don't really know how much they sell for. Nothing to it really though, it's very plain and simple. I'd be surprised if it costs more than $50. Click the image for the link. If someone calls them, let me know what it's worth.


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