Introduction: Building a Precision Current Reference Using REF200
I don’t have any current source in my lab, and these are useful for testing multimeters for example. I was looking on ebay for a current source but I wasn’t able to find an affordable one so I’ve decided to build one using the REF200 from TexasInstruments. This should be a fairly easy build that uses few parts so anyone can do it.
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Step 1: Checkout the Video!
The video describes the entire build so I recommend watching the video first to get an overview of the project. Then you can come back and read the following steps for more detailed explanation. If you enjoyed the video don't forget to subscribe to my channel and maybe hit the like button.
Step 2: Parts List
Here you can find a list with links to all the parts I used in the project. Feel free to adjust and make changes according to your own needs.
- Plastic Enclosure http://voltlog.com/y/jp146
- 4mm Banana Jack http://voltlog.com/y/e71g0
- Battery Charging Protection Module http://voltlog.com/y/e71g0
- 3P4T Rotary Selection Switch http://voltlog.com/y/e71g0
- 18650 Lithium Battery Cell http://voltlog.com/y/e71g0
- SO-8 Adapter PCB http://voltlog.com/y/e71g0
- REF200 - you should get it from one of the big distrubuitors Digikey, Element14 or directly from TexasInstruments.
As shipping these items from China to your location might take some weeks it is recommended to order them ahead.
Step 3: How Does It Work?
The heart of this circuit is the REF200 integrated circuit from TexasInstruments.This ic combines three circuit building-blocks on a single chip: two 100uA current sources and a current mirror. This makes it ideal and very simple to use in our project because we could use a single current source and get 100uA on the output, we could parallel the two current sources and get 200uA output, we could use a 100uA current source put it through the current mirror, get 200uA out and add it to the initial 100uA for a total 300uA output or we could also parallel the two current sources for 200uA, put that through the current mirror to get another 200uA which added to the initial 200uA would gives us an output of 400uA.
I don’t like having to replace batteries in my lab equipment so if we take a look at the REF200 datasheet we notice it can work with voltage from 2.5 up to 40V. That's quite a wide voltage range which makes it compatible with a single rechargeable lithium cell.
The idea is to use an 18650 cell recovered from a broken laptop battery because it's free and it should should have over 2000mAh capacity which will last the current source for years but just in case the battery gets discharged I will also add one of these small lithium cell charger modules complete with protection circuitry. If you don't have any of these salvaged cells you can just purchase a new battery.
Step 4: Working on the Enclosure & Wiring
First step was to wire everything up according to the schematic(attached in PDF format to this step). I will start by soldering the REF200 IC on it’s small adapter pcb, next I will make the required connections on the rotary switch. I’m also soldering some wires to the battery terminals, care should be taken here to avoid excessive heat being applied to the battery so I’m always quick when soldering to batteries.
I’ve used eagle cad to draw a front panel for this enclosure just to get some markings where holes should be drilled and to make sure everything fits inside.
Next I took care of the enclosure and drilled the required holes and cleaned them up a bit. Once I had the enclosure ready I started fitting all the parts inside. I used some double sided tape to hold the battery in place. Same with the battery charging module, it was secured with some double sided tape and a slot was made in the side wall of the enclosure to allow connecting a micro usb cable for charging the battery. I then made the final connections to the output terminals and everything is ready now for the first test.
Step 5: Testing
Testing the current source was as simple as connecting it to a precision current meter, in this case the Keithley 197A. I got some pretty good results, less than one microamp measurement error and it was very difficult to tell if the meter was out or if the reference was out (mostly because I don't have a recently calibrated multimeter) but the REF200 has a spec sheet accuracy of plus or minus 0.5%.
I'm pretty sure with how the project turned out and I will be using this piece of kit for testing all the other multimeters in my lab. I hope this inspires you to build your own. Don't forget to checkout my youtube channel for more interesting videos: