4-20ma Sensor Simulator Part 5

by on Feb.23, 2012, under Electronic Projects, SCADA

Now to the fun stuff…

This is the temperature probe I use for watching the temperature in the oven, it also has a serial output that can be logged to a computer.

Multimeter with temperature Probe

This is the solder paste I bought from Ebay for about $4 for 50 grams.

Solder Paste from Ebay

Cheap 3 mil syringe from a animal husbandry supplier.

The 18 gauge needle was full length, so I cut it down to about 1/2 inch to allow more control. Only fill the syringe with .5 mil or less of solder paste, anymore than that it will be to hard to push out.

The black tape is my way of keeping the end sealed when not in use.

Oh, when not using the solder paste, store in the fridge in a ziplock bag. Less chance of getting solder on things you eat.

3 mil Syringe loaded with Solder Paste

Practice squeezing out the solder in a spare PCB board to get a feel for how it comes out. If you get to much out at one time a toothpick or a dental pick is a good way remove extra paste. The toothpick or dental pick is also a good way of applying the paste if you find the syringe awkward to use.

Starting to Apply Paste

Now the board is ready for parts placement. A good set of tweezers are your friend here. Just set the parts on the solder paste and it will hold it into place. The amazing thing about solder paste and heat, it turns into a liquid and the parts actually float for a moment and the parts will align with the pads. So being a little off on the placement is not the end of the world.

Paste done

Adding Parts

Below are the pictures of the reflow process.

1 is before heating

Board and Temperature probe before preheating.

Preheat of the oven and board for 2 minutes to about 125 degrees C.

Reaching the peak of the reflow process temperature of 225 degress C.

Cool down of the oven.

After the cool down process you can start to check out the solder joints.

This picture all looks great….

This picture…not so good. Enlarge the photo and you will see the bridge between the last two pins of the IC.

If you look at the picture below you will see where the solder paste was applied you will see to much solder paste as added at these pins.

Now to fix the bridge I used a Solder Sucker and a fine point soldering iron. By heating the two pins with the iron and then quickly sucking the excess solder out, the bridge was removed without damaging the board. Did not have to resolder the joints this time either. Sorry no picture of this…would need a third hand.

Tools that help me do the things I do.


Pretty much the end of this process, I am confident you can do this to.

As one of my favorite sayings goes “God Hates a Coward” which to me means, go for it, the worst thing you can do is fail.


Till Next time, thanks for your time.


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4-20ma Sensor Simulator Part 4

by on Feb.21, 2012, under Electronic Projects, SCADA

Reflow Oven…. Or better known as a cheap toaster oven.

After soldering the first few boards by hand I did some reading on on using a toaster oven or skillet to reflow the solder on the SMT components. Looking each method I decided that a toaster oven would work better for me.

After buying the oven that was on sale and less then $40.00, ordering solder paste from Ebay and picking up 5ml syringes with needles I was ready to start the adventure.

Now I did a test to see if the oven was good enough to reflow solder.

You will need a empty toaster oven(just the rack), temperature meter, a stop watch( or at least a clock with a second hand).

Setup the oven for dual element heating, on mine this is the Toasting setting and have it on all the time or set the cooking timer to max on time. The temperature measuring senor like the thermocouple probe that comes with some multimeter should be placed about the center of the oven.

Now to test the oven. Watch the temperature meter and increase the temperature in the oven till you get to a temperature between 100-125 degrees C (212-257 F). Leave it here for about 2 minutes. Do this to make sure the oven has warmed the internal parts for the next temperature increase. After the two minutes crank the oven on full, start watching the time and temperature on the meter to see how much time it will take the oven to reach a temperature of 225 degrees C (437F). If the oven does not reach the 225 degrees C in 80-90 seconds empty, it will not be able to do it when you have it full of components and boards. Return it and try a different one.

When you have a toaster oven that will do the above, then it is time to test with a PCB.

Here is a generic Sn/Pb solder paste curve:

You do not need a fancy controller to do the above curve. In fact as long as you meet the plateaus of 100-125C and 225C for the times periods shown above graph you will achieve the desired effect.

Now if you are like me…you will in time build or buy a reflow controller.

Next Time Pictures of laying out the solder paste.

Till Next time, thanks for your time.


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4-20ma Sensor Simulator Part 3

by on Feb.16, 2012, under Electronic Projects, SCADA

XTR117 Multi MSOP

Above is the schematic of the unit.

As you can see this is simple circuit. The XTR-117 is a current amplifier, plan and simple. By using a the 5 volt reference built into the chip and a resistor you can control the current that will be amplified.

If you have a 125kohm resistor between the 5 volt and the Iin pin you will have  current  flowing at this pin. This chip will amplify the current to 4 ma on the output when a the circuit is built as in the PDF above.

Every 125kohm resistor you put in parallel with the first 125kohm resistor will increase the input current and increases the current by 4 ma.

So as you decrease the resistance you increase the output current of the chip. Below is the prefect world resistor values to achieve the 4-20ma output in 4 ma steps.

125kohms=4 ma, 62.5kohms=8 ma, 41.67 kohms=12 ma, 31.25 kohms=16 ma, 25kohms=20 ma

Now in my world it was hard to find a 125Kohm resistor so I used a 124kohm and get pretty good results. Each step is a about .03 ma short of the 4 ma step I wanted to achieve.  This was good enough for my requirements at this time.

If I was to build one for calibrating or tuning a current loop I would spent the time and get 125kohm .5% resistors. There are out there, just not something you find at the local electronics store.

The input through a bridge rectifier makes it so there is no polarity required. Positive can go on either terminal. Made it this way to handle any transients that may occur.

So far I have 45 units in operation and find them very helpful in testing analog systems with either a 4-20ma or 1-5 volt input.

Oh, for some of you who didn’t know, if you need a 1-5volt source, just put a 250 ohm resistor in the return leg of the unit you will get the desired 1-5 volts across it.

Got to Love that Volts(V)=Current(I) x Resistance(R)

Till Next time, thanks for your time.






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4-20ma Sensor Simulator Part 2

by on Feb.14, 2012, under Electronic Projects, SCADA

The units Specs:

Sink 0-70ma

Input voltage 9-40 volts

No input polarity

Size 1.250″ x 1.500″

That is it. Very simple.

Now my mistake I make on this unit. The mounting holes for the board. I did not leave enough room to use a a standard 6-32 metal standoff (the only size of metal standoffs I had in my tickle trunk of parts). I had to resort to plastic cylinder standoff with plastic bolts. This way I will not short with any the components in the board.

A 4-40 standoff would work fine as a standoff for this board.

I also epoxied some together in groups of 2 and 6 for bench testing of a development system.

Embedded standoffs in the epoxy for mounting.


Next Post will show the design and the math to pick the resistor value get the output current one wants.

Till next time thanks for your time.


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4-20ma Sensor Simulator

by on Feb.13, 2012, under Electronic Projects, FOR SALE, SCADA

Having a  need for a cheap simple 4-20ma sensor simulator and not finding any on the web, decided to design and build one.

To achieve this I used the XTR-117 driver by Texas Instruments

XTR-117 Data Page

I learned a few things when building this unit. One is that surface mount soldering is not as easy as one thinks, Not impossible, but harder then the good old through hole I was trained on oh so many years ago. Two, after learning I can do surface mount soldering and I am pretty good at it. But decided that the idea of a reflow oven would make the job even easier. So did a lot of reading and then bought a cheap toaster oven and downloaded the temperature curve for the solder paste I bought of Ebay.

After getting the toaster oven out and doing a test run of the oven to make sure it would heat up fast then the curve described by the temperature curve of the solder paste. It did and then decided to go for a actual board test.(used my handy multimeter with its K type thermocouple as the temperature indicator.)

Laying out the solder paste using a tooth pick and placing the components on two test boards it was time to bake me some boards and solder.

With a my Iphone as the timer, temperature probe and the handy reflow curve I stared the process of my first reflow.

The whole process took less then 5 minutes for the reflow, but had to wait for about 15 minutes for the cool down…

They looked great, but the real test would be to finish solder in the other components and power them up.

The pictures above are of the one of the first units I did in the toaster oven, worked way better than I ever expected.


The next post on this little unit

The unit I built gives one approximate 4 ma steps from starting from 0 ma to 20 ma with couple of extra features, one being a variable output and a way to add in a external resistive sensor.

The idea I had for the external resistive sensor was to use a photocell in the 40-100 kohm range. This allowed the unit to give a different reading through the day.

The only problem with the unit is it can not be calibrated.(way to small to add in calibration pots for each level. 1.5″x1.25″)

More to come on this project, such as the simple design, using a toaster oven as a reflow oven, and if you really need some what it will to get some.

Till next time thanks for your time.




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RS-485 Test Switch

by on Feb.12, 2012, under Electronic Projects, SCADA

ABSwitch for RS-485 testing

Above is a PDF of a simple switch setup that I use almost everyday when testing RS-485 communications on a new or unknown system.

The best result when building one is to make sure you label the switch positions and wires. After that, testing goes much faster.



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