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Topic: noise 4-20 ma output transducers, question about capacitors, (added schematic) (Read 504 times) previous topic - next topic

allanhurst

Don't put your capacitor there! - it won't let ANY current through!

Put it in parallel with your load resistor ie between the arduino analog input and ground, close to the arduino pin. And a series 4k7 or so would be an even better noise filter - see enclosed.

A higher resistor value - eg 220 or 240, would make better use of the arduino's dynamic range.

EDIT - sorry - overlooked you were using a 3.3v Arduino. So that should be about 150 or 160 ohms.  nedit.

You should calibrate against known weights ( and none!) to make sure......


Allan



TomGeorge

Hi,
@allanhurst  diag.

Tom... :)
Everything runs on smoke, let the smoke out, it stops running....


JoeJosefo

another 2 questions to add to the noobness of my thread to try to understand some things better:

Im trying to understand how the 4k7 resistor would (not) alter the analogread that corresponds with the current drawn by the transducer (I thought that  for example when the transducer senses 0 psi, it would draw 4 ma of current from the battery, then because of the resistor in the circuit and ohms law, I will get a reading of .66 volts (with 165 ohm resistors) on the corresponding pin, etc but I must not be getting the real idea behind this reasoning fully

Im getting that the current will come from the capacitor instead of the battery, but the capacitor and the 4k7 are connected anyway, is it then that the capacitor and the 165 ohm resistor are connected to the battery ground directly but not the 4k7 resistor?

Also, I have read that the 4k7 resistor can protect the arduino in case I turn it off but not the power supply for the sensors, so in that case would a 4k7 resistor for each sensor be necessary?


thanks in advance have a nice monday,

Josefo

:)
:)

allanhurst

The input impedance of an arduino analog input is effectively infinite.

The 4k7 and 100nF make a single pole low-pass filter which is 3dB down ( half power ) at about 340 Hz.

At dc it has no effect whatsoever.

So long as your signal doesn't vary significantly in <5mS ( 10 time-constants)  , this will be fine. Unlikely in your application, unless your animal is leaping around a lot, in which case you will have more pressing problems!


The 4k7 does indeed protect the arduino from many transients. Internal diodes in the AT chip clamp the voltage to within about 0.7 volt of the rails if the current isn't too high. The 4k7 limits this current. Anything up to about 10k would be OK - it isn't critical.

If worried it may be worth adding a 4.7v zener in parallel with R1. I wouldn't bother.

My circuit is correct.

Repeat it ( using seperate analog inputs of course ) for each sensor.

165 ohms is theoretically the correct value for R1, but in practise because of the variations in the arduino's
3.3v regulator ( a few percent) you will have to calibrate anyway, so it's not too important. 150 or 160 would do.

Allan

JoeJosefo

Ok! I will repeat it for each sensor like you kindly suggested, I thought of asking my last question just in case I was interpreting the schematic wrong, 

Thank you Allanhurst!

Josefo

:)

JoeJosefo

Hello all I'm adding a picture of my Permaproto board with the capacitors and resistances in parallel, just in case anybody in the future will want to see it for any reason. Hope I didn't make any big mistake

Resistances are 165 ohms .1% error and capacitors are .1uf 10% error

In the picture, at G9 I soldered underneath the wire that goes to analogpin1 and at G14 I soldered the wire that goes to the negative of the rca male adaptar where you plug in the the pressure transducer. Similarly for the other 3 transducers.

The results im getting are much more stable than before, i take 10 samples and the average remains almost constant, Ill try to calibrate it tomorrow. Thanks to allanhurst for the schematic corrections!

Josefo
:)

TomGeorge

Hi,
We need a schematic, picture do not show the whole picture.
Where do the wires go, where and what is your power supply, what pins of your controller are the components connected to.

I know that this can be tedious for some but its necessary, especially if sometime in the future you want to build another or need to trouble shoot a fault.

Thanks.. Tom.. :)
Everything runs on smoke, let the smoke out, it stops running....

JoeJosefo

Hi!

Well the schematic was is two posts above but I will update the previous post to include it as well. I understand what you say, however when I was about to build this I couldnt find an example, people would just say put a capacitor from the analog pin to ground, which was very cryptic to me because this is my first project.

:)

TomGeorge

Hi,
Sorry, but a schematic shows ALL your power supplies, labels pins and components.

It needs to explain to anyone looking through this thread, one day or 12 months from now, for some answers to their query using how your project was put together.

This is were you give back to the forum, by showing your working solution.

Tom... :)
Everything runs on smoke, let the smoke out, it stops running....

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