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Using Arduino => General Electronics => Topic started by: aidanb17 on May 13, 2020, 01:08 am

Title: Protecting from Reverse Polarity
Post by: aidanb17 on May 13, 2020, 01:08 am
Hey,

i am very new to Arduino and creating my first project.

I am creating a circuit that takes an small analog meters reading (HY gain rotator degrees position) and creates a digital reading and then displaying on an LCD.

im using an ADS1115 to convert the signal. i am measuring the differential between the two sides of the meter.

with a diode installed on the positive for VIN on the nano, i plugged the supply in backwards and blew the meter everything else was fine.

i am wondering how i can protect against this in the future, i tried a bridge rectifier but for some reason loads up the CAP and puts voltage onto the meter and maxes the meter out.

Thanks!
 
Title: Re: Protecting from Reverse Polarity
Post by: jremington on May 13, 2020, 01:22 am
Sorry, your description makes little sense.

What other circuitry is driving the meter, and what are the voltages on the meter terminals?

You MUST obey the restrictions on the ADS1115 inputs, and the ADS1115 itself may have been destroyed by whatever you did.
Title: Re: Protecting from Reverse Polarity
Post by: larryd on May 13, 2020, 01:24 am
Why not use a polarized plug ?


Title: Re: Protecting from Reverse Polarity
Post by: aidanb17 on May 13, 2020, 01:28 am
meter is rated for 55mV

i have attached schematic
Title: Re: Protecting from Reverse Polarity
Post by: jremington on May 13, 2020, 01:30 am
The schematic shows that the meter (assumed to be "I") isn't grounded, so you probably can't use the ADS1115 to measure the voltage across it.

Again, what are the voltages on EACH of the meter terminals, relative to ground?
Title: Re: Protecting from Reverse Polarity
Post by: aidanb17 on May 13, 2020, 01:40 am
it is grounded through the pot below it, which is used to calibrate the meter.

the terminals are the same voltage when i measure them relative to ground.
0V to ground at home position
3V to ground at high position

the circuit does function well i just need to protect the meter
Title: Re: Protecting from Reverse Polarity
Post by: Wawa on May 13, 2020, 02:06 am
What is commonly misunderstood is that the differential input voltages of the ADS1115 must both stay within supply/ground of the ADS.
If any of your 'meter' terminals carries a voltage outside those limits, then things could blow up.
Leo..
Title: Re: Protecting from Reverse Polarity
Post by: jremington on May 13, 2020, 02:25 am
It sounds like you may have already destroyed the ADS1115 and/or the Arduino when you blew the meter. 

If the ADS1115 is functioning properly AND you obey the input voltage restrictions, it cannot possibly supply current to the meter.

Check both the Arduino and the ADS1115 out very carefully, by measuring some known voltages, before proceeding.
Title: Re: Protecting from Reverse Polarity
Post by: SteveMann on May 14, 2020, 05:46 am
Are you looking to replace the meter movement with a digital readout?
If so, just remove the meter and use the Arduino to measure the voltage at pin 3 of the 8-wire cable.
Title: Re: Protecting from Reverse Polarity
Post by: MarkT on May 14, 2020, 11:19 am
You need protection circuitry on the inputs to the ADC.

Some series resistors, say 10k, then schottky protection diodes to the rails.  This is standard in
pretty much all circuitry that interfaces analog signals from the outside world.

(http://sphinx.mythic-beasts.com/~markt/protect_circuit.jpg)

The resistor limits currents to very low levels, the diodes prevent over- or under- voltage on the
delicate ADC inputs.
Title: Re: Protecting from Reverse Polarity
Post by: MarkT on May 14, 2020, 11:23 am
And my other little tip is always colour code your wires and connectors, red for +ve, black for -ve.  And always
measure a voltage if you are not sure, ie always check whenever doing a new circuit.
Title: Re: Protecting from Reverse Polarity
Post by: aidanb17 on Jun 29, 2020, 01:11 am
You need protection circuitry on the inputs to the ADC.

Some series resistors, say 10k, then schottky protection diodes to the rails.  This is standard in
pretty much all circuitry that interfaces analog signals from the outside world.

(http://sphinx.mythic-beasts.com/~markt/protect_circuit.jpg)

The resistor limits currents to very low levels, the diodes prevent over- or under- voltage on the
delicate ADC inputs.
Having these diodes in this configuration, would it affect the 5V+ power? the voltage coming from the meter isnt the cleanest.
Title: Re: Protecting from Reverse Polarity
Post by: JohnLincoln on Jun 30, 2020, 07:22 am


(http://forum.arduino.cc/index.php?action=dlattach;topic=683952.0;attach=363189)


The meter by it's self is not a voltmeter.

It measures a current (of up to 1mA). That is why the schematic has a symbol with the letter 'I' for current on it.

It is converted into a voltmeter by adding resistor R2 and the variable resistor in series with it.

You should be measuring the voltage across the combination of the three of these components in series.
This is going to be a voltage between 0 and 13V, so you will need an appropriate potential divider to reduce this to less than 5V.

You can make this measurement between pin 1 (GND) and pin 3 of the  connector. There is no need to even open the case.
Title: Re: Protecting from Reverse Polarity
Post by: aidanb17 on Jul 04, 2020, 06:23 pm
I actually originally tried to measure voltage here using a voltage divider, however the controller doesnt do a good job filtering the voltage and it got pretty complex filtering the voltage enough to get an accurate reading without it jumping around, i have a working version using the ADS1115 to measure the differential between the two pins on the meter. i am now trying to put this on a single PCB and eliminate the Arduino (see attached). Maybe you can help verify this schematic for me as well i am trying to add the over voltage and over current protection to the ADS1115 inputs.

This circuit takes a differential reading from the ADS and displays it on an OLED.

I also added pins to upload to the 328 using TTL.