Current measurement with Hall effect sensor

Large AC/DC current measurement using Hall effect sensor

Some applications require us to measure and display large current like motors, amplifiers, transmission lines or current collectors without actually disturbing the actual setup. In this post I will describe how to make large current measurement using non-contact method.

We will use hall effect based current sensor for this purpose.

Hall Effect
When a current-carrying conductor is placed into a magnetic field, a voltage will be generated perpendicular to both the current and the field. This principle is known as the Hall effect.

The Hall voltage is proportional to the amount of current and magnetic field strength.

Vh = Kh* BI/t

Kh is Hall coefficient, B is magnetic field strength, I is the current through the conductor, t is the thickness across which voltage is generated.

The current carrying wire passes through the aperture of the sensor. A hall element is fitted inside the core  of these sensors. The magnetic flux around the wire interferes with the Hall element and generates a voltage signal. The scheme of measurement is as shown:

The Hall voltage is a low-level signal on the order of 30 microvolts per unit gauss magnetic field. This low-level output is prone to noise and requires an amplifier to amplify it to suitable level..

Signal details of CSLA2CD sensor

CSLA2CD Hall effect current sensor

The main advantage of this sensor is its form factor which makes it easier to put on a PCB. Other advantage being current measurement range- which is upto hundreds of ampere.


Thus by reading analog signal at AI pin, we can measure current through a target.

Optoisolator cum voltage level converter circuit using 4N35

All digital devices work at a voltage level of 5V/3.3V but many field devices work at a much higher voltage level (12V, +/-15V, 24V).

For example 8051 microcontroller input works in 0-5VDC signal and a sensor may work at 24VDC. Interfacing the sensor with the microcontroller will irreversibly damage the microcontroller. In such situation we are required to change the signal level of sensor from 24 VDC to 5 VDC to make the interfacing possible.
Also it is very important to isolate the signal from field devices as they may carry high voltage spike picked up from machines like motor.

An optoisolator can offer both solution at a time. It can provide isolation voltage of the order of few kV and convert the signal level. Here we are talking only about digital isolation.

I mostly use 4N35 IC for isolating digital IOs. It offers isolation of 1.5kV between input and output.
I will share the schematic and selection of suitable values of components.

The circuit in Fig-1 perform an invert operation. When sensor o/p is present, photodiode will be ON and emit light towards phototransistor. This will turn the phototransistor ON. Resultant output to load will be short to ground.

We have 24VDC output from sensor which has been converted to 5VDC by using 5  volt supply. It can be changed to whatever level just by replacing the output side supply.

Also note that the Ground label is different. We shall not short the ground of sensor supply and output side supply for better isolation.

Fig-1

In fig-2, the output follows input albeit at different signal level. When input signal from sensor is present, the circuit outputs 5V (actually 3.5V considering 1.5V drop across collector-emitter CE) and when input signal is absent, output is connected to ground via 10k resistor.

Fig-2

Input resistor R1 is selected after looking into the datasheet of 4N35 as well sensor.
The maximum forward current I(f) for 4N35 is 50mA. For 24VDC and 1k resistance at its input side, it will draw 24/1k- 24mA current which is less than maximum rating.

We must also check if sensor output is capable of providing this much current.

For very low value of I(f), voltage drop across CE will be large.

Nano second delay pulse train generation circuit using 74LS123 IC

For some applications we need to generate a trigger signal which is in nanoseconds width range. For example, you want to trigger a camera immediately after an event and keep the exposure time minimum to get the best possible image.

I needed this circuit for similar application. Requirement was to get pulse width of the order of tens of nanosecond. For this purpose I used 74ls123 which a dual retriggerable monoshot IC.

The schematic was prepared and simulation result gave pulse width down to 10 ns.

However when the circuit was fabricated, even with different combination of R and C, we could not get pulse width lower than 60 ns. This may be due the parasitic capacitance of the wires and solder.

Interfacing Arduino MEGA with 3.2 inch TFT display

In this post I am going to discuss about interfacing an Arduino MEGA with a 3.2 inch color TFT display. We will be discussing both hardware connections and software requirement to successfully complete the interface.

Pre-requisites

(1) Arduino MEGA with USB cable
(2) TFT display. I am using one from the link below.

https://www.amazon.in/Robocraze-320X480-Color-Display-Modul/dp/B077T9X2VQ?tag=googinhydr18418-21&tag=googinkenshoo-21&ascsubtag=_k_EAIaIQobChMI_9TDqJSb4wIVRAwrCh2TmAoLEAQYByABEgKf6vD_BwE_k_&gclid=EAIaIQobChMI_9TDqJSb4wIVRAwrCh2TmAoLEAQYByABEgKf6vD_BwE


(3) Power supply for Arduino

(4) Library file for TFT display. For IL9341 driver, the library from link below worked for me. I hope they work for most of the drivers.
https://github.com/Bodmer/TFT_HX8357

(5) Jumper wires (Male to Female termination). You can find them here.
https://www.amazon.in/Electrobot-Jumper-Wires-120-Pieces/dp/B071VQLQQQ/ref=pd_sim_328_5/262-0718414-5292569?_encoding=UTF8&pd_rd_i=B071VQLQQQ&pd_rd_r=ec3986de-9e3c-11e9-b4b8-2f2e1d937197&pd_rd_w=IOUIG&pd_rd_wg=qirB8&pf_rd_p=3ba80840-2950-4d64-ba61-c68a14bd0939&pf_rd_r=2YFMXD30JCRRMSRDTHHM&psc=1&refRID=2YFMXD30JCRRMSRDTHHM


Steps

(1) Make the hardware connection between Arduino and TFT display.

Use jumpers wire as shown below. This make the connection between TFT and arduino easier.

(3) Install the library file from the link below:

https://github.com/Bodmer/TFT_HX8357

(2) Insert the Arduino MEGA into the USB port of PC/laptop.

(3) Upload an example program from the library file into the arduino. If you see some output, you are good to go.


Happy Duinoing.

My NTPC interview experience

It was back in 2010 when I was called for interview at NTPC based on my performance in written test conducted by them. Yes. Back in 2010, GATE score was not considered for interview call in PSUs (IOCL was the only one who started it then).
Ok. here I am going to post my experience of facing NTPC panel. The interview was conducted at PMI, Noida.

Member 1: What is your college project? What is it about?
Me: answered it.

Member 2: Where have you done your internship?
Me: At IOCL, Haldia

Member 2: What is the role of an engineer there?
Me: He is responsible for plant operation. If something is not functioning, he is suppose to make it work.

Member 2: Are you sure this is what an engineer does there?
Me: (This time I was more blunt) No sir. He communicate with junior colleagues to get status update, attend meetings and work on files.

Member 2: Yes. Now you are correct.

Member 3: Okay. What have you seen at IOCL?
Me: Various types of sensors , PLC, DCS...

Member 3: Which DCS is used in IOCL?
Me: Honeywell, model ...

Member 3: Can you explain something about DCS?
I made a block diagram showing different blocks with their name and simultaneously explained their role.
Member 3: What Data Highway do?
Me: answered.

Member 4: Okay. Thats all from you now Arihant. You can go. Ask the next person to come in.

After about a month, I got a final selection confirmation mail.

BARC interview questions for instrumentation engineering

Hello budding instrumentation engineers.
I have been asked about types of questions asked in BARC interview by a lot of aspirants. In this page I will be making most accurate list of questions asked in BARC. This list has been prepared from my personal experience as well as stories shared by other applicants. Follow me.

(1) List down your favorite subjects.
(2) What is the working principle of Thermocouple?
(3) How do you measure the current in T/C?
or
If I give you a multimeter, where will you place the probe to measure developed voltage?
(4) Which flowmeter can work without power?
 or
If you are posted in a jungle where there is no power and given a task to measure flow through a pipe, which flowmeter would you use? How does it work?
(5) Explain Sample and hold circuit.
(6) Solve RC, RL, RLC circuit. Draw the voltage vs time, current vs time plot for these circuits.
(7) What is transfer function?
(8) Derive the transfer function for RC, RL, RLC circuit.
(9) Define pressure and derive the formula?
(10) Opamps as instrumentation amplifier.
(11) P, PI, PID controller
(12) Why there is permanent offset in P controller? How can it be eliminated?
(13) Which controller is best for use in noisy environment? Why?
(14) How rotameter works? Why is it tapered?
(15) Can we use rotameter for any liquid?
(16) U-tube manometer working.
(17) What are the various process parameters in industry?
(18) How do you measure temperature?
(19) What do you understand by NTC?
(20) If I need to measure temperature of a moving body, how will I do that?
(21) What will happen we use single material instead of two materials in thermocouple?
(22) What are the different types of flowmeters?
(23) Explain the working of orifice meter? Why is orifice plate bevelled? Which side will be placed upstream?
(24) What is term ρ*g*h in Bernoulli equation
(25) Why 4 to 20 mA is used in industry but not 0 to 20 mA?
(26) Draw an orifice installed in a pipe. Why there is pressure drop from upstream to downstream?
(27) Draw the pressure profile and velocity profile across orifice meter. What is vena contracta?
(28) What is Bernoulli's equation. Explain.
(29) What is discharge coefficient? Typical values of Cd for all flowmeters.
(30) Why actual flow is different from ideal calculated flow?
(31) Compare between venturi and orifice meter?
(32) Can venturi be used when installed in opposite direction?
(33) How a differential pressure transmitter (DPT) is installed? Draw.
(34) Why do we use a 3-way manifold valve?
(35) What is cold junction compensation.

(36) Ideal characteristics of op-amp.
(37) What is CMRR? What is slew rate? Their mathematical derivation/ equation.Their units.
(38) 2-wire, 3-wire and 4-wire configuration of RTD. Comparison between them.
(39) What is the effect of vapour pressure in orifice meter flow measurement?
(40) What are the different types of T/C? Specify their measurement ranges.
(41) You have to measure the temperature of a chemically reactive substance. If its maximum temperature is 800, which T/C will you use?
(42) Why Thermowell is used?
(43) Draw the circuit of V/I and I/V converter using Op-amp.
(44) What do you understand by Pt-100?
(45) What is self heating error? How to minimize it?
(46) How far we can carry 4-20 mA? What limits the distance?