Current Transducer

[The Joker]

I have purchased a current transducer from Farnell. I am connecting the output to a 13bit A/D converter.
https://uk.farnell.com/lem/htfs-800-p/current-transducer/dp/9135758
https://uk.farnell.com/microchip/mcp3302-bi-p/adc-13bit-2-7v-2ch-spi-14-dip/dp/1852022
I don’t understand how the voltage output for the current transducer works. Please could someone help?

 

[dougy83]

Measure the voltage between Vout and Vref (which is Vcc/2). This gives you a voltage proportional to the current.

For the 800-P model, the measured difference voltage will be 1.25/800*I, i.e. 1.5625mV/amp

 

[KeepItSimpleStupid]

The current transducer can measure both positive and negative currents depending on the direction.

Some A/D converts are ratiometric and have a pin for the reference to go. The effective range is +- 2.5 volts from the reference pin.

The output from ground is 0-5 V, so you would need to know the center point. Ratiometric converters depend less on the absolute accuracy of the power supply. Noise on the power supply is effectively removed. If the measurement was done when the 5V supply was instantaneously 5.1 V and another time it was 5.25 V, and the reference to the A/D was set properly, then the zero would change appropriately. The gain of the sensor would always be fixed.

 

[The Joker]

Hi guys,

Sorry for the late reply and thanks for replying.
I've had to hold off this project due to other commitments.
Anyway I'm back to building this circuit. So I'm trying to connect the output of the Hall Effect to the A/D convertor and get a Digital output. This is what I've got so far:

For the Hall Effect sensor:
Pin 1 is going to 5volts,
Pin 2 is going to 0v
Pin 3 (output) is going to pin one on the A/D convertor.
Pin 4 (VRef) is going to 2.5v.

For the A/D convertor:
Pin 1 (channel 1) Is connected to the output of the Hall Effect sensor.
Pin 2,3,4,5 and 6 I'm guessing I don't need?
Pin 7 is going to ground
Pin 14 (vdd) is going to 5v
Pin 13 (VRef) is going to 5v?
Pin 10 is the digital output.
I am confused what pin 8 (chipselect/shutdown) is for?
Also what do i do with pin 12 (Analogue ground), i don't have an analogue ground plane?

Please could someone share they knowledge on what these pins are used for and if i am on the right track?
Many thanks in advance

 

[Norator]

If you want to check the part first, the locked rotor current for a 12v starter motor may be 400A, or you can use a foot or so of iron coat hanger wire immersed in oil or water as a load resistor. Iron has ~6x the resistance of copper.

 

[KeepItSimpleStupid]

I didn't look at the datasheet, but I want to comment on:

chip select/shutdown: I didn't look at the datasheet, so I don't know if it's active low/active high. The purpose of such a signal is so you can have multiple A/D converters. SPI typically uses chip select to select the chip your communicating with. My guess is it takes the digital bus and puts it a high Z.

Analog Ground: Grounding is a subject all to itself. One can usually assume that the analog grounds and digital grounds are connected independently and then together AT ONE POINT. Another way to look at analog ground in this case is the reference your measuring to, so connecting it close to the hall effect 0V point would be the right thing to do.

Every PC trace has resistance and you can think of analog ground not carrying any current and digital ground is the power supply return path. So, with all of the analog grouds connected together, they SHARE the same reference which will hopefully be the (-) of the power supply.

To the analog ground, it doesn't matter what the (-) supply is doing because all of the analog grounds are connected to the same point effectively.

The PERFECT situation is to have every IC ground/analog ground connection to be a single wire to the (-) of the power supply. This is impractical, so we break it up into current carrying ground (Digital Ground) and a reference ground (analog ground) and tie them together at one point.

 

[The Joker]

Thanks mate that was helpful. I think i know what to I'm doing with the AGND.
I'm still a little confused on the on the chip select/chip shutdown. I will be using multiple channels in the future but for now I only need the one channel. It initiates communication when the device is pulled low so how would I go about connecting this?

 

[KeepItSimpleStupid]

The ____ over CS means NOT, or can be abbreviated as ~CS as well.

Note that in Fig 6-4 it is being used, so therefore I think you have to use it so the device doesn't get confused.

~CS allows you to have multiple A/D chips. SPI uses ~CS to address IC's on it's bus rather than a complex form of hidden vendor ID's and fixed addresses.