monitoring current for dc motor near stall

[tony_g]

so after finally venturing into mosfets and making my customized rc winch controller which came out much better than expected i have decide to make an alternative board as i can get the size even smaller and with fewer components and via's seeing as now i have a couple of friends who have developed a keen interest in wanting one of my boards.

initially whilst looking around for all in one h bridge solutions i came across several soic8 mosfet h bridge packages, one of which is now the drive of my current board, i also stumbled across another dc driver chip the vishay SI9986 buffered h bridge.

View attachment si9986.pdf

after testing my first design and seeing the current draw was alot lower than i expected under a good load i want to try these chips.

after reading around online i have seen in some chatter that people state that the pins Sa and Sb on the chip can also be utilized potentially by a microcontroller to monitor current.

as this chip would free up 2 pins on my 08m2 i would like some advice how i can do this so i can add a section in my program code to watch either of the pins from the si9986 to stop the winch if its current starts to get within a predetermined range of the chips peak or even max continuous current ratings so as to try and avoid killing it if its held near the motors stall point.

the main reason for this is that this chip does supposedly get warm when running so i dont want to risk exceeding its thermal limits or damaging the sillicon inside if the winch is pushed to hard.

thanks,
tony

 

[Bill.b]

Hi Tony.

I am not familiare with the h drive unit you are using,

Here is an portion of a bot I am bulding using a L298N module.

to trip on over current I am using a 0.2 ohm resistor in the ground supply to the
module and an opamp to amplify the voltage and to set the trip level. I required a digital input to the 40x2 as
I initiate and interrupt when a overload occures. the opamp circuit as a fixed delay of 2 seconds. this
can be changed by repalcing one resistor with a pot to change the delay.



Bill

 

[SAborn]

Sa and Sb are just supply inputs (source) for both low side mosfets.
You would couple both Sa and Sb together and use a shunt to connect them to GND, then read the shunt to monitor current draw.

 

[tony_g]

thanks for the repsonses guys, that should give me something to read up on to pass some time lol

 

[SAborn]

If it was me i would look to using something like this.

http://www.ebay.com.au/itm/1pcs-ACS712-5A-range-module-current-sensor-module-/251042028107?pt=LH_DefaultDomain_0&hash=item3a73455a4b

It would simply go in series between Sa + Sb and Gnd.
They give a reasonable linear voltage out (0-5v) and very easy to use Readadc10 to monitor current. (about a adc of 200 per amp) you might find lower rated modules.
Sometime its just easier to add a module to read current than all the work to save $1.50

Edit............ It would be only a ADC of 100 per amp, as the module reads both +/- 5 amps, so zero amps would be at 2.5v (half rail voltage) this might reduce the resolution for your low current range.

 

[tony_g]

ah, nice find, and smt so i could take the chip and caps off that board and place it on the revers of mine seeing as their is plenty of space left for a few extra bits.

thanks saborn

tony

 

[SAborn]

Search for the chip, as they are for a fraction of the price as the module.

Allegro products work very well for current monitoring with a picaxe from my view, the best is a unipolar sensor and not the bipolar sensor, as the unipolar sensors only read + amps and not +/- amps, which means zero amps are around zero volts output, giving 0-5v for the full amps range, this doubles the resolution of the adc reading.

I have worked with shunts, and used CT's for current, but now days use allegro current sensors as they are cheap for the simplicity of intergrating them direct into a circuit, from 1-200 amps its a easy product to use IMO.

 

[tony_g]

just had a look for them on digikey, $4.13 cad, not to bad, i will add it to my next order from them for some more fets and bulk resistors

 

[Dippy]

An option to CT/Hall, where appropriate, is shunt + diff amp.
These are usually cheaper and sometimes better - depends on app.
Example: TI/BB IN20x family.
There are many others from different manufacturers. Search Farnell inventory to see some options.

 

[g6ejd]

So all you need do is either:

a. Join Sa to Sb and then link to a series R (depending on current), say 0.1R and then use an ADC to measure voltage across that R = current x10 (for R=0.1R), so if you measure 1volt, your current flow = 10A (1/0.1)
or
b. Add two R, each in series with Sa and Sb, then measure the individual bridge currents with two ADC.