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Overccurtent detection on circuits

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Sink0

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Hi, i got a designe with bunch of sensors, coils, leds and so on. I am using an STM32F102 as the controller. How can i designe some sort of auto-diagnstic tool that can see when i go some sort of overcurrent on some line? Current Sense Resistor with ampop on the ARM AD?

Thank you!
 

Sink0 said:
Hi, i got a designe with bunch of sensors, coils, leds and so on. I am using an STM32F102 as the controller. How can i designe some sort of auto-diagnstic tool that can see when i go some sort of overcurrent on some line? Current Sense Resistor with ampop on the ARM AD?

Thank you!
Click to expand...

use ct. Ct o!p rectify and use adc
 

Can you reapeat with no abreviations? haha could not understand a word.

Thank you!
 

use current transformer . current transformer output rectify and use adc as u want to u configure as Ur program .
 

I was looking for a more compact solution. If possible with just ICS and small passice elements... any idea?
 

Hi,

You can use a resistor to act as a "shunt". Measure the voltage (potential difference) across the resistance. This can be used to detect the amount of current flowing: V=IR, so, I = V/R

Measure the voltage drop using ADC and convert this to current measurement in your code. Compare the read value against a preset current value and turn off a switch (transistor maybe) when the current limit is exceeded.

For compactness, you could also use resettable fuses:
en.wikipedia.org/wiki/Resettable_fuse

Tyco, Littelfuse, Bourns and a few more manufacturers make such resettable fuses.

Hope this helps.
Tahmid.
 

i don't know the currents or the topology of your circuit but you can probably use a resistor in series with the load connected in the ground side and measure the voltage drop using the A/D,
depending on the current and the acceptable losses for your circuit if you use a very low resistor (like 0.02 ohm) you need additional amplification
to be measured by the A/D , for example a differential opamp with a gain of 50x to give 1v/amp

Building a differential amplifier : OPERATIONAL AMPLIFIERS

Alex
 

You didn't give any specification for the voltage level for that line,
if you can use an opamp that has an common-mode input voltage range that can work with that then yes.
Is there a specific reason that you want to implement a high side current sense?

Alex
 

Oh.. sorry..Ignore the H on my post haha. Its not a high side.. is just to sense the current on the line. But as you told i have to check the input range of my OPAMP.
 

When you use a low side resistor then things are much easier, you still need an opamp that can work with 0v in the input but you don't have to worry about the positive range.

Alex
 

What would be a high side resistor? My system have no high or low side...
 

I don't know your circuit, do you have floating outputs?
How many lines?
maybe you can measure the current in the input stage, it is all theoretical i don't know what circuit we are talking about.

Alex
 

Basically the Cortex-M3 will be drinving some Transitors to activate or not some coils. The current itself would be less than 50mA on the colector.
 

is this a NPN sinking current (provides gnd to the coil), is so you can use a resistor between the transistor and coil.

Alex
 
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    Sink0

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Ok.. thats probably what i will be doing. Any sugestion of rail-to-rail 3.3V veeery cheap opamp?

Thank you!
 

You don't need a rail to rail opamp, for example an LM358 has an input range of 0v up to Vcc-1.5v.
I'm also thinking now that a resistor connected between the collector and coil will have full power supply when the transistor is turned off because of the voltage connected to the upper side of the coil,
maybe it would be better to connect the resistor between the emitter and gnd, if it a small resistor it will not disturb the bias of the transistor.
or on the other hand you can use a rail to rail opamp that can get Vcc as input.

Alex
 

I was thinking. I am going to have a 5V on my board. I could power the opamp with 5V and clamp the output with a zener so i can use it on the VCC with no problem.
 

Your problem will be with the input if the opamp can't handle the 5v input with 5v supply ,clamping the opamp output will not help on that.
Using a clamp on the output could be useful to avoid voltages above 3.3v to go to the A/D input (which is 5v tolerant but will not give correct reading),
in that case it is better to use a diode with low forward voltage (BAT85) connected to the 3.3v of the arm, the zener doesn't have a sharp enough turn on slope.
what is the power supply of the coil?

Alex
 

Yea.. forget that.. that was a real dumb idea.. the power supply of the coil is 12V. I could use the ressitor on the emiter, but if for some reason i get some overcurrent, i am going to have a big voltage on the emiter, what would be a big problem.. But if i use a 1 ohm resistor .. with 50mA i migt get a big imprecision becouse of the OPAMP offset. Got stop and think on that.
 

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