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Hall effect vs Shunt resistor for feedback

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davoud

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Hello guys,
I am trying to design an IGBT based welding system and i have some questions :

For current control, we need to a feedback and i know that usually Shunt resistor used for these systems.
1) would be better that we use hall effect instead of shunt resistor (at output) or not?
2) How about CT transformer?
any suggestion would be helpful.
Davoud
 
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I think CT is better in this case; resistor may involve problems with dissipation and risk to stop equipament because resistor failure.Hall effect is expensive and not justified for current limiting only (it's good in measuring). CT may be a toroidal with 10-20 wires passed by main cable. Risk of saturation may be avoid choosing a larger toroidal.
 

Thanks Guys ,
Dear Frank :
Thus what do you suggestion? Shunt or CT ?
Drear iop95:
Power consumption of shunt resistor is very low and i think there is not worry for restore failure but i think when the circuit is open (stand by ) , any spike and shock maybe affect circuit connected to the shunt. what is your opinion
 

My concern is about resistor heat dissipation and risk that heat to affect around cmponents or resistor itself. For exemple, at 100A current, with resistor of 0.01ohm will be about 100W power dissipation, sure in intermitent pulse but may be a problem. I recommand CT like best solution for this application.
 

iop95, .01 ohms and 100A gives a voltage drop of 1V, in the old days moving coil meters were calibrated using 100mV, i.e. .001 ohms at 100A giving dissipation as 10W. If you are indulging in electronics, then using a opamp, the shunt can be as low as .1 milliohm (10mV), gets to be a problem to find a resistor low enough in value. One advantage of the other systems is that the current is floating, i.e.not tied to an output line.
You don't say what the main welding supply voltage is, but what ever it is its going to drop by a significant amount at a current of 100A. So it could be an idea to monitor this and use this as the "current" sensing.
Frank
 

Yes, 0.01ohms will give 1V drop at 100A and 10W dissipation; problem is to find a so low value resistor and resistor value to be stable at temperature step-up; can be used as low value possible resistor + OP AMP to reduce voltage drop and power dissipation but this involve a good resistor (mean expensive) so I think cheapest solution remain CT for this app.
 

It's not hard to find current sense resistors rated for hundreds of amps at sub-milliohm resistance, with built in kelvin sense connections and non-inductive design. Such resistors are preferred for high bandwidth precision applications. They're not more expensive than equivalent hall effect sensors.

The choice between a sense resistor and a hall effect or CT sensor usually comes down to which option satisfies the measurement criteria and which is easier to implement in the circuit. A sense resistor can satisfy practically all measurement criteria, but may be difficult to implement if you need isolation or level shifting. Magnetic sensors offer the isolation, but may not have acceptable bandwidth or accuracy.
 

In this app bandwidth response it's not so important, accuracy also; It's about a current limiting circuit control not precision current measurement for wide range and fast transient answer. If resistor is cheaper than few wires on a toroidal then resistor is the best method.
 

Thanks all,
Other question :
Can we use resistor with opto isolator for rising safety ? in the other word which methods are suitable for connecting shunt to control circuit?
I should pointed , I use tl494 for control pwm and Fly-back technologies.
 

If you use an opto isolator, you need to overcome the threshold of the LED, before you get any change in output current. So either your shunt has to drop > 2V or you will need an auxilliary bias on the diode - another small PSU?
Frank
 

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