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Choosing the right current sensing resistor.

Leggyownz

Feb 7, 2017
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I have a resistor that has gone bad and need to find a replacement. But it's so badly burnt up that it's no longer readable. I have an L620 that mentions the use of a current sensing resistor on its' sense line.

What do I need to do to figure up a proper replacement for this? The motor that this is running isn't pulling much for amperage and doesn't mention voltage or maximum amperage draw but I know the circuit is running on 12v. With the resistor down I can't see the maximum output.

Any help is appreciated!
 

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danadak

Feb 19, 2021
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Is that a L6201 ?

That datasheet does not show any real info about how to manage that Rsense value.....
Maybe post directly at ST or contact their FAE to get an answer ?

Regards, Dana.
 
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Delta Prime

Jul 29, 2020
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R (sense) it's for voltage feedback
for motor current control.
Source or sink current delay time versus input chopper. Typically 35 ohms resistors used. Providing a feedback voltage of -1 to 4 v.
The high side Power mosfet DMOS transistor maybe at fault, the intrinsic diode between the source and drain has probably failed and blew up your
R (sense)resistor. High current spikes occur on the current sense line (as you put it) during the transition between the transistor being on and off overtime the component stresses and eventually fails . I would replace that ( I C ).
 

Minder

Apr 24, 2015
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Generally the resistor is in the common return for the H bridge , in many application this is 0.5ohms or less.
The dual comparator in the L297 detects this voltage, and controls the output accordingly.
Typically there is a sense level pot on vref pin of L297 to set the OC point.
The L297 has the sense1 & sense2 (p13 &14} to detect any over current.
 

Leggyownz

Feb 7, 2017
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Sorry for the delay in getting back with everyone. Quoted everyone below for less posts at once. Posted this as I was leaving for work and then went home to work on other circuit boards I was sent to repair. As a preface, I'm no engineer, I have no formal training and everything I've learned is completely self taught. I'm trying to learn as much as I can and very rarely do I get a component that I don't have a datasheet to tell me the value or a spare board, a picture of a board or something to go off of. This is one of those cases. Most of my repair work is within centrifuges and I can acquire service manuals for those. This is an Eppendorf liquid handler and I'm not sure I've seen a service manual for any liquid handler I've worked on. Anyway...

Datasheet for anybody that needed it

Is that a L6201 ?

That datasheet does not show any real info about how to manage that Rsense value.....
Maybe post directly at ST or contact their FAE to get an answer ?

Regards, Dana.
It is a 201, saw your response last night while I was at home but without the board in front of me, I felt I couldn't give an actual response. The lettering so faded off at that spot.

Wasn't aware ST would even help out with that.

Makes one wonder about the working or rather, non-working remaining circuitry.
I had already replaced a mosfet chip that had a hole in it. Currently hoping that was my only problem. Checked what I could close and around to this circuit and everything seems to be alright. I got passed the error I was having and managed to get a step further. But then this motor still wouldn't spin so when I located this resistor, I pressed down on it while the machine was initializing and the motor spun. It's broken in the middle as it turns out and pulling up on it will cause the motor to stop, pushing in on it will get it to go again. The largest problem is that the machine is still apart so I can't finish initialization due to this being part of a shaker platform so after I get past this step, the shaker begins shaking on the table and jostling my live board around *eek*

R (sense) it's for voltage feedback
for motor current control.
Source or sink current delay time versus input chopper. Typically 35 ohms resistors used. Providing a feedback voltage of -1 to 4 v.
The high side Power mosfet DMOS transistor maybe at fault, the intrinsic diode between the source and drain has probably failed and blew up your
R (sense)resistor. High current spikes occur on the current sense line (as you put it) during the transition between the transistor being on and off overtime the component stresses and eventually fails . I would replace that ( I C ).
So you're recommending a replacement of the l620 while I'm at it, correct?


Generally the resistor is in the common return for the H bridge , in many application this is 0.5ohms or less.
The dual comparator in the L297 detects this voltage, and controls the output accordingly.
Typically there is a sense level pot on vref pin of L297 to set the OC point.
The L297 has the sense1 & sense2 (p13 &14} to detect any over current.
I see the mention of the L297 for a stepper motor but we have neither l297 nor a stepper motor in this system. In this case we just have a DC motor spinning a belt. We seem to have more of setup like page 14 of the datasheet

If I misunderstood anything here, my apologies. I don't get to talk to anyone about this kind of stuff and with no formal training, there's a lot that can easily go over my head!
 

Minder

Apr 24, 2015
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The L620 has a sense pin which is the bottom of the H-bridge, a low value resistor is normally connected to this and common and the volt drop monitored for any overcurrent. Typically use 0.5ohms.
Although I see the L620 has a sense pin for setting the current?
The likes of the L297 monitor this voltage and lower the current accordingly. Your circuit may have a similar setup.
If it is this sense resistor that has failed, I would replace the resistor with a 0.5, disconnect the motor (or load) , no signal applied, and see if the resistor fails, i.e. detect for shorted bridge.
 
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Delta Prime

Jul 29, 2020
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If I misunderstood anything here, my apologies. I don't get to talk to anyone about this kind of stuff and with no formal training, there's a lot that can easily go over my head!
Now you have a whole community to talk to about this kind of stuff. And I am not discounting other suggestions which are based not only in theory but experience. We're all just trying to help.
 

Leggyownz

Feb 7, 2017
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Now you have a whole community to talk to about this kind of stuff. And I am not discounting other suggestions which are based not only in theory but experience. We're all just trying to help.
In speaking of, on a totally different board I have an odd question and maybe I should be making a new thread on it, but you saying this got me thinking about this.

I have a board that "won't turn on" well it powers but the program won't initialize. I get good voltages across the board on the various power supplies. But in the schematic attached, if I take a single multimeter probe, with the other probe attached to nothing (or touching anything), and I poke V20 on pin 1, a few seconds later the program initializes and it starts up. This can also occur anywhere on that line on L5 or beyond or any of the through hole traces along the way. This can also occur whether I use the positive probe or the negative probe.

My suspect is C27 not charging up enough to give the circuit enough to draw from to start up. I've seen this on various other models of this equipment and it's generally a capacitor that's not swelled and looks just fine to the naked eye. I have yet to desolder it so I can get my capacitor tester on it to prove this theory.

Basically my question is, does this make sense with the multimeter? Is it possible the multimeter is acting as a capacitor? I've referred to some of my technical books but can't find anything overtly helpful in explaining a multimeter. Didn't even know how I would google this issue either.

As for the actual main issue of this thread, I popped in a .5 ohm current sense resistor and replaced the l620 driver and I do get the motor turning now but the software still gives me an error *Unable to reach operating current*. I didn't get that when I forced the original current resistor to work by pressing on it so I've purchased a range of resistors up to 20 ohms which is the same as the reading I get when I press it together and check resistance. I'll keep you guys updated on what ends up working for me.
 

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Delta Prime

Jul 29, 2020
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if I take a single multimeter probe, with the other probe attached to nothing (or touching anything), and I poke V20 on pin 1, a few seconds later the program initializes and it starts up. This can also occur anywhere on that line on L5 or beyond or any of the through hole traces along the way. This can also occur whether I use the positive probe or the negative probe
When the probe is connected to the circuit, the impedance matching of the circuit and probe determines the voltage at the input of the probe.
You are loading your circuit with your probes they present not only a resistance but an inductance and a capacitance.The symptoms are exactly what you have described. When you probe a circuit you change the electrical characteristics of that circuit in your case you're loading your circuit.
 
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