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Current monitor/sensor


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The opamp , BD139 or an output transistor is shorted.

I was also wondering this because before when I used PNP output transistor in common emmiter orientation if you remember circuit all works fine, the main probmel was as you said ringing because voltage gain.

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Although it is difficult to build the current sensor but still it has many ways to monitor the current. For small current applications the most preferable circuit is building using the OP-AMP.

OP-AMP has good applications int he current sensing and monitoring. Also you can build the circuit using the Microcontroller board using its ADC feature.

Although it is difficult to build the current sensor but still it has many ways to monitor the current. For small current applications the most preferable circuit is building using the OP-AMP.

OP-AMP has good applications int he current sensing and monitoring. Also you can build the circuit using the Microcontroller board using its ADC feature.

turnkey pcb assembly

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Today i tried to measure some values, at first i was measuring voltages, when i remove BD139 out of circuit voltage on output pin of OP is 22,8V although the input voltage is 0V.

This is not OK?

If i put BD139 back in circuit and try connect it to my power supply unit (0-30V power supply from this website) off course i have short circuit, so unlimite current wanted to flow through BD139, so what do you think now what should goes wrong there??

 

 

Edited by denci
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Like I said, maybe an output transistor is shorted (or wired wrong) making the output voltage of the power supply as high as it can go. Since you removed the BD139 transistor then the diode D10 on the original schematic connected to it and to the project's output conducts and forces the output of the opamp to a high voltage and maybe destroying the opamp. 

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Like I said, maybe an output transistor is shorted (or wired wrong) making the output voltage of the power supply as high as it can go. Since you removed the BD139 transistor then the diode D10 on the original schematic connected to it and to the project's output conducts and forces the output of the opamp to a high voltage and maybe destroying the opamp. 

I tried to replace OP and BD139 and there is the same story, when i connect base on BD139 then try to flow unlimitec current from power supply, i have not connected battery at all.

I also chech the output transistor and seems ok.

Its logical that current want to flow through output transistor because its forward orientated (base voltage higher than colector voltage) but i put 0V from negative feedback and there should no current flow at start???

 

 

Edited by denci
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I do not know what you are talking about. The 0V to 30V power supply in this website does not use a battery and unlimited current cannot flow if there is no load and the transistors are not shorted. It is impossible for the base voltage of the output transistors to be higher than their collector voltage.

Opamp U2, the BD139 emitter-follower and the output transistors emitter-followers make a DC amplifier with a voltage gain of about 3.07 times. If the input to U2 is 0V then the output of U2 is about +1V and the output of the project is 0V. If the input to U2 is +5V then its output is about +6.5V and the output of the project is(5V x 3.07=) +15.35V. if the input to U2 is 9.77V then the output is about +11.3V and the output of the project is +29.99V.

 

Oh, maybe you are talking about the battery discharger circuit we discussed a few years ago? I do not think its schematics are here any more.

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A few months ago you said, "I don't know why OP folower doesn't work properly any more, if i put on OP + input pin zero voltage from MCU, is should be the same value on - input pin of OP but it isn't, there is always some value, in last case was 0,6V so 1,2A current was flow through BD139 and power transistors."

But that is impossible. If the (+) input of the opamp is 0V and the (-) input is a positive voltage then the output of the opamp will be as low as it can go which is about 0.01V. Then the BD139 is completely turned off and the output transistors are also completely turned off.

I see why the BD139 draws a high current in your battery discharger circuit: There should be a current-limiting resistor between the emitter of the BD139 and the bases of the output transistors.

The new software for this forum keeps making all my repies a quotation from you!!!!

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schemeit-project (3).png

So, this is circuit i want to make controled battery discharge with and as i said don't work as i expected.

When i connect power supply unlimitet current want to flow through circuit bit i don't know why?

 Maybe you are confused with my comment about power supply, no, i just use power supply from this forum to power this discharged circuit!

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Transistor is burned out again so i just wondering that the main problem is that at power supply present and before output of MCU provide zero voltage to + input of OP very high current through BD139 and in bases of output transistor because battery not connected at start.

As is said i have no problem with circuit where i use PNP uput transistors because at start no current was flow through circuit because no base current flow when battery was not connected to PNP emmiters.

But i eliminated that version of circuit because problem of voltage gain and oscillations like you said but i must introduced some solution in this circuit to avoid high current flow when battery is not  connected onto collectors.

 

 

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When the battery voltage is low or when the battery is not connected to the collectors of the output transistors then when the BD139 turns on its load is the base-emitter diodes of the output transistor in series with the very low value resistors causing a very high current in the BD139. I said yesterday to add a resistor between the emitter of the BD139 and the bases of the output resistors to limit the current. 

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Thanks!

I tried this and works fine, i put the 680E resistor between emmiter and bases, but there is one think, it can't possible to make no current flow throught circuit if battery disconnected, some current always flow into transistors, around 1.5ma regarding my simulation.

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  • 2 weeks later...

If there is a positive input voltage to the opamp (+) input, the current flows from the emitter of the BD139 through the base-emitter diodes of the output transistors and through the low value resistors to ground. The 1k emitter to ground resistor of the BD139 also has current in it. When there is 0V on the discharge power resistor then there will be no current in it and no current in any other part.

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Yes, thats thrue, but when i put resistor between BD139 emmiter and two output transistor bases this mean that if no zero voltage on power resistor (beacause of some unfourable reasons) current flows through output transitor in any case also without battery because there is forward biased.

I hope that i am right.

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  • 1 month later...

If there is a positive input voltage to the opamp (+) input, the current flows from the emitter of the BD139 through the base-emitter diodes of the output transistors and through the low value resistors to ground. The 1k emitter to ground resistor of the BD139 also has current in it. When there is 0V on the discharge power resistor then there will be no current in it and no current in any other part.

When i have 0V on + input pin of opamp LM324 then i have 24V on ouput and bd139 is turn on end also when i put resistor 220W between BD139 emmiter and ouput transistor bases. This is all againts how i want to circuit works, maybe something wrong with opamp?

Is there any chance to no current flow at all throught circuit even battery is no conected and regardless what in input voltage on + input pin of OP, because anyway there is additional relays which turn on battery and it starts discharging?

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When i have 0V on + input pin of opamp LM324 then i have 24V on ouput and bd139 is turn on end also when i put resistor 220W between BD139 emmiter and ouput transistor bases. This is all againts how i want to circuit works, maybe something wrong with opamp?

Is there any chance to no current flow at all throught circuit even battery is no conected and regardless what in input voltage on + input pin of OP, because anyway there is additional relays which turn on battery and it starts discharging?

No, the opamp and transistors are a follower. 0V input gives 0V output at the 0.5 ohm resistor. +1V input gives +1V output at the 0.5 ohm resistor. +10V input gives +10V output at the 0.5 ohm resistor producing a current of 20A.

 

220 ohms between the emitter of the BD139 and the bases of the output transistors limits the current in the BD139 and reduces the base current in the output transistors.

If the battery is not connected then if the input of the opamp has a positive voltage then the BD139 will more current than when the battery is connected.

 

I cannot remember why you want to kill a battery by discharging it. 

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No, the opamp and transistors are a follower. 0V input gives 0V output at the 0.5 ohm resistor. +1V input gives +1V output at the 0.5 ohm resistor. +10V input gives +10V output at the 0.5 ohm resistor producing a current of 20A.

 

220 ohms between the emitter of the BD139 and the bases of the output transistors limits the current in the BD139 and reduces the base current in the output transistors.

If the battery is not connected then if the input of the opamp has a positive voltage then the BD139 will more current than when the battery is connected.

 

I cannot remember why you want to kill a battery by discharging it. 

This is exactly the main problem in circuit, if battery not connected and positive voltage apply in + iput of OP by chance, so this mean that high current try to flow through BD139 and 220E resistor and buri it.

Circuit is for battery test purpose, programmable cotrol discharger to test battery healthy and compare quality between each battery pack.

So i have a big problems because PCB is already made and no big chance are allowable there any more, so i try to find solution to current flow in ouput transistor base.

Maybe other solution is available there because as i said there is relays too and when relays is not activated i wan to no current flow except we select program to start discharge the battery.

 

TNX for your helps!

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  • 2 weeks later...

It's maybe batter to add 100E restor from 24V supply to BD139 collector and onother 100E resistor between emmiter and output transistor bases?

Or there should be additional relay to provide open circuit while battery is disconnected and no current flow when battery isnt present at all?

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Of course the transistors will not work when you need them to work if you use resistors to limit their current. Also the current will still be fairly high and the added resistors will get hot.

Why not use Mosfets instead? They use NO base current. When the battery is dead or is missing then the circuit will use only the 1mA to 2mA of supply current for the opamp.

battery discharger.png

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  • 1 month later...

Hello again, i have had problem with RC filter from PWM but i solve it with proper resistor and capacitor value and i get DC signal into positive pin of OP, see picture below.

20160224_171559.thumb.jpg.1fbf42015ba736

When i put 100 bit value i get around 0,5V which is OK.

There is one onother problem which i have still not solve till now, when i connect battery and start to discharge it the signal from RC is like below:

20160224_171643.thumb.jpg.8a1cfee8914ec0

Do you know why this happening??

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