Current monitor/sensor

[denci]

The IC is called and is shown as a high side current sensor but you connected it as a low side current sensor so of course it WILL NOT WORK!

Hi...a long time ago i asked a question about ZXCT placement in circuit and i started a project again and i will continue where i stayed, so i have one question.
In circuit below, which represent microcontroler (PWM) regulated discharge current/power with OP folower as we can see, where where shpuld i place the ZXCT1107 and shunt for measuring in circuit to ensure me relevant information about current??
View attachment 42491

 

[audioguru2]

Your camera is horrible. Why don't you sketch the schematic using Microsoft Paint program or another program?

Your opamp has no power supply so it will not work. If it has only a single positive supply and ground then it must have inputs that have a common-mode voltage range that includes ground like an opamp in an LM358 dual opamp.

It looks like you are heating the load resistor by discharging the battery into it. The battery current also includes the base current of the PNP transistor.
Connect the sensing resistor for the ZXCT1107 between the positive terminal of the battery and the emitter of the PBZXCT110NP transistor.

 

[denci]

Sorry I took a picture quickly with phone and bad light i just want to show example where to put ZXCT because i did not get a reliable resault before at output pin of ZXCT.
I didn't draw a power supply but i have it off course, it is 24V and i took LM324 which have common-mode voltage range.
I took into account of base current of battery discharging also, this is the reason why i want to measure current via current monitor.
so i should put the ZXCT as mentioned, this is what made me confused.

 

[denci]

Your camera is horrible. Why don't you sketch the schematic using Microsoft Paint program or another program?

Your opamp has no power supply so it will not work. If it has only a single positive supply and ground then it must have inputs that have a common-mode voltage range that includes ground like an opamp in an LM358 dual opamp.

It looks like you are heating the load resistor by discharging the battery into it. The battery current also includes the base current of the PNP transistor.
Connect the sensing resistor for the ZXCT1107 between the positive terminal of the battery and the emitter of the PBZXCT110NP transistor.

Should i place shottky diode between transistor collector and load resistor for protection?
Voltage from OP folower feedback can occur also on transistor collector or no?

 

[audioguru2]

Your circuit is missing a resistor from the collector of T2 to the base of the PNP output transistor to limit the base current. T2 will probably overheat without it.
The PNP output transistor is missing a resistor from its base to its emitter to help turn it off.
Both transistors add delay and additional voltage gain so the entire circuit will probably oscillate.

I think you should use a fast driver emitter-follower driving a fast power transistor emitter-follower since they have no voltage gain and their very small delay will not cause oscillation:

View attachment 42526

 

[denci]

thanks for your help!
i will thinking about my circuit and which material i have available for this project and maybe i should corrent my current circuit

i have onother PNP transistor and how should i connect each other in parallel to decrease dissipation of single transistor and therefore obtain less temperature on my heatsink, i think that it's quite same than with npn which must have emmiter resistor to equalize current through both transistor, it isn't?

 

[audioguru2]

If you connect two transistors in parallel then each needs an emitter resistor so that they share the current fairly equally.
The opamp will probably not have enough output current to drive output transistors without adding a driver transistor.

I sketched a circuit with a PNP driver transistor and a PNP output transistor. They have enough current gain for the opamp to be able to drive.
BUT since they are both emitter-followers with a Vbe of about 0.7V and 1V then the output voltage will not go any lower than about +1.5V to 2V.

View attachment 42537

 

[denci]

Is this OK if we look back to my first version of circuit?
This is what I was thinking about and I think this is the most easy for correction my current version of circuit, I just want to add another transistor on my heat sink to decrease temperature on it.

 

[audioguru2]

Your amplifier will oscillate because its transistors provide voltage gain. Then at a frequency where the opamp's phase shift would cause oscillation, its gain is less than 1 so it does not oscillate, but the total gain with the transistors is higher than 1 so the entire amplifier will oscillate when negative feedback is applied (the phase shift causes the negative feedback to become positive feedback at high frequencies).

Also, common emitter transistors have more phase shift than emitter-followers due to the Miller effect (the capacitance from collector to base is amplified).

Also, your PNP transistors are missing resistors that will turn them off quickly. So yours turn off slowly which adds additional phase shift.

 

[denci]

So, which circuit you finnaly suggest for me with two power transistor if it's need?

 

[audioguru2]

So, which circuit you finnaly suggest for me with two power transistor if it's need?

Your circuits with an opamp plus common-emitter transistors have too much voltage gain so they will probably oscillate.
I showed two circuits with emitter-follower driver and output transistors that will not oscillate.
But if you use PNP transistors the output voltage will not be able to go near 0V unless the circuit uses an additional negative supply.

I showed a circuit with NPN emitter-followers that can go down to 0V if the inputs and output of the opamp can, without an additional negative supply. Add a second output transistor and emitter resistors for both output transistors and it will work perfectly.

 

[denci]

Is this ok?

 

[audioguru2]

Is this ok?

It will work well if the driver transistor has a fairly wide bandwidth like a BD139.
In this forum there is a 0V to 30V at a few mA to 3A power supply. The improved version has two 2N3055 power transistors in parallel with emitter resistors and a driver transistor. When a slow TIP31 driver transistor was used the transient response showed bad overshoots and ringing. The BD139 fixed it. The original version used a very old maybe obsolete driver transistor that was fast.

I corrected and made notes on your circuit:View attachment 42539

 

[denci]

TNX for suggestions, i use 24V Vcc supply voltage in my current version so i must calculate R2 for proper voltage drop on it, i think so.
Should i use MJ15003 NPN output power transistors in combination with some kind of BD139 transistor?
MJ15003 transistor has a small thermal resistance factor so it can decrease heat dissipation to heatsink.

 

[denci]

OP is LM324, i think that it should be ok

 

[audioguru2]

i must calculate R2 for proper voltage drop on it

You cannot change the voltage drop of R2 by changing its value. R2 quickly discharges the base-emitter capacitance of the output transistor and discharges its leakage current. 1k ohms is fine.

Should i use MJ15003 NPN output power transistors in combination with some kind of BD139 transistor?
MJ15003 transistor has a small thermal resistance factor so it can decrease heat dissipation to heatsink.

A small thermal resistance allows a power transistor to increase its transfer of the heat to the heatsink. Then the transistor will be cooler and the heatsink will be warmer.

 

[denci]

If i use just one power transistor for max. 10A discharched current i should use emmiter resistor also for voltage drop on it, i think that 0.22 or 0.33E is ok.
What about noise from negative feedback of OP, i think that its better to use capacitor between negative pole of OP and ground?

 

[audioguru2]

If i use just one power transistor for max. 10A discharge current i should use emitter resistor also for voltage drop on it, i think that 0.22 or 0.33E is ok.

No. An emitter resistor is not needed for only one output transistor.
Are you discharging a 12V battery? My circuit with an NPN driver transistor and NPN output transistor is a battery charger, not a discharger.

 

[denci]

yes, I want use in both direction, one circuit is provide battery discharge for analyze it, other is for charge bit only optionally, the main purpose is only discharger, since is not big difference between circuit.

 

[denci]

Instead of power supply we use battery for supply circuit and it stars discharged over load resistor and npn transistor.
Because of adjustable pwm voltage we regulate fixed discharged current, I think that this circuit is ok with npn follower circuit and transistors mentioned above.