Maker Pro
Maker Pro

Measuring Voltage Across Power Resistor

Hi, please i need help measuring the voltage across the power resistor(PR6) by hand without using a multimeter. i tried using ohms law to determine the current and used the current to determine the voltage afterwards but the results was 0.01028....... volt so i think i am doing it wrongly. Because that will mean that this 0.1028.... volt is moving into LED1 which is absolutely wong.
 

Attachments

  • vaccrossR.PNG
    vaccrossR.PNG
    30.5 KB · Views: 14

Harald Kapp

Moderator
Moderator
Where does pin 8 of CN7 go? Without knowing what's connected there (and how) one can say only one thing for sure: When transistor PR6 is "on", the voltage across R6 will be 5 V.
When transistor PR6 id "off", the voltage depends on the circuit connected to CN7, Pin 8.
 
I believe pin 8 is connected to a capacitor on the diagram. You can check pin 7 on this image
 

Attachments

  • vaccrossR.PNG
    vaccrossR.PNG
    32.1 KB · Views: 9

Harald Kapp

Moderator
Moderator
I have seen that. The point of interest is: what's behind connector CN7? This is not shown on this schematic, but relevant to your question.
 
i am finding it difficult to get you. But its a laptop DC jack with 2 LED of different colours. One for charging and the other for charging completed. i actually cant see anything behind because from my perspective its just a dc jack port that is removable. i dont have the board currently with me, just the schematics. sending you a sample of the jack port i mean. kindly explain what you mean by whats behind the connector
 

Attachments

  • dcjack.jpg
    dcjack.jpg
    92.3 KB · Views: 2

Harald Kapp

Moderator
Moderator
explain what you mean by whats behind the connector
The circuit that is within this box:
upload_2020-10-29_10-46-0.png
I assume, there is an LED connected to Vdd via a series resistor. That would turn on the LED when the signal AC_LED_ON is high and turn off the LED otherwise. If that is the case, the voltage across PR6 is either 0 V or Vdd.
But: If there is a circuit within the box that connects Pin 9 of CN7 to ground somehow, then there will be a current through PR6 even with transistor PQ3 being off. Bu t I don't know what's in there :(
 
ohkay i get your point now. It means that without dissasembling the DC jack to see its circuit i wont be able to know by ink and paper how much voltage is at the power resistor when the transistor is turned on
 

Harald Kapp

Moderator
Moderator
No, it does mean the exact opposite. As stated in post #2:
one can say only one thing for sure: When transistor PR6 is "on", the voltage across R6 will be 5 V.
That's because Vce of a saturated bipolar transistor (PQ3) is only a few hundred millivolt.
You can't know the voltage across R6 when the transistor is off.
 
ohkay pls can you clarify this for me -> +5vpcu is EC standby power supply according to a book i read. And i guess this 5v is connected to a resistor and the other end of the resistor is connected to a junction where current moves to the transistor and also the LED. So assuming the transistor was not present i guess i would possibly see 5v present across the resistor because it is connected also to the LED line. Please correct me if i'm wrong cus i am just a baby learning electronics.
 

Harald Kapp

Moderator
Moderator
As indicated by the red arrow: Vce is approx. 0 V (in the ballpark of 100 mV):
upload_2020-10-29_12-56-2.png
When the transistor is fully on, think of it as pins 3 (collector) and pin 1 (emitter) being short circuited.
Note that this is a very simplified representation of a bipolar transistor used as a switch and being fully saturated. This can not be generalized to all situations/circuits.
So assuming the transistor was not present i guess i would possibly see 5v present across the resistor because it is connected also to the LED line.
As said before: without knowing what's in the box hiding behind CN7 it is impossible to say what will happen.
 
Thank you very much i think i understand now. God bless you so much. I have other questions but i think i will create a different thread for that because it's not related to this post.
 
Top