# Measuring Voltage Across Power Resistor

Discussion in 'General Electronics Discussion' started by philipshadow, Oct 29, 2020.

20
0
Oct 29, 2020
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.

File size:
30.5 KB
Views:
14
2. ### Harald KappModeratorModerator

10,806
2,437
Nov 17, 2011
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.

20
0
Oct 29, 2020
I believe pin 8 is connected to a capacitor on the diagram. You can check pin 7 on this image

File size:
32.1 KB
Views:
9
4. ### Harald KappModeratorModerator

10,806
2,437
Nov 17, 2011
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.

20
0
Oct 29, 2020
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

File size:
92.3 KB
Views:
2
6. ### Harald KappModeratorModerator

10,806
2,437
Nov 17, 2011
The circuit that is within this box:

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

20
0
Oct 29, 2020
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

8. ### Harald KappModeratorModerator

10,806
2,437
Nov 17, 2011
No, it does mean the exact opposite. As stated in post #2:
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.

20
0
Oct 29, 2020
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.

10. ### Harald KappModeratorModerator

10,806
2,437
Nov 17, 2011
As indicated by the red arrow: Vce is approx. 0 V (in the ballpark of 100 mV):

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.
As said before: without knowing what's in the box hiding behind CN7 it is impossible to say what will happen.