# Drop voltage on transistor base

Discussion in 'General Electronics Discussion' started by Bigz1, Aug 12, 2017.

1. ### Bigz1

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0
Aug 12, 2017
Hi

I am using a NPN transistor as a switch. I am taking the base from a LED. My problem is their is a constant voltage of 1.87v when the LED is off(1.97v when it is on). I was thinking of a zener diode of 1.9v to drop the voltage but am unable to source one. Is their an alternative way? My knowledge of electronics is very limited and appreciate any help.

2. ### Bluejets

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Oct 5, 2014
Show the circuit.

3. ### Bigz1

3
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Aug 12, 2017
My problem is the base always has voltage.

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4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,482
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Jan 21, 2010
Show us the circuit (the whole circuit)

5. ### Bluejets

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Oct 5, 2014
Bit shown is all backwards...

6. ### dorke

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Jun 20, 2015
The obvious problem is the 12v supply is connected in the opposite direction.
Change it and recheck ,like so:

7. ### Bigz1

3
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Aug 12, 2017
Sorry my mistake drawing (12 v supply is right way in circuit). 12v LED is constantly on. I can't draw all the circuit. Its a UPS that's not accessible. Its remotely turned on but I want an indicator to show its status. The D1 Led is on the UPS(shows when battery is on).

8. ### dorke

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Jun 20, 2015
Try measuring the on/off voltages on the junction of R1 and D1.
What led is 12V?
Another problem is ground reference between the 12V battery and the UPS.

9. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
If you completely remove the connection to the base of the transistor, does the LED stay illuminated?

If so, your first problem is one of:
1. The transistor is connected incorrectly
2. The transistor is PNP
3. The transistor is damaged
4. It is not a transistor
Do you have a part number for Q1? Values for your resistors might be nice to see also.

With the circuit you've shown the base of the transistor is effectively not connected because the rest of the circuit does nothing. However I'm assuming there is more to the circuit that you've shown. However we have no idea what it is, or even what reference you used in reading your voltages. As @dorke has said, there needs to be a ground reference, and we also need to know what the floating connections are connected to. If drawn conventionally, that diode on the far left is reverse biased, but I can't be sure that you're following any conventions in drawing your diagram.

10. ### BobK

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Jan 5, 2010
Are you saying when D1 is on the voltage is 1.97 and when it is off it is 1.87? I find that hard to believe.

Bob

11. ### CDRIVEHauling 10' pipe on a Trek Shift3

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May 8, 2012
If your merely desirous of switching Q1on and off why are there two paths biasing the b,e junction?

Are these components that you're adding or are they OEM?

Chris