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new to electronics.. a little help?

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Arynock

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Okay so I am rather new to electronics and i'm trying to get the whole grasp on transistors.. I have a couple projects I have been wanting to that would require some switches and I figured transistors would be my easier best, such as using a small source to switch on a larger source..

The only problem I have is that I am using a NPN transistor.. I get the logic behind a transistor... a small current gain at the base will make a larger current gain through the E-C... The only problem I am having is that i will apply a load through the E-C.. And my voltmeter is showing current flowing through the E-C without any connection or current at the Base... Is this right?!?
 
I attached a picture of an example i found online and tried to create. The only problem is that I will apply power and push the switch, but neither of the lights will light. I have checked my circuit 10x and have found nother wrong or anything backwards... Its hard to learn about these when half the example schematics i try dont work :(
 

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Arynock said:
I attached a picture of an example i found online and tried to create. The only problem is that I will apply power and push the switch, but neither of the lights will light. I have checked my circuit 10x and have found nother wrong or anything backwards... Its hard to learn about these when half the example schematics i try dont work :(

Well the circuit looks correct to me. If nothing is connected backwards (LED or Transistor) then your down to a defective component or a wiring error that you keep missing. Maybe if you could take a picture of your construction we might find something you missed. The LEDs could be tested using just the power source and the 470ohm resistor in series.

Lefty
 
If you are using high brightness leds they drop more voltage than standard ones which can cause your problem.

Also, are you sure your transistor is working? It is possible that you may have fried it by accident in the process of building the circuit. You can test this out if you meter has a transistor tester, or check out the LED's by powering them directly from the battery with a resistor in series.
 
How did you measure current flowing through the E-C with a voltmeter?
Usually a current meter is used in series to measure current.
A voltmeter measures voltage, not current.

The circuit is correct. The LED with the 10k resistor will be dim and the LED with the 470 ohm resistor will be bright if the transistor is connected properly and the LEDs and battery are not backwards.

Check the datasheet for the transistor to see the part number, the picture of its pins labelled and if it is an NPN transistor.
 
Here is a way i use PNP AND NPN Transistors,im not entirly sure if its the way it is ment to work, but it works perfectly everytime.

the PNP Output(Emitter) is connected to the anode(long pin) of the led and the cathode goes to ground(with a resistor of course)

and the NPN Output(Emitter) is connect to the cathode(short pin) of the led and the anode goes to VDD(+V) (with a resistor of course)

I tried to add a schematic but it said i have reached my max in uploads? so what do i do there?
 
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It sounds like your NPN and PNP transistors are wired upside down. Then they act like a 7V zener diode.

A few years ago I also reached my upload limit. I deleted all my attachments then the site's owner made the the limit unlimited. But only for me?
 
audioguru said:
It sounds like your NPN and PNP transistors are wired upside down. Then they act like a 7V zener diode.


its still working how a transistor is supposed to though, right???
i hook the collector up to +v, and the small current at base,for testing i used a 10k resistor, and connecting base gives me an high output at the emitter of whatever the +v is, it works.
 
You said you connected the emitter of a PNP transistor to the anode of an LED. That is backwards.

The emitter of an NPN transistor connects to the anode an an LED. Then its collector connects to +v. The cathode of the LED connects to ground through a resistor of about 1k for the LED to be not bright and not too dim. 10k is much too dim.
The LED turns on when the base is connected to +v.

Your transistor is an emitter-follower without voltage gain. Its output voltage is less than the supply voltage.

if the LED is at the collector of the transistor then the transistor can have voltage gain and the output is almost the same as the supply voltage.
 
audioguru said:
You said you connected the emitter of a PNP transistor to the anode of an LED. That is backwards.


Yes thats right the anode of LED to emitter, and cathode to 0V, and the base goes to 0V with a 10K resistor and the collector goes straight to +V

the emitters output once the base is connected is around the same as +V very high,inuf to kill a LED Quick.
 
Your schematic is correct and you could destroy the LED without a series resistor. If the transistor is inserted correctly it should work, what PNP transistor are you using ?
 
I just forgot to add the resistor in the schematic(just added it then), the transistor is a multi-replacment Common transistor pn200 Case(TO-92)
 
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Then according to the schematic you posted, the transistor as shown should have the 'flat' side of the case facing away. Looking at the transistor with legs down and flat with part number facing forwards the pin out is e,b,c.
 
in the schematic you looked at, that is the flat side with part numbers on it, and its working???????? so is it set up right and i have called it wrong(C,B,E) instead of (E,B,C) which is right
 
Gordz said:
Your schematic is correct
No it is not! Look again. The PNP transistor is backwards.
If its supply voltage is about 9V or higher then the transistor would conduct all the time like a 7V zener diode.

A backwards transistor has a current gain of about 5 instead of a current gain of 200. So it might light the LED dimly.
 

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Instead of guessing about the pins, why don't you look at the datasheet?????
 

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Okay, so i managed to figure out that i did have a bad npn transistor.. So i started up the same project as was actually able to make it work... Now my only question is.. Does anyone know a good way on how to use a transistor to switch on 12V source using only a 5V source? What all would i need to take into account?
 
Take a look at Nigels tutorials. They are for PICs but you can use the ideas anyway:
 
Arynock said:
Does anyone know a good way on how to use a transistor to switch on 12V source using only a 5V source? What all would i need to take into account?
You need to know how much current the 12V load uses.
You need to know how much current the 5V signal source can supply.
Then the load current divided by the signal current and multiplied by a fudge number gives the gain and specs for the transistor.

The base voltage of the transistor must go near 0V to turn it off and near 0.7V to 1V to turn it on.
A current-limiting resistor must be in series with the transistor's base.
 
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