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A transistor

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WG1337

New Member
Hi!
I have one transistor (NPN, I think, C945).
Also I have a battery that I yesterday filled with new electrolyte (the battery was dry). The battery is very similar to a car battery (12V), but I took it from an old UPS.
What I want to make is make a LED light on when 1V is added.
What I tried is add the +12V to Collector, -12V to Emitter and a lightbulb right after the transistors Emitter, but it didn't light on when I gave +1V or +12V to Base.
Maybe someone can explain bit how it is supposed to be done?
 

MikeMl

Well-Known Member
Most Helpful Member
Post a schematic.
 

WG1337

New Member
I found a nice app called Multisim, I used it to get the result, but not sure if this is also true in the real life...
8900-sch.jpg

What I don't have is a ground (a armature in the ground) and nice looking wires, also not the 5V bulb, it is only for testing if there are some volts in the Base.
 

smanches

New Member
Place the transistor after the bulb, so you're switching the ground side of the bulb.
 

WG1337

New Member
But I have a problem by the start.
If I put Collector to + and Emitter to - the I also get the 12V, even without Base.
Is this OK?
And how do you mean the ground side of bulb?
 

smanches

New Member
Place the +12 to the bulb, the bulb to the collector, then the emitter to the negative side of the battery.

This way you are making a "low-side" switch, which will do what you are looking for. The way you have it is called a "high-side" switch, which requires more voltage at the base before it turns on.
 
Last edited:

Bob Scott

New Member
You had better check that your C945 is still good. Max collector current is only 100mA; max base current is 20mA.

I'll bet its blown. It is not capable of driving those light bulbs. All you guys take note.
 
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sheldonstv

New Member
plus your circuit shows no resistor at the base of your transistor either-theres a very good possibility your transistor is now faulty
 

audioguru

Well-Known Member
Most Helpful Member
Of course the transistor is blown up since the OP knows nothing about electronics.
 

WG1337

New Member
Ahh, I didn't check the max current. It is now dead... I guess, but the multimeter still shows some value, so not dead yet?
I have a battery with 12V, like I said, and max current of the battery is... well I don't know exactly, but is is said 12V8.2AH/20HR so it can do 8.2A for 20 hours?
What would you recommend me to do? The power of the bulb is unknown, but I will get a new one anyway, so you can choose... 1W, 5W...
 

audioguru

Well-Known Member
Most Helpful Member
The 5V/1W light bulb in series with the base of the transistor will draw about 200mA in the base of the transistor that will blow up the transistor.
The 12V/25W light bulb will draw 2.1A which will also blow up the transistor.
Actually, the light bulbs draw about 10 times the current when they are cool.

Your battery is 8.2Ah at a 20 hour rate so it supplies 0.41A for 20 hours.
It might supply 8.2A for a minute or two.
 

WG1337

New Member
So the best would be to add the bulb before the transistor and place a 80 Ohm between bulb and transistor?
80 Ohms = 12V/0.15A

Just had an idea to use the Nokia battery charger as the power source, it has 5V, but haven't check max amps.
 

audioguru

Well-Known Member
Most Helpful Member
Don't use light bulbs as a resistior. Their value is 1/10th when cool.
 

WG1337

New Member
No, not the light bulb (sorry if I typed it wrong back there), but a another piece - resistor - just before the transistor.
 

audioguru

Well-Known Member
Most Helpful Member
"Just before the transistor" means nothing.
Where is the resistor and what is its value?
Is it in series with the base of the transistor?
Are you still trying to use a puny little transistor when you need a power transistor?
 

audioguru

Well-Known Member
Most Helpful Member
I would switch to 1.5 volt LEDs and lower both battery voltages. I think those transistors can only take about 8 or 9 volts.
Blue and white LEDs are about 3.5V.
Red LEDs are about 1.8V.
IR LEDs are about 1.3V to 1.5V but you cannot see them.

Most little transistors like the 2N2222 are rated at 30V to 40V.

You don't need two batteries. One battery can be used.
 
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