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Help with transistors

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yxqjzz

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I was trying to drive an LED with a TTL and a transistor circuit. The circuit is set up as shown in the picture. As I turn on the dc voltage, the LED is constantly on instead of pulsing like the TTL. Then I ground the TTL input and weirdly the LED is still on. This is the first time I've learned to use a transistor so I don't know what I did wrong.

p.s. the transistor is a 2N2369A and the TTL is 3V 1kHz and 100ns wide
and the LED is 20mA @ 3.8V forward voltage

Thanks
 

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Sceadwian

Banned
What do you mean 100ns wide? That's a 10mhz pulse.
 

yxqjzz

New Member
sorry for confusion, 100ns is the pulse width and the pulses are 1ms apart.

By the way, if I try to amplify the TTL by using a non-inverting op amp, what characteristics should I look for when choosing an op amp? I am wondering if it would be easier to build a circuit with op amp or transistor
 

Sceadwian

Banned
That still makes no sense yxqjzz. 100ns is still 10mhz that's a duty cycle of .0001 % the LED shouldn't show any light at all, are you sure the transistor is good, and where at you gettting 100ns wide pulses from?
 

yxqjzz

New Member
yes thats what I thought, and the led shouldn't be on when the base is grounded. I tried a couple other transistors but it's still the same, so I'm pretty sure its not the problem of the transistor unless I'm using the wrong type. the ttl is from a nim gate generator, this is for a project at school.
 

crutschow

Well-Known Member
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yes thats what I thought, and the led shouldn't be on when the base is grounded. I tried a couple other transistors but it's still the same, so I'm pretty sure its not the problem of the transistor unless I'm using the wrong type. the ttl is from a nim gate generator, this is for a project at school.
If it's not the transistor, then you must have it connected wrong.
 

unclejed613

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audioguru

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your 20mA LED fried the 10mA transistor.
No.
The 500 ohm current-limiting resistor limits the current to (8V - 3.8V)/500 ohms= 8.4mA or a little less due to the saturation voltage loss of the transistor.

I think something else fried the antique transistor, or it is connected backwards.
 

Sceadwian

Banned
The basic circuit he posted is sound. As AG and Crutshow said though something is either wrong with the transistor, it's hooked up wrong, or there's something that's being missed about the driving signal.
 

MrDEB

Well-Known Member
not connected right

he said he can ground the base and the LED still lights??
he has the connections backwards.
recheck the data sheet and look at the transistor view top or bottom??
more than likely he has the emitter and base crossed.
just my opnion
 

Roff

Well-Known Member
as i scan my "silicon crystal ball" ( Datasheet Archive - Free Datasheet Search Engine - PDF Datasheets - Data Sheet - Datasheet - Application Note - Free Texas Instruments Samples ) i see that the 2N2369 is an NPN device, max Vceo 15V, max Ic 10mA. your 20ma LED fried the 10mA transistor. try a 2N3904 or a 2N2222 instead.

after that, fix your output pulse width. with the duty cycle you have, the LED will not be visible.
2N2369 collector current is 200mA max, continuous. Not 10mA. Where did you get that?

Way back when I was designing circuits for a living, 2N2369 was my tranny of choice for high-speed saturated switching. It's overkill for an LED that is supposed to flash at a visible rate, but it's pretty inexpensive, so I don't see any reason not to use it here if it's at hand.
 
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yxqjzz

New Member
Thanks for all the help. I double checked the transistor pins by comparing to this datasheet, but it seems the connections are correct, assuming the picture is a view from bottom. Maybe I will try mounting everything on a circuit board instead of having all the wires hanging around and see if that helps
 

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unclejed613

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the 10mA came from one of the data sheets (actually a selection guide) that came up in the search. i usually open specific data sheets rather than use selection guide charts, but this time i opened the selection guide.......
 

audioguru

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You need to put a 1k resistor between base and 0v rail to turn the transistor off.
Adding another resistor will not make any difference.
The transistor did not turn off with a TTL low input and when its input was shorted to ground.
 

Roff

Well-Known Member
The circuit is so simple, it doesn't need to be simulated. It just needs to be repaired, and then to have the input pulse rate reduced below about 30Hz, with a substantial duty cycle, so it can be seen.
 

yxqjzz

New Member
Thanks again for all the inputs. I finally found out the problem! The LED which is in a metal box is connected to my circuit through a BNC cable, and the negative of the cable is grounded by the box, which makes the 8V dc to be applied across just the 500 ohm resistor and the led.

Now that the reason is figured out, I am trying to design a different circuit that does the same job but allows the led to be grounded to the negative of the dc power supply. Any advice will be appreciated.
 

unclejed613

Well-Known Member
Most Helpful Member
use the transistor as an emitter follower. there's no gain that way, but it does buffer the TTL output going into the LED.
 

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