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serial port control ir transmitter, low transistor Hfe

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3dOptics

Member
I am having problems with the following circuit I built. I am getting +5v from my computers molex 4 pin power supply connector red wire. I have Pin 3 on the circuit hooked up to pin 3 on my computers serial port. I have Pin 5 hooked up to both molex power supply ground black wire and to pin 5 on the serial port.

I am using the transistor here:
2N3904TAR

The Infrared led here:
AM4457F3C

I am using a program called WinLirc that sends a signal to pin 3 on the serial port.

I measured voltage from the power supply red wire to the black ground wire it is +5.18v. I removed the led from the circuit and measured the voltage from the collector pin on the transistor to the pin on the 220 ohm resistor and it measures 7.08v. Every time I click the button on the program the voltage on my meter goes up about .20v and then drops back to 7.00v. I put the led in the circuit and connected the meter to each of the led's pins. The voltage reading is around 0.68v. When I click the button the voltage between led pins sames about the same.

Before I put the 3904 transistor in the circuit I tested the Hfe with my multimeter and it measured 160. Later, I took it out of the circuit and tested it again and it measured 74. Why is the Hfe dropping? Should I use a different transistor? Is my circuit designed correctly?

Thanks.
 

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Nigel Goodwin

Super Moderator
Most Helpful Member
The circuit is completely wrong - you need a resistor feeding the base, and the emitter directly to chassis.

Check the 'hardware extras' section of my PIC tutorials, or the IR board.
 

Hero999

Banned
Replace the bulb with the LED and current limiting resistor.
 

3dOptics

Member
Replace the bulb with the LED and current limiting resistor.
I measured the voltage from the TX(Pin 3) and GND(Pin 5) and it measures -11.02v. I measured the transistor Hfe before putting it in the circuit and it measured 160 Hfe. After using the circuit for a while I took it out of the circuit and the Hfe measured 82. Is that normal? Otherwise the circuit works great. Thanks.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
I measured the voltage from the TX(Pin 3) and GND(Pin 5) and it measures -11.02v. I measured the transistor Hfe before putting it in the circuit and it measured 160 Hfe. After using the circuit for a while I took it out of the circuit and the Hfe measured 82. Is that normal? Otherwise the circuit works great. Thanks.
If you're connecting that directly to a serial port, then you need to add a reverse connected diode across base and emitter of the transistor, or the transistor will break down. I would also suggest changing the base resistor to 10K or so.
 

3dOptics

Member
If you're connecting that directly to a serial port, then you need to add a reverse connected diode across base and emitter of the transistor, or the transistor will break down. I would also suggest changing the base resistor to 10K or so.
Before reading your reply I was thinking the negative voltage from pin 3 was killing the transistor. So I thought if I put a diode between Pin 3 and Base pin of the transistor that should block the negative voltage. This has solved the problem. The transistor Hfe no longer drops. See first image.


You suggest "add a reverse connected diode across base and emitter of the transistor". Could you explain what that would accomplish and should I do this instead of having a diode between Pin 3 and the Base? Would it be connected like in the second image.
 

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mneary

New Member
In the second image the diode is in the correct place but it is reversed. Its function would be to divert the negative current from the base.

In the first image, the diode blocks the negative current.

In the end, they accomplish almost the same thing. I prefer the second circuit; Its load on the comm port is more symmetrical.
 
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