transistor question

Other Hardware General Electronics
1

hello guys Im trying to do this project, so I did this setup as in the diagram I uploaded on my breadboard. It is basically sending very very fast ir pulses at same frequency as tv remote controls, so it blocks the other person who has the remote control from changing channels etc.

So I did this test and you can see all the results in the picture, those numbers are like the actual measurements I got with multimeter. What my question is

  1. Why is my Vce so high at 0.98v, I read everywhere its 0.2v in saturation, so isn't my transistor in sat if Im giving it 10mA base current?? Even without base resistor and increasing/decreasing ir leds didn't affect it too much.... I used 2n4401 for this but also tried other transistors, best I got was 0.6v.. I want to know so I can increase my current little bit more because according to ohms law V=IR if assuming Vce is 0.2, then my current on first two leds alone should've been 4.65v - 1.21v-1.14v-0.2= 2.1v ÷ 10ohm = 0.21A or 210mA, but instead I am getting only 82mA because 0.98v is being wasted at Vce and 0.46v between E to ground, shouldn't this be like 0v??If I removed one string(only had two leds) then this emitter to ground voltage reduced to 0.23v and my current increased to 100mA, so basically my question is why am I not getting the Vce=0.2v, and why I have this emitter to ground voltage of 0.46v.

  2. Also is it better if I use a 3.3v mini pro or a 5v for this project? I want to use a single 3.7v 600mah lipo battery for this, so assuming that same setup as in the diagram with 80mA running through each side, in this case is it better to use 3.3v mini pro or use a boost converter and run it on 5v mini pro ??

  3. Also why Im I only getting 3v on pin 3, ? does it have to do with the code?? but I guess it doesn't really matter too much



20181121_050106.png

2

The 2N4401 is claimed to have a saturation voltage of 0.4V with 150mA load and 15mA base current.
That's not really a switching transistor spec, its just a regular general purpose device, switching devices
have optimized saturation voltages. If you want the ultimate a ZTX851 could give you well below 100mV
Vce(sat), (or a suitable MOSFET, ie logic level for your logic supply voltage)

Just try 100 ohm resistor on the base you'll probably see an improvement on Vce(sat).

You are drawing 10mA from pin 3, so its voltage is expected to drop 0.4V or so - check
the datasheets (output pins have a finite output resistance).

3

80mA or so for a led seems quite high. Have you a link to the data sheet ?
With a 3.3 volt Arduino, you'd probably put the leds in parallel, each with its own series resistor.
The voltage you measure at pin 3 (which I guess is a PWM signal at say 66% duty cycle) will depend on how you measure it. With an Oscilloscope, you'll see a 0 to 4.65 volt square wave. With a digital volt meter you'll see 3.03 volts.