Having hard time to figure transistor output current

[bulak]

Hello!

**broken link removed**

Studying this simple schematic I couldn't figure out how many amps the capacitor will feed to the PUT transistor when it discharges. And how many amps will the transistor feed to the LED? Is there some calculation for this?

Also, by my calculations, the transistor GATE is getting 2.26v, so when 470k gets to 2.26v the capacitor will discharge 3.74v(6-2.26) to the transistor, the transistor(AK) will drop more 2.26v and the rest will go the LED. Is that correct??

I just wanted to know how to calculate the amps going the LED...

Any help on this would be greatly appreciated. I've been thinking hard on this.

Thanks!

 

[ericgibbs]

hi,
What is the colour and forward voltage drop of your LED.???

EDIT:
Look at the test circuits in this datasheet.

 

[ericgibbs]

Also, by my calculations, the transistor GATE is getting 2.26v, so when 470k gets to 2.26v the capacitor will discharge 3.74v(6-2.26) to the transistor, the transistor(AK) will drop more 2.26v and the rest will go the LED. Is that correct?

Hi,
Look at this image, there is an error in your circuit and calculations.

 

[bulak]

Hi!

I'm using a standard green 5mm LED, I don't know what it is its forward voltage. How do I check it?

I got this rigged in a breadboard and I'm getting odd voltage readings:

Transistor GATE->COLLECTOR voltage is constant at 2.56v (shouldn't it be at 3.85v like you said?). While ANODE->CATHODE voltage flashes the LED at 2.85v.

Also, what about the current, how do I know how much current the capacitor is discharging?

And what is this software you used?

ps.: sorry to throw all this stuff at you at once! I really'd like to understand what is happening

 

[Roff]

The max peak current Ip(max) is 5uA, so the largest value timing resistor in this circuit that will insure oscillation is
Rt(max)≈(6V-4.4V)/5uA≈320kΩ.
Ip(typ)=4uA. I tried the sim with various values for Rt. It oscillated with 420k, but not with 430k. To be safe, keep it below 320k.
A simple way to keep the time constant the same is to use 220k and 4.7uF.

 

[bulak]

Hi!

But for this kind of LED to light doesn't it need a current of at least 10mA?

This circuit is from the book Make eletronics: learn by discovery... why did you remove the LED and put Rt instead?

I don't know why I'm getting so different readings from what they are supposed to be

EDIT: I see something now, I'm using 4 x 1.5v cells, but their total voltage is 6.35v. So I correctly measured gate voltage now, and it is 4.08v, which makes sense.

I just don't understand how much current the capacitor discharges and to where it goes? And how can a LED light (flash) with just 5uA, if they work with 10mA!

 

[Roff]

Hi!

But for this kind of LED to light doesn't it need a current of at least 10mA?

This circuit is from the book Make eletronics: learn by discovery... why did you remove the LED and put Rt instead?

I don't know why I'm getting so different readings from what they are supposed to be

EDIT: I see something now, I'm using 4 x 1.5v cells, but their total voltage is 6.35v. So I correctly measured gate voltage now, and it is 4.08v, which makes sense.

I just don't understand how much current the capacitor discharges and to where it goes? And how can a LED light (flash) with just 5uA, if they work with 10mA!

Peak LED current is not related to the Ip spec. Ip is the current required to trigger the PUT. It is the current through the timing resistor at the point where the PUT fires.
You will get hundreds of milliamps of peak current in the LED, if the simulation is correct.