You can check it yourself. If of course you have my model library.

 

hi A,
Sims for, R2 to 0V and R2 to Emitter.
They look the same, showing approx 8nSec D1 pulse width [ 0% to 0%], however I would choose the bootstrap option.
E
BTW:
I have had your model library for some time, finally decided today, to create a composite of my models and yours, all works fine.
Thanks again for your LTSpice work.

 

At another load (not LED), there may be pulses of several tens of volts at the transistor emitter. Therefore, connecting a R2 resistor to 0V may cause a breakdown of the transistor emitter junction. Therefore, I connect the lower pin of the R2 resistor to the emitter.

 

I need to pulse a LED at 200ps. Using the below circuit, i can pulse led at ns intervals. Can anyone help me to tune the RC components in the collector terminal of transistor to get 200ps pulse.
Thanks in advance

Hola, I do not intend to derail this thread but I would like to know how do you actually intend to use this once you get it working.

Trying to recall how it was applied in a distance measurement equipment (Swiss origin) which, IIRC, compared a precisely calibrated timing loop against the actual TOF.

 

hi meena,
This is using a generic NPN and LED
Stepping the Ctune.
Shows the LED current and Voltage.
E

First - let me qualify my knowledge and experience on this - I have none. So I'm wondering what would happen if you reversed the LED and let the inductance kickback from the coil fire the LED? What kind of voltage would be seen there? And that would answer what resistance would be needed to protect the LED - IF the resistor is required at all.

Again, I plainly state I don't know the answer to your question. I'm just wondering if reversing the LED would achieve the result you want. To mitigate the length of the LED flash you would have to choose the proper coil.

[edit] Here's what I'm thinking (wondering) Invert D1, Change L1 to a tunable choke and remove R4 (not shown in drawing). Q2 charges L1. Q2 shuts off and L1 discharges through D1.

Partial schematic:

 

AFAIK the EB breakdown voltage (forward and reverse) is fairly low -- 6V perhaps.

 

Testing my hypothesis: Using a green LED (Vf ?) and a 33mHy choke; connecting four AA batteries (6V) and simulating the switching off of Q2, the LED will flash. I am unable to measure the duration of the flash - my scope is not a memory scope and my computer based scope is not presently functioning (computer board needs some repairs). But I DO get a flash. Next will try with a super bright LED just to see what happens. But for sure, the flash is visible; so it must be longer than you want.

 

Using 10mHy, my green LED would flash dimly. Using a scrap transformer, using just one set of the coils as an inductor I get a much brighter flash. However, this arrangement has also blown out my red Super Bright LED (Vf = 1.95V).

I guess it comes down to how frequently you want your LED to flash and at what current level you want going through it. I didn't set up tests with any resistors, I just wanted to prove to myself that an LED will flash when the magnetic field of a DC charged coil breaks down.

 

hi meena,
This is another option you can try.
This is the method I used with ZTX451 transistors and LD163 laser diode.
Very fast Rise Time, which is needed for distance measurement.
E
@Bordodynov

 

Also, very high overdrive currents are used for these very short pulses to overcome capacitance and to get enough energy per burst to make a detectable signal. 20 to 50Amps is possible if the duty cycle is low (High kHz to low MHz) with pSec pulses.