Help, please, selecting some Electronic Components

Inductance in R1 will make a difference. It will slightly increase the voltage on the capacitor. But given the relative values I'd agree it can probably be ignored.

Robert.

Thanks Robert for the update about the rise and fall rates for light emitted from a white LED.

CS

Another update :

Probably my final one on this thread; but those who spotted the big error in the Fig. 3(a) schematic are welcome to proclaim “I told you so !”

Over the weekend, Susan Parker kindly had a look at the schematic,

and, in answer to my questions, advised me:

1. The drawing is indeed wrong, C3 [at the input] is a decoupling cap.
2. If the diode is to do anything useful, then a Schottky part should be used.
3. The resistor should be low inductance as a matter of principle, but otherwise is immaterial as to the tolerance etc. since is is the far side of the main reservoir cap. It does need to be of sufficient power as it is recharging the 3300uF cap.
4. Note however this circuit is NOT suitable for your application, the 100R and 3300uF time constant is way too slow for your frame/strobe rate.

So … my understanding is that the circuit is good for the originally intended purpose: BUT the input is wrongly drawn and needs to better reflect the diagram in the data-sheet [which I posted earlier].

But unfortunately it is not fast enough for my proposed frame-rate.

Thanks, everyone, for your comments.

I will probably build one anyway, just for the short flash-duration, even if it doesn’t cycle at the rate I want.

MichaelG.

If you want a faster cycle time reduce the value of the storage capacitor and increase the voltage at the supply end of the 100R resistor. Keep the energy the same. Bear in mind the stored enery is half the capacitance times the SQUARE of the voltage so if tou double the voltage use a quarter of the capacitance. So for 12V you want about 560uF. In practice the preferred value of 470uF will be fine. This just sets the flash duration IF the pulse is longer than the RC time constant.

In practice you can have a high flash rate as long as you keep the pulses short. Which is what you want anyway. This is not xenon strobe, you can turn it off. The original designer did not do a good job.

Robert.

I agree with Robert, except if the repetition rate is much faster then you don't want to risk pumping too much energy through the LED. As a rough guide the resistor should be chosen to limit the average current through the LED to its normal operating current or rather less.

The LED will get a rough ride, but the idea is that the heat generated in the LED by the narrow pulses will dissipate between the pulses enough to avoid overheating, but the pulse energy integrated over time should not exceed the normal operation or it will overheat.

Neil

I think the bonding wire that connects the LED terminal to the top side of the diode die may be the feature that limits the max pulse current that can be achieved.

CS