PIC18F4520, IrLED want to get high Amps - help with capacitor value

[wanna-be-engineer]

Hi guys,

I am trying here with a project which is more or less an optical smoke detector working with IrLeds. My problem is that I need to send pulse current to the IrLED in order to get a peak pulse of almost 1 Amp and I cannot figure out which capacitor to choose for that case - as a matter of fact I am really confused how one chooses a capacitor at all. I know the formulas and all the theory around but in practice .... Please, give me a hint or share your knowledge with me as I am stuck with that for almost a month.

I am using PIC 18F4520, got a 5V voltage regulator, the IrLED is HE1-220AC, the phototransistor is BPW96. I am also not sure about the 2N2222 transistor that I am using. I would be VERY VERY Grateful for any suggestions!

View attachment 64368

The R9 resistor is just for simulating the IR light. Here I am attaching also the datasheet of the IrLED from Harvatek View attachment 64369

 

[colin55]

Firstly, to get 1 amp through D1, the resistor R1 will have to be 3R (or less).
The transistor Q8 will have to be a transistor such as BD 679.
The electrolytic C1 will have to be 470u.
The pulse from the micro will have to be just a few milliseconds.

 

[dr pepper]

Further to Colins comment you might want to use a low esr 470u cap, a standard one would be ok for a while butyou might find it'll start to get hot due to internal resistance.

You could rob one out of a dead switching supply, they are the tall thin ones.

 

[colin55]

but you might find it'll start to get hot due to internal resistance.

How on earth do you work that out when the duty cycle will be only a few percent. The IR LED is about 100mW!!!!!

 

[Nigel Goodwin]

Check out my PIC IR tutorial, the hardware of which does exactly what you need (your circuit above is completely wrong on various points).