Electronics > Projects, Designs, and Technical Stuff
Questions about infrared LEDs for remotes
<< < (4/7) > >>
oPossum:

--- Quote from: Benta on September 22, 2019, 08:46:12 pm ---6: 1 A is completely over the top for IRLED current, which is why I suggest around 100 mA. It can be altered easily by changing the emitter resistor (and, if necessary, the base resistor as well). Less current might work just as well, a practical experiment is necessary.

--- End quote ---

It isn't for a pulsed application like this.
Benta:

--- Quote from: oPossum on September 16, 2019, 05:19:35 pm ---I usually just rely on the internal resistance of AA batteries and the PN2222A to limit current to about 1A.

--- End quote ---

After this example of brilliance, nothing more needs to be said...

Peabody:
Benta, I think I understand your circuit, but I'm confused by your statement that Vce is negligible.  I'm not sure how you meant that, but if the voltage at the emitter is 0.7V and the LED voltage drop is 1.2V, then the CE voltage has to be 1.4V, and the transistor is acting like a 14Ω resistor, which also makes it the hot spot in the chain.

But I understand the advantage of keeping the LED current constant as the battery voltage drops.
Benta:
Yes, the phrasing was perhaps a little unclear.

Your voltage calculation is quite correct. What I meant to say was, that the collector-emitter voltage automatically adjusts to the IRLED forward voltage. With a low Vce you are able to use LEDs with a higher forward voltage, or you can tolerate battery voltage drop to a higher degree. I hope this makes sense,
Kleinstein:
For pulsed operation like in a remote 100 mA pulse current is not much and would result in low power, low range. So some 500 mA is well possible as it would be only with a low PWM ratio: some 30% at 38 kHz for a few ms at most followed by a little longer off. 1 A may actually be on the upper limit. The remarks on the PWM ratio are independent of the drive circuit - just a point that may be overlooked and it makes clear that it is a low PWM ratio that allows high peak current.

The BC327 is a PNP, the correct NPN brother is BC337.  At low current the gain is 250 - at high current like 500 mA it is much lower. The low VCE spec is for a rather high base current, like IC/10.  At 500 mA and with 10 mA base drive, expect some 0.3 V for VCE. At 5 mA base current it may be in the 1 V region.
This can still work with a 3.2 V supply. However the resistor to the driving side needs to be much smaller to get 10 mA of base drive. So more like 150  Ohms.
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod