| Electronics > Projects, Designs, and Technical Stuff |
| Questions about infrared LEDs for remotes |
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| Benta:
Damn! Of course it's BC337-40, not BC327. My bad... Concerning the hfe discussion: you're presuming that the transistor is working as a hard switched device. That's not the case. In my quickly sketched circuit it is operating in it's linear region, which means the high hfe is applicable. At 100 mA collector current it's 250 min. |
| iMo:
A sim.. |
| Benta:
@imo: Looks nice. Could you do a sim with 1.8k as base resistor, please? |
| iMo:
1k8 base resistor. |
| Benta:
@imo: Thank you very much, very helpful. The second plot shows that the circuit still works perfectly, but at reduced current: ~64 mA instead of ~100 mA. Why? The 1N4148 datasheet gives the answer: at low forward currents (<1 mA), VF is down to 0.5...0.55 V. This explains the lower collector current =IRLED current. Solution: an even lower power diode. The curvy collector voltage is due to the fact, that the simulation LED is not suited for high currents and the chip heats up. Substituting the correct IRLED should solve the issue. |
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