| ??? 06/14/09 00:49 Read: times |
#166067 - He doesn't really have to consider voltage Responding to: ???'s previous message |
Andy Neil said:
Quite so.
But doesn't it go to show that he's missed an important point about the ULN - or any other - driver? (viz, the additional voltage drop that it introduces) His specific question was why the LED was dimmer with the ULN driver than without it. Understanding why this is so would also help to understand why a proper constant-current drive is so much better. Wouldn't it? Perhaps, but it also points up that, with a constant-current source, the collector-emitter voltage really doesn't matter, provided he has, say, a 24-volt supply. The ULN-whatever, would sink it regardless. The LED would "see" the required current. The brightness would match. All this because all the parameters are within specified limits. This allows the LED forward voltage to be ignored. It also allows the collector-emitter voltage to be ignored. It simply means that he hasn't considered all the relevant parameters ... yet. He's just trying to wring a bit too much utility out of his 5-volt supply. He should design LED circuits using current as the controlling factor, and not voltage, and use a large enough supply to make the various forward voltages irrelevant. RE |
