Super informative post. Thanks for sharing.<p>One thing I didn't understand was the author's comment that "I bought the part off eBay, not from a reputable supplier, so it could have come from anywhere."<p>A 5-pack of quality 7805's can be found on Amazon for $5, including Prime shipping, (e.g. <a href="http://amzn.com/B00H7KTRO6" rel="nofollow">http://amzn.com/B00H7KTRO6</a>), so what's the incentive to buy parts of unknown provenance on eBay or the like?<p>I ask not being a hardware guy myself, so genuinely curious, as I've heard stories like this before.<p>Again, fantastic overview of the chip, though. I learned a lot.<p>[Edit: spelling]
This is a beautiful post, and I am reluctant to try and say anything profound. Still, all of HN's a stage so I'll attempt a brief explanation for the general reader about why we need voltage regulators.<p>A logic chip like a microprocessor is designed for a particular supply voltage, if this voltage drops too much the logic circuitry will switch falsely. Say we had only a capacitor and we tried to power the logic chip with it. As the chip draws current the capacitor discharges - this is because current is movement of charge, so the charge (and energy) can come only by draining the capacitor. For an ideal capacitor the voltage is directly proportional to the charge across it, so as the charge drains the voltage falls. To hold the voltage constant we need to keep 'topping up' the capacitor with charge. This is what a voltage regulator does - it uses a negative feedback loop to sense the capacitor voltage and when that voltage falls the circuit provides just the right amount of charge 'juice' for the top-up.<p>As we take the foot off the clutch pedal in a car, the load gets engaged to the engine and if we sense a stall we press the gas pedal a bit. That's the imagery of a voltage regulator in action.<p>The capacitor plays a key role because the regulator feedback loop isn't very fast - one trouble with fast feedback circuits is chatter, or responding to every blip. Negative feedback circuits are designed to be more like ship wheels - they like to steer sedately and not respond to every excited cry from the mast. But what happens if a current blip arises because a logic circuit block turns on all at once (in response to some block of code)? That local current blip is provided by the capacitor, it acts like an ATM to provide local draws - but it still depends on the regulator to top it up.<p>In fact you can think of a battery as a capacitor that tops itself up via electrochemistry, it works as long as there are ions in the electrolyte. If instead of 'bandgap energy' we used the chemist's terminology of 'electrochemical potential difference', then the system similarity becomes evident.
Prior to reading this the only thing I knew with certainty about a 7805 was never to use it to pick up your PCB. They get a little toasty when shorted, and I've burned myself on them more often than on my soldering iron.
I know almost nothing about electronics - why don't the thin wires going to the die melt like a fuse, are they just made out of the right material?
I am compelled to echo the the first statements by quarterwave and kjs3. I was personally saved from a morning of mindlessness, and I will be soon opening up an IC or two to see for myself. You said I could do it. Thanks kens.
Well, the 7805 is sold by multiple vendors, so, it may be a counterfeit, or just a subcontracted part?<p>Very, very nice explanation of the 7805 though. This is the bread and butter of linear regulators.
This is with all seriousness the most informative thing I read this week. Took me right back to my abortive flirtation with EE as an undergrad. If thing had been explained this clearly, I might have stuck with it.