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I got a SainSmart digital oscilloscope kit for christmas, and put it together according to documentation I found on the Amazon.com listing for the item.

The markings on the board directed me to connect the input pin and output pin opposite of how they should have actually been connected. That is, it was receiving 9v to the output pin instead of the input pin, and the input pin was connected to where the regulated 5v output was supposed to go. I discovered this by nearly burning my fingertip on the overheating voltage regulator after plugging in the 9v power supply, and then looking up the voltage regulator's datasheet and comparing it to the board diagram.

I removed the voltage regulator, turned it around, and soldered it back in place. When I plugged in the power supply again, the regulator got hot again, and I still didn't get the expected 5v at the output. Is my voltage regulator fried?

(Note: it's either a 78L05 or a 79L05 - the board has one of each and I don't recall offhand which one was the one I was dealing with.)

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    \$\begingroup\$ If it is a 79L05 then it is a -5V regulator, so trying to use it to get +5v will end in tears. You would need to connect its input to the -ve terminal of the supply, its ground would go to the +ve terminal (which is also your 5V supply), and the output would be your 0V for your supply. \$\endgroup\$ – Tom Carpenter Jan 5 '16 at 0:23
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    \$\begingroup\$ The SainSmart oscilloscope kit comes with excellent instructions, follow the instructions at the bottom of the second page and measure the voltages at the test points shown, that will tell you if the regulator is working \$\endgroup\$ – Jasen Jan 5 '16 at 5:38
  • \$\begingroup\$ @TomCarpenter Huh. So it was wired correctly the first time (provided it was 79L05 and not 78L05)? If that's the case, why would it burn out? \$\endgroup\$ – mouseas Jan 5 '16 at 21:08
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If it was hot enough to burn you, there's a very good chance it's dead. Especially if it gets hot when correctly connected.

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    \$\begingroup\$ The maximum operating temperature for these regulators is about 150°C, isn't it? But touching something at only 50-60°C can produce burns. So this is not at all a reliable method for determining whether the regulator is dead. Nonetheless, it probably is, although who cares anyway--it costs so little that there is no harm in replacing it just because it might be damaged. \$\endgroup\$ – Oleksandr R. Jan 5 '16 at 1:47
  • \$\begingroup\$ touching somehting 60°C becomes uncomfortable after a minute or so, I don't think it can cause burns. 100°C will not burn instantly - as a stunt I have used my bare hand to snatch spaghetti from the pot. (you have to be quick). On the Sainsmart oscilloscope the regulators do not not normally get hot. \$\endgroup\$ – Jasen Jan 5 '16 at 5:31
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    \$\begingroup\$ @OleksandrR. William is not saying that the heat fried it, but rather that being fried caused it to heat up. \$\endgroup\$ – user253751 Jan 5 '16 at 6:07
  • \$\begingroup\$ What immibis said. High temp is a symptom of it dissipating power that it probably shouldn't be (unless the normal load current is significant!), which is an indication of damage and current flowing where it shouldn't. \$\endgroup\$ – William Brodie-Tyrrell Jan 5 '16 at 10:10
  • \$\begingroup\$ @immibis I have not suggested otherwise. This is simply an unreliable way to determine whether it is dissipating "too much" power or not. It could be operating perfectly normally and still be hot enough to burn you. Measuring its output voltage would be the most useful and obvious test of its operation. \$\endgroup\$ – Oleksandr R. Jan 6 '16 at 17:13

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