Where is this smoke coming from? No... seriously

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So I'm happily working away on a bit of code and I see some wisps of white-ish smoke waft by.

Holy Crap!

So I shut down the power supply and think to myself... what is causing this. I'm using a buck converter (switching power regulator) for the first time and I've so far been happy with the setup. The AVR (mega8) is warm but not hot.

So I turn it on again, wait a second, and smoke again. It looks like it's coming from a jumper wire. Sure enough, the insulation on it is melting. On my breadboard I have a power rail that has a separation in the middle, essentially two separate sets of rails that I just jumper over in order to have power and gnd available the entire length of the board.

Interestingly enough, I have the gnd from my bench supply plugged in about 12 holes from this jumper wire that is smoking. I moved the gnd connection from that rail to the connection row for the gnd on the buck converter. Now I don't have a hot wire and I don't have the smoke.

Why did this happen? I'm only pulling about 60mA off the bench supply which is outputting 7V.

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Update:

Ok, I'm prepared for the flaming that's sure to follow this post.

So I'm using a ghetto power supply that I made by converting an ATX psu. I figured this would be just fine, I'm only looking for a source of 'dirty' DC because my target circuits will be further regulated down to a cleaner 5v. I figured I might as well run off of 7v as I'm planning to use a 9v battery for this project when it is finished. So I plug my positive supply into the +12v and my ground into the +5v. This should yield +7v, and it does.

After my smoking problems as described above I decided to check the current draw again. With described 7v setup I'm pulling 160mA off the bench supply. When I move the plugs to +12v and GND I'm pulling only 76.1mA

Why the difference? Why the smoking wire with the +12/+5 setup?

ps- here's the supply I'm using:
http://www.instructables.com/id/...

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Power supplies are designed to put out current, not sink it. The set up you describe is not recommended as on the return path the current would be flowing into the 5V output. What happens then depends on the design of the power supply, but it will be unpredictable, and generally undesirable. Even RF oscillations are not unheard of in such cases.

If you have to use such a set up, put a heavy load on the 5V supply so that all the return current passes through that load, instead of into the 5V output.

If you think education is expensive, try ignorance.

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While I try to see some effort in thought, which I understand your thought on this, your bench supply setup is a bad one. Switching power supplies are/can be a pain in the ass when it comes to grounding, AND regulation, depending on design. One thing you NEVER do is try to use one rail to supply ground to subtract from another rail as you have done. What ends up happening in ATX/computer supplies is that both rails are trying to regulate each other. The reason the problem goes away when you move the jumper is the buck converter is doing some isolation. The AVR getting warm is also a concern to me as it is probably taking a punishment from this setup.

Suggestion is to get rid of the ATX supply. Pweriod, unless you want more headaches with the signals that need to be fed it to operate properly, and purchase a simple wall wart or two at +12v and then use your buck regulator.

Jim

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Depending on the frequency of your buck converter, a breadboard with all its huge
parasitic capacitances might not provide an ideal hookup solution for your
switchmode converter. General rule for switchers is to keep all parasitic
impedances as small as possible, bypass supply rails within a few millimeters of
the switching circuit etc. pp.

Jörg Wunsch

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ATX PS is grounded. All black wires are also grounded. PC is grounded. Programmer is connected to PC. Ouch. Your virtual ground (the 5V) is shorted to ground via the other devices. Got it ? That's the most likely reason for the smoke. So don't use the PS like that.
ATX PS is a very bad PS for this purpose. But you were told that already by the fellow posters.

Nard

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you should definitely not be using an ATX power supply for powering a 7V, 60mA circuit.

Use a regular transformer (wall wart) and regulate it as necessary.