smoke test - failed!

[richiereynolds]

Hi there, looking for some hardware help.

Had constructed a little power supply break out board - battery -> 220uf electrolytic cap -> LM7805 -> 220uf electrolytic cap followed by some swiches to bridge the 5V output to some different circuits.
Got quite late last night and of course I plugged the battery in the wrong way ...
Saw smoke and pulled it out again, couldn't have been more than a few seconds. Went to bed ...

So, how likely is damage, what am I likely to have damaged, and how could I test the components?

If the LM7805 is outputting 5V can I assume it's ok?
Is there any way I can test the caps in circuit? Obviously I can test that current is flowing through them but how can I test they're actually doing their job?

Thanks guys,
Richard.

[ethought]

If one of the caps was smoking, from my limited experience, you can sometimes tell which one it was by visually inspecting. Anything with burn marks or bulging casing (swollen looking) should be replaced.

As I understand it is usually not good practice to reverse positive and negative / ground on electrolytic capacitors as they tend to smoke, or explode (loud).

[Joker94]

As I understand it is usually not good practice to reverse positive and negative / ground on electrolytic capacitors as they tend to smoke, or explode (loud).

not only loud but dangerous.

I am not sure what happens to a cap when you hook the power back up the right way but i dont think it will do anything out of the norm. If nothing has happened already i dont think any thing will.

if the regulator is out putting 5v it is fine to still use. and even if you have to and or need to replace the caps they are less than 10 cents each and if anyone reports that it may be dangerous i wouldnt hesitate replacing them out of safety.

Regards

Joker94

[little-c]

if the capacitor failed, it would short and smoke regardless of direction of current.

if it is an electrolytic capacitor, I wouldn't place any bets on it functioning within the range that is stated.



most important thing, replace the electrolytic capacitors to insure that they do what they are ment to.

[richiereynolds]

Thanks all, actually it all appears to be working ok but I can't be sure the caps are doing their job so I guess I'll replace them. Was just hoping for an easy way to test them in circuit to avoid more soldering!

[Soeren]

Hi,

Replace the regulator as well and trash the old one completely, whether it appears to work or not.
If it ain't dead allready it will die and you don't want it to take a microcontroller and other circuitry with it if it suddenly outputs too high a voltage. Regulators and caps are cheap compared to what you use the supply for, so go with safe (and connect it the right way 'round next time )

[cyberfish]

If you can spare 0.65V (input > 7.65V), it may be a good idea to add a reverse-polarity protection diode.

[richiereynolds]

Given the likelihood that I'll do the same stupid thing again at some point that sounds like a good idea!

Thanks all for the advice!

[Soeren]

Hi,


For polarity protection without a voltage drop, use this:


For a very small drop (20mV/A), use this - if the battery voltage is 12V or more.

[little-c]

Dead links, don't know if its me or not>

[Soeren]

Hi,

Dead links, don't know if its me or not>

Might it be the propagation that is/was too low causing a timeout?
I allways check links and pics in my browser when uploaded to a server and they show good here (or I would have corrected it), but they're placed on a fairly local server.
Perhaps if you try again later, they should get through.

[little-c]

still dead. not a clue why.

[Soeren]

Hi,

Strange, but here they are in a different way:

[little-c]

those work. thanks.