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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 28th Jan 2016, 8:03 pm   #1
merlinmaxwell
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Default Save yourself a quid or so...

Building a power supply for my radios/antenna/usb link that needs 13.7/12 and 5V. Got all the bits, built, checked and powered up, BANG, the 5V regulator lost its smoke.

I thought I was being clever by getting a 3A 7805 style regulator for the 5V (you can't have too much current available), wrong, the pinouts are not the standard in/common/out but in/out/common oops!! And of course the tab was grounded (so much for the current limiting).

Moral of this story, read the datasheet first.
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Old 28th Jan 2016, 9:06 pm   #2
TonyDuell
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Default Re: Save yourself a quid or so...

On the 'traditional' 78xx and 79xx regulators the middle pin and tab are the most -ve connection. So common for a 78xx and input for a 79xx.

What is the number of the regulator you used wth the odd pinout?
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Old 28th Jan 2016, 11:45 pm   #3
MrElectronicman
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Default Re: Save yourself a quid or so...

I recently built a similar regulator, but since I had some problems in the past I actually read the data sheet, and lo and behold the pinout is different from the usual 3 terminal devices.

Not too happy with the linear regulator however, I prefer to use a switching type as they are more efficient, i.e. less heat, wherever possible.
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Old 29th Jan 2016, 4:59 pm   #4
Biggles
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Default Re: Save yourself a quid or so...

The smaller 100mA versions are bad for this. Different manufacturers seemed to follow different pin outs, although the middle leg is usually common. I once wasted a bit of time when building a 13.2V power supply by using diodes to "jack up" the common leg of a 7812 which was fine out of the case but then returned to 12V every time I bolted it all up. Yes, you've probably guessed, I'd forgotten to isolate the tab and it was returning to 0V via the aluminium case. I now always tend to check the exact part number (prefix/suffix) and manufacturer of the 100mA ones against an RS or Maplin catalogue to confirm the pin out as I have had too many overheat and fail on switch on by idly guessing which is the input connection.
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Old 29th Jan 2016, 5:57 pm   #5
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Default Re: Save yourself a quid or so...

Quote:
What is the number of the regulator you used with the odd pinout?
It was a... STMICROELECTRONICS LD1085V50
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