Save yourself a quid or so... - UK Vintage Radio Repair and Restoration Discussion Forum

28th Jan 2016, 8:03 pm

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.

TonyDuell · 28th Jan 2016, 9:06 pm

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?

MrElectronicman · 28th Jan 2016, 11:45 pm

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.

Biggles · 29th Jan 2016, 4:59 pm

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.
Alan

29th Jan 2016, 5:57 pm

Quote:

What is the number of the regulator you used with the odd pinout?

It was a... STMICROELECTRONICS LD1085V50

28th Jan 2016, 8:03 pm	#1
Guest Guest Posts: n/a	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.

28th Jan 2016, 9:06 pm	#2
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,219	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?

28th Jan 2016, 11:45 pm	#3
MrElectronicman Heptode Join Date: Feb 2011 Location: Washington DC, USA Posts: 619	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. __________________ David

29th Jan 2016, 4:59 pm	#4
Biggles Rest in Peace Join Date: Feb 2004 Location: Hexham, Northumberland, UK. Posts: 2,234	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. Alan