which one is right - pin outs on a split rail 7912cv - Page 1
Forum section: electronics / beginners

author: Deathwish
author_rank: Unknown
timestamp: August 13, 2015, 10:02:07 pm
content: Ok this should be easy, I pulled a couple of diagrams of the net to build a split rail supply from a centre tapped transformer. the 7912 is an ST 7912CV which according to the DS is a TO220 with pins 1=gnd, 2= in and 3= out. I take gnd to be taken from the centre tap of the transformer and connect up as such, i took In pin 2 to be from the negative of the Bridge rectifier, but oh no, all I get is a hot regulator and naff all else. so I take a look at the 2 diagrams I have and suddenly my mind goes into a spin , which pin outs are the right ones ..... Do they play pass the pins with these things ?

author: Paul Moir
author_rank: Frequent Contributor
timestamp: August 13, 2015, 10:08:15 pm
content: Yes, the negative (79xx) versions have a different pinout.The centre pin on a to-220 is normally connected to the centre tab. This necessarily changes the pinout of the negative regulator since the silicon is directly and conductivity attached to the tab.

author: Deathwish
author_rank: Unknown
timestamp: August 13, 2015, 10:10:58 pm
content: The 7812 is playing nicely as it should its the 7912 that is not.but both of those diagrams have different pin outs to different places yet the transformer is still a centre tapped one in both. So which is right.I lost the plot with it and am now watching serial killers on youtube till I calm down

author: MarkF
author_rank: Super Contributor
timestamp: August 13, 2015, 10:22:20 pm
content: Do you have the tabs of the two regulators isolated? Otherwise you'll short the input to the negative regulator to gnd.

author: Paul Moir
author_rank: Frequent Contributor
timestamp: August 13, 2015, 10:25:12 pm
content: Oh sorry, I think I'm following you now. I've never seen them switch the pins on the 79xx. It should be Ground-input-output (1-2-3), so your first diagram following the red numbers is right.You don't have both of 'em screwed to the same heatsink do you?

author: Deathwish
author_rank: Unknown
timestamp: August 13, 2015, 10:26:42 pm
content: No both are on seperate heatsinks.

author: Deathwish
author_rank: Unknown
timestamp: August 13, 2015, 10:59:40 pm
content: Its running ok now. Oh well, stupid pin outs. got to put a 5V rail on next.

author: apis
author_rank: Super Contributor
timestamp: August 13, 2015, 11:13:24 pm
content: It's also a good idea to add some protection diodes for different fault conditions:

author: ludzinc
author_rank: Unknown
timestamp: August 14, 2015, 03:24:37 am
content: Quote from: Deathwish on August 13, 2015, 10:02:07 pmRTFM. Read The Formatted Datasheet for the parts

author: Deathwish
author_rank: Unknown
timestamp: August 14, 2015, 08:02:45 am
content: "the 7912 is an ST 7912CV which according to the DS is a TO220 with pins 1=gnd, 2= in and 3= out."Kind of tells you I DID RTFMMy gripe is with common parts such as the 78 or 79xx series devices I would expect them to be drop in replacements for each other, but if you are going to draw a circuit diagram use the pin designator not numbers. The use of G, I ,O on those circuit diags rather than 1,2,3 would of been much more use to a beginner or anyone else as far as I am concerned.