Is there any benefit in using L7912 if I have transformer with 2 output windings - Page 1
Forum section: electronics / beginners

author: dusan
author_rank: Regular Contributor
timestamp: February 29, 2024, 03:13:32 pm
content: I'm making power supply. My transformer (MYRRA 44238) has 2 independent 15V outputs. This means I can either use 2x 7812 and make 2 independent 12V supplies, or 7812+7912. Is there any benefit in using 7912? Am I correct that 7912 is only used for center tap transformers?

author: Zero999
author_rank: Super Contributor
timestamp: February 29, 2024, 03:30:30 pm
content: The benefits to using an L7912 are only one bridge rectifier is required, which saves a component and reduces the voltage loss in the rectifier, so smaller filter capacitors can be used.

author: Andy Chee
author_rank: Super Contributor
timestamp: February 29, 2024, 06:12:38 pm
content: The 7912 is used to generate a negative referenced voltage rail, for example:Your 2x independent 15V outputs can be connected in parallel (same phasing), to use the above circuit topology.However you can also connect your 2x independent 15V outputs in series and create a center tap, for example:Lots of other configurations as well.

author: edavid
author_rank: Super Contributor
timestamp: February 29, 2024, 07:15:54 pm
content: Quote from: Zero999 on February 29, 2024, 03:30:30 pmOn the other hand, you would expect the 7812 to have lower output impedance (not that there is any evidence for this in the datasheet).And with the arguably too high transformer voltage, you might prefer to drop a bit more voltage in the rectifiers.

author: Zero999
author_rank: Super Contributor
timestamp: February 29, 2024, 07:39:24 pm
content: Quote from: Andy Chee on February 29, 2024, 06:12:38 pmI would not recommend connecting the secondary windings in parallel and using #2. Larger capacitors for the same current are required, as it's only half wave and if significantly more current is drawn from one rail, than the other, it will induce a net DC flux in the transformer core, causing it to saturate. That configuration should be reserved for when there's only one transformer winding.Use #1, which is perfectly fine, or the two L7815.Quote from: edavid on February 29, 2024, 07:15:54 pmBoth points are valid.The L7812 should be more stable than the L7912, given it has an emitter follower on the output. The L7912 has a common emitter output which is more likely to oscillate.Quote from: dusan on February 29, 2024, 03:13:32 pmI've just done a quick search for the transformer. Note it's only rated to 160mA, so you won't be able to draw the full current from L7812/L7912, more like around half that, as the RMS current in the transformer is always higher than the DC output. The data sheet recommends a 160mA fuse on each secondary. It should be time delay/slow blow, to avoid nuisance tripping. A resettable PTC fuse might be a good idea, if you don't want to have to replace it, after it's tripped.Refer to the table at the bottom of page 14:https://docs.rs-online.com/54d1/0900766b80f3c2a1.pdfAnother data sheet, which also shows the mechanical layout.https://www.farnell.com/datasheets/41164.pdfAnother thing is the unloaded secondary voltage is 29V, so the filter capacitors must be rated to at least 35V, given it might be even higher if the mains voltage is on the upper end of the tolerance band.

author: Andy Chee
author_rank: Super Contributor
timestamp: February 29, 2024, 08:11:54 pm
content: Quote from: dusan on February 29, 2024, 03:13:32 pmWhat is the purpose of your power supply?Is it a generic bench power supply?Or will it be built as part of a larger overall project? What is the project? Digital MCU? Analog amplifier?

author: dusan
author_rank: Regular Contributor
timestamp: March 01, 2024, 06:57:48 am
content: generic bench power supply