Author Topic: Is there any benefit in using L7912 if I have transformer with 2 output windings  (Read 453 times)

0 Members and 1 Guest are viewing this topic.

Offline dusanTopic starter

  • Regular Contributor
  • *
  • Posts: 67
  • Country: sk
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?
 

Offline Zero999

  • Super Contributor
  • ***
  • Posts: 19578
  • Country: gb
  • 0999
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.
 
The following users thanked this post: Ian.M

Offline Andy Chee

  • Frequent Contributor
  • **
  • Posts: 707
  • Country: au
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.
 

Offline edavid

  • Super Contributor
  • ***
  • Posts: 3386
  • Country: us
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.

On 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.
 

Offline Zero999

  • Super Contributor
  • ***
  • Posts: 19578
  • Country: gb
  • 0999
The 7912 is used to generate a negative referenced voltage rail, for example:

(Attachment Link)

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:

(Attachment Link)

Lots of other configurations as well.
I 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.

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.

On 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.


Both 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.

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?
I'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.pdf

Another data sheet, which also shows the mechanical layout.
https://www.farnell.com/datasheets/41164.pdf

Another 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.
 

Offline Andy Chee

  • Frequent Contributor
  • **
  • Posts: 707
  • Country: au
I'm making power supply.
What 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?
 

Offline dusanTopic starter

  • Regular Contributor
  • *
  • Posts: 67
  • Country: sk
generic bench power supply
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf