Dual secondaries -- quick question

[redrabbit]

I have a circuit that wants +12v, and +10v... regulated, and the negative pins tied together on the circuit board.

I have a 12+12v dual secondary transformer (and dual primaries).

Can I regulate each secondary -- separately, and have their negative pins tied at the output ?...without conflicts or trouble ?



thanks,
=RR=

[sawreyrw]

Yes, if the secondaries are isolated from one another. BTW, you need filter caps after the rectifiers.

[mag]

Yes, this will work but you can do the same with only one rectifier brigde.

Put the two secondaries in parallel and connect them to the rectifier.

Regulate the 12V rail with a 7812 (or something similar).

For the 10V rail you can add a regulator directly after the rectifier (in parallel to the filter capacitor) or after the 12V regulated rail.
depending if the regulator can work with a dropout of 2V.

How much current do you need on those supplies?

[redrabbit]

Originally posted by mag


How much current do you need on those supplies?

For the 12v... I need to pull around 1.8A , possibly 2A
For the 10v, I need only around 5 mA.

Upon further reading of the CIRCUIT (link), I realized that I only need one supply at 12v (the former 10v can be upped to 12v).

"" Note that VDD and VS can
be driven with the same power supply in the range of +5V
< VDD = VS <+12V. ""


My transformer is....
LP-24-2000
48 VA
24VCT @ 2.00A
12V @ 4.00A

So if I parallel it, I'd be playing with 4 Amps @ 12v.
Maybe it is better for regulation to drain 2A from an available 4A......rather than trying to drain 2A from a 2A max ??


The Vdrop on my LT1084 regulators (TO-220) is about 1.3v at full load (less at lower load).
I also have 7912K, and LM338K.

This circuit will be running 24-7, always.

[woody]

Using those secondaries in parallel you get 12v @ 4a. Then when
you rectify it you should get 12v X 1.41 or 16.92v - ~2v lost in
the bridge rectifier for ~ 3A at 15V dc. I would be tempted to
wire the secondaries in seriese and use 2 diodes as a full wave
rectifier instead of a bridge to give the regulator another volt
or so.

[megajocke]

That transformer is unsuitable for that output voltage and current. You need more headroom for the regulator. Seriesing the windings won't give enough current. Iac RMS = about 1.8 times the DC current, so you need about 3.5A RMS from the transformer. A low dropout regulator might be possible but I'm skeptical it will be enough when you consider ripple, line voltage tolerance and rectifier drop.

[redrabbit]

You may be correct.
I built it already, with the 2 separate windings individually, full bridge each, and 2 LT1084's set for 11.5v......and the voltage and regulation diminishes as the load is increased.
By the time it was pushing close to 2A....the volts fell to under 10v, and poorly regulated.


I will save this PSU for another project, maybe adjust it for 2 x 9v (or +9v, with a big heatsink).

I will try again with a more robust xformer.

=RR=

[megajocke]

For 12V 2A you need about 15V or a little more from the transformer. It needs to be about 3.5-4 amps.

[AndrewT]

Hi,
for 12Vdc with 2A continuous load the transformer must be rated >=12Vac 4Aac.
But at this loading it will run hot.
Most recommend that the continuous demand should be <50% of maximum rating.
I recommend you use 12Vac 8Aac (100VA) transformer and use >4700uF smoothing.
If you buy a toroid and find that the maximum drop out @ maximum current coinciding with minimum mains voltage degrades the regulated output then add a 1Vac winding to give 13Vac 8Aac transformer. This should meet your requirements.

15Vac will make the regulator run hotter than necessary.

[megajocke]

You have to consider mains voltage drop too. It might work at 13V though at nominal line voltage and with a bigger than absolutely necessary transformer.