Convert 21V to 15V dc to dc

[geminni]

I need a schematic to get 15V from 21V dc.
I know how to get that with LM7815 but with that I can have only 1A. I need something for 4Amps!
I need that for small power amplifier. I already have transformer with 15V secondary that gives mi 21V dc. Power IC's are rated at 15V so 21V will probably destroy them 🙂

 

[FoMoCo]

I need a schematic to get 15V from 21V dc.
I know how to get that with LM7815 but with that I can have only 1A. I need something for 4Amps!
I need that for small power amplifier. I already have transformer with 15V secondary that gives mi 21V dc. Power IC's are rated at 15V so 21V will probably destroy them 🙂

Some of the older 7815 datasheets have a circuit that will allow you to "wrap around" a power transistor and get more current.

 

[Sorento]

see pg. 20 / 21 of LM7815 pdf, LM7815 description, LM7815 datasheets, LM7815 view ::: ALLDATASHEET :::

 

[sofaspud]

Why not just use an LM338?

 

[Rundmaus]

15V from 21V @4A via a linear regulator will mean you burn 24W in the regulator. Do you plan for the appropriate heatsink?

Andreas

 

[sofaspud]

Yes, you do.

 

[FoMoCo]

see pg. 20 / 21 of LM7815 pdf, LM7815 description, LM7815 datasheets, LM7815 view ::: ALLDATASHEET :::

That's similar to what I was thinking of. Except the circuit I'm thinking of has an extra diode and resistor. I'm thinking it's in an older National datasheet.

 

[hilli_billi]

If you measure the ac voltage of the transformer you get 15V eff. If you rectify it and put a cpapacitor you get approx. 21V (with no load). That's the peak value. If you draw some current it will decrease. If you put a linear regulater, it will dissipate about 15W @ 4A.

 

[hilli_billi]

21V To 15V

I simulated a simple regulator with LT-spice.

 

[geminni]

Of course I will use someheatsink 🙂

payloadde, I will try your suggestion from datasheet, it sounds simple for a start.

 

[tomchr]

Have a look at the LMZ23605. I've used some of the LMZ regulators for filament regulators in my tube amps. They're excellent regulators.

~Tom

 

[geminni]

I was not sure what was good for audio because some years ago I have made something with transistors but it was not good because it was bad for the sound so I did it with lm7815 and used it wisely! 😀

Someone mentioned LM317 so I have found a schematics here:
24v to 12v converter | Power Supply Circuits
I did not used that much transistors of course 🙂

It worked fine under load of 3 amps so I just need to test it with amplifier and music.