Lm7812/7912

kinser · 21st November 2004, 04:04 PM

Hi to all,
I am in the last step of finishing my OPA541 (bridged 2 op amps) based subwoofer amp....
and now I need 12vdc +/- for the active filter and fan for the heatsink.
Now my power supply is 36.6vdc and i would like to use the LM7812/7912 regulators, but on the datasheet its writen that the max input volatge is 35v
I dont whant to use a small transformer for it...
can I use these regulators? if not maby i could use the LM317/337?
Thanks,
Kinser

kinser · 21st November 2004, 04:40 PM

Noooooo one?

Kinser

DcibeL · 21st November 2004, 05:24 PM

Since you are using the regulators on a low power circuit, why don't you just put a 5W or so resistor in series with the input of the regulator to drop the input voltage?

Reklats · 21st November 2004, 05:27 PM

I'm not sure about the 337, but the 317 datasheet says that the maximum "input-output voltage differential" is 40v, so it seems like it would work.

kinser · 21st November 2004, 06:28 PM

Hi,
so i can put 5w resistor, how many ohms?
and about he 317 - can i use it on the negative rail also?
Thanks,
Kinser

peranders · 21st November 2004, 06:59 PM

If you have 35-37 volts is real life I wouldn't be afraid to use 7x12 regulators but as said above you can burn away a few volts with series resistors or you could use LM3x7 which only have the voltage restriction from input to out, not ground. If you are lazy to calculate you can check my QRP02 or QRV07 amps. I use +-12 Volts LM317/337 in those designs.

If it was a commercial product I would not chose to have more than 35 volts worst case as input voltage. Then I would have played it safe.

HFGuy · 21st November 2004, 07:00 PM

use an LM317 with a bypass transistor. Dont use a resistor , its wasteful and kinda stupid

peranders · 21st November 2004, 07:14 PM

HFGuy, I think you misunderstood. The load was only a couple of opamps for an active filter. Not a heavy load.

kinser · 21st November 2004, 07:25 PM

ok, I just was notified that I need this amp finished in two days...
So I wont have time to go get the lm337/317 because I nee to go to Tel-Aviv to get the parts and i got shcool, i only have the lm7x12, and resistors i can get from a friend. should the resistors be 5-10W and between 80 to 800 ohm?
Thanks,
Kinser

carlmart · 22nd November 2004, 09:55 AM

Using a series of RC filters before the regulator, whatever the type you are using, is always a good idea.

To better implement it you should use a large capacitor, with no series resistor, after the regulator. Works very good.

Of course use separate bridges for this low current supply, not the same as the GC.

Carlos

21st November 2004, 04:04 PM	#1
kinser diyAudio Member Join Date: Jan 2004 Location: R'aanana	Lm7812/7912 Hi to all, I am in the last step of finishing my OPA541 (bridged 2 op amps) based subwoofer amp.... and now I need 12vdc +/- for the active filter and fan for the heatsink. Now my power supply is 36.6vdc and i would like to use the LM7812/7912 regulators, but on the datasheet its writen that the max input volatge is 35v I dont whant to use a small transformer for it... can I use these regulators? if not maby i could use the LM317/337? Thanks, Kinser

21st November 2004, 05:24 PM	#3
DcibeL diyAudio Member Join Date: May 2003 Location: Saskatchewan	Since you are using the regulators on a low power circuit, why don't you just put a 5W or so resistor in series with the input of the regulator to drop the input voltage? __________________ The power of Science compels you!

21st November 2004, 06:59 PM	#6
peranders Electrons are yellow and more is better! diyAudio Member Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4	If you have 35-37 volts is real life I wouldn't be afraid to use 7x12 regulators but as said above you can burn away a few volts with series resistors or you could use LM3x7 which only have the voltage restriction from input to out, not ground. If you are lazy to calculate you can check my QRP02 or QRV07 amps. I use +-12 Volts LM317/337 in those designs. If it was a commercial product I would not chose to have more than 35 volts worst case as input voltage. Then I would have played it safe. __________________ /Per-Anders (my first name) or P-A as my friends call me

21st November 2004, 07:14 PM	#8
peranders Electrons are yellow and more is better! diyAudio Member Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4	HFGuy, I think you misunderstood. The load was only a couple of opamps for an active filter. Not a heavy load. __________________ /Per-Anders (my first name) or P-A as my friends call me

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21st November 2004, 04:40 PM	#2
kinser diyAudio Member Join Date: Jan 2004 Location: R'aanana	Noooooo one? Kinser

21st November 2004, 05:27 PM	#4
Reklats diyAudio Member Join Date: May 2004 Location: Ohio	I'm not sure about the 337, but the 317 datasheet says that the maximum "input-output voltage differential" is 40v, so it seems like it would work.

21st November 2004, 06:28 PM	#5
kinser diyAudio Member Join Date: Jan 2004 Location: R'aanana	Hi, so i can put 5w resistor, how many ohms? and about he 317 - can i use it on the negative rail also? Thanks, Kinser

21st November 2004, 07:00 PM	#7
HFGuy diyAudio Member Join Date: Sep 2004 Location: Toronto, Ontario	use an LM317 with a bypass transistor. Dont use a resistor , its wasteful and kinda stupid

21st November 2004, 07:25 PM	#9
kinser diyAudio Member Join Date: Jan 2004 Location: R'aanana	ok, I just was notified that I need this amp finished in two days... So I wont have time to go get the lm337/317 because I nee to go to Tel-Aviv to get the parts and i got shcool, i only have the lm7x12, and resistors i can get from a friend. should the resistors be 5-10W and between 80 to 800 ohm? Thanks, Kinser

22nd November 2004, 09:55 AM	#10
carlmart diyAudio Member Join Date: Mar 2001 Location: Brazil	Using a series of RC filters before the regulator, whatever the type you are using, is always a good idea. To better implement it you should use a large capacitor, with no series resistor, after the regulator. Works very good. Of course use separate bridges for this low current supply, not the same as the GC. Carlos

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