7812 TO220 regulator little help please - diyAudio
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Old 20th May 2003, 03:28 PM   #1
strulon is offline strulon  France
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Default 7812 TO220 regulator little help please

hello

i have one little question, if someone can answer it would be nice

so i ve got a 12.6 V 1A output from my transfo and i d like to feed a 7812 12V 1A TO220 regulator with it, will the 7812 output be 12V regulated?

or should i use a adjustable trim before?

thank you

fred
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Old 20th May 2003, 08:36 PM   #2
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You have 12,6 AC transformer right...so then you have to rectify it...and smooth it with a cap..will get you to around 16V..
That is enough to use a 7812. Remember it needs more voltage than 12,6V to be able to give 12 regulated.


The 7812 is fixed at 12v out regulated...so you need no trim!

Why did you post this at in the Tubes section by the way?

Cheers,
Bas
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Old 20th May 2003, 08:45 PM   #3
EC8010 is offline EC8010  United Kingdom
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Perhaps if he added a 1N4148 between the regulator's ground lead and ground, which would cause the regulator to produce 12.6V, the forum of choice would become clear?
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Old 21st May 2003, 05:24 AM   #4
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How much current (DC) will you need? And how much current totally?
You cant get 1A 12 DC from this winding , maybe 0,7A .
And to load a 7812 with 1A is in the danger zoone, and it probably will get very hot.
Is it for DC heater to one or two tubes ? Then I say you'd better throw in a diode from pin two (and that is the case too) to - (might be the ground) as EC8010 suggest.

björn
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Old 21st May 2003, 07:41 AM   #5
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Quote:
Perhaps if he added a 1N4148 between the regulator's ground lead and ground, which would cause the regulator to produce 12.6V, the forum of choice would become clear?
Once again your ruthless intellect has made it all clear...crystal clear.....

Guess I should get more sleep...

Cheers,
Bas
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Old 21st May 2003, 10:00 AM   #6
strulon is offline strulon  France
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hi, thanks for your replys

in fact it is for the relay board of my tube preamp, which relays are 12V, i guess i am more concerned with tube amps that s why i ve been using this forum and not another, sorry

so i ve bought a transfo for a ridiculous price to make my preamp and i have one 12.6V 1A output left after building the preamp, and i want to use it to drive my relay board, which relays are 12V,


so i ve been picking up a relay board scheme from a magazine which has a 7812 regulator feeded with 12V and i ask this question because i want to use my 12.6V output to feed the 7812

so hello Bas, can you explain me how can i rectify my transformer?

by the way here is a picture of the preamp, it is a 6 ch preamp

Click the image to open in full size.

cheers

fred
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Old 22nd May 2003, 03:33 AM   #7
PMiczek is offline PMiczek  United States
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Default 12 V regulator

Yes, the 7812 produces a regulated 12V output, but needs a higher, rectified voltage which is at least 14.5 volts at the input, NOT the output of the transformer directly. Good news is, all of these parts are cheap compared to your trafo.

So: unregulated DC in, regulated 12V DC out.

I would say first add the currents for all of the relay coils that can be on at the same time, to calculate the maximum current needed. That will be the worst case for the DC regulator, and it will allow you to calculate if you need a heatsink (you can also use more than one 7812, they are cheap enough).

The AC output of the transformer is rectified first, using a bridge rectifier, or four diodes, followed by one or two capacitors. This gives you the so-called un-regulated supply somewhere between 14 and 18 volts DC, depending on your transformer and the load etc. You need about 14.5V with this part to maintain regulation, but this is not so critical with relays. Then the 7812 regulator, with or w/o heatsink.

Whether you need a heatsink for a linear regulator can usually be calculated based on the type of case (TO-220 here), the voltage across the regulator, and the maximum current. There is probably a formula for this in the 7812 datasheet.

Wild guess: for this transformer voltage and 1A out, you need a bridge rectifier rated at 1-3 amps, 2 or three cheap electrolytic capacitors around 2200-3300uF each and rated at 25 volts minimum, and a heatsink rated at 10 degrees C/watt for the regulator.

Nice picture, btw.

PM
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Old 22nd May 2003, 09:03 AM   #8
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Quote:
The AC output of the transformer is rectified first, using a bridge rectifier, or four diodes, followed by one or two capacitors. This gives you the so-called un-regulated supply somewhere between 14 and 18 volts DC,
Like PMiczek said..but here is a little diagram to help you..


Quote:
why i ve been using this forum and not another, sorry
No need to be sorry.. ;-) I was just curious!

Cheers,
Bas
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Old 22nd May 2003, 11:51 AM   #9
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I just have to say, the PCB looks AWESOME!!! WOW!!! Have you done the layout yourself?
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Old 22nd May 2003, 02:20 PM   #10
strulon is offline strulon  France
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thank you Bas and PMiczek


Bas, for the diagram, it looks familiar to me now, thanks

concerning the caps, should i use one 2.2microF 25V, like on the diagram or should i use three of them?


PMiczek, i am not sure of which current to add for the relays, but i have one old heatsink, ill be using it, anyway thank you.

in fact i am using three relays at the same time, and i think each one needs 10 mA as "detection current", i guess it is quite low in fact,

the datasheet of the relays can be found here
NAIS DS2E-S-DC12V http://www.soyter.cz/pdf/nais/DS_E.PDF


thanks Mikael, i did the layout myself from a stereo preamp pcb, just adjusting the different spaces to fit my components, you can find the pcb from beginning in my site under "protocircuit.jpg" to see the change,

bye

fred
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