How to choose appropriate transformer

[vinod chandran]

Hi,
i am in the middle of a sub-woofer amp project. the components of this project are, a fm radio board, sub-woofer filter board, 20w stereo amp board for satellite speakers, an led vu meter board. My problem is how to choose an appropriate transformer for this project. all are12 volt boards My doubt is how much voltage and how much amperage
help me plz.
-vinod

 

[TheLaw]

Hi,
i am in the middle of a sub-woofer amp project. the components of this project are, a fm radio board, sub-woofer filter board, 20w stereo amp board for satellite speakers, an led vu meter board. My problem is how to choose an appropriate transformer for this project. all are12 volt boards My doubt is how much voltage and how much amperage
help me plz.
-vinod

The boards require a regulated 12VDC? Or do they have their own power supplies?

Anyway,

A 12V transformer would be adequate. I'd say a 2-3A transformer would also probably be decent. Not sure of the power consumption on the other boards, but you should have a little headroom on the transformer regardless.

The transformer will produce 12VAC. You'll need to convert that to DC by using a bridge rectifier. When AC is converted to DC, you multiply 1.41 x the AC voltage. So a 12VAC transformer would produce roughly 16.9VDC after rectification.

If the boards require a regulated 12V on the spot, you'll need to a pretty high power regulator. I searched Digikey and found the LM1085IT-12-ND...which is a 3A 12V regulator. You'll need a heatsink on that, I'm sure.

I don't want to explain too much because I might be leading you on the wrong path if this is not how your boards are set up.

 

[davelectronic]

Power reqirments

I would go with calculating total peak and possible surge current, you have already said the voltage, i might go + 5 % up on the power due to line and load regulation, a second easy rgulator is the LM338K TO3 type steel case, and only needs a couple or so external components for its function, its good for 5 amps if your in put is not more than a few volts higher. Dave.

 

[vinod chandran]

The boards require a regulated 12VDC? Or do they have their own power supplies?

Anyway,

A 12V transformer would be adequate. I'd say a 2-3A transformer would also probably be decent. Not sure of the power consumption on the other boards, but you should have a little headroom on the transformer regardless.

The transformer will produce 12VAC. You'll need to convert that to DC by using a bridge rectifier. When AC is converted to DC, you multiply 1.41 x the AC voltage. So a 12VAC transformer would produce roughly 16.9VDC after rectification.

If the boards require a regulated 12V on the spot, you'll need to a pretty high power regulator. I searched Digikey and found the LM1085IT-12-ND...which is a 3A 12V regulator. You'll need a heatsink on that, I'm sure.

I don't want to explain too much because I might be leading you on the wrong path if this is not how your boards are set up.

Hi TheLaw,
Thanks for the information. i'm going to build this with a 2A transformer. and of course there will be 12v regulator also. once again thanks to share the knowledge about rectifier output(v in*1.41). because i don't know anything abt rectifier's output. So thanks.

 

[vinod chandran]

Thanks Dave,
5A transformer is not affordable for me to this project. i'm planning to buy a 2 or 3A transformer

 

[vinod chandran]

The boards require a regulated 12VDC? Or do they have their own power supplies?

Anyway,

A 12V transformer would be adequate. I'd say a 2-3A transformer would also probably be decent. Not sure of the power consumption on the other boards, but you should have a little headroom on the transformer regardless.

The transformer will produce 12VAC. You'll need to convert that to DC by using a bridge rectifier. When AC is converted to DC, you multiply 1.41 x the AC voltage. So a 12VAC transformer would produce roughly 16.9VDC after rectification.

If the boards require a regulated 12V on the spot, you'll need to a pretty high power regulator. I searched Digikey and found the LM1085IT-12-ND...which is a 3A 12V regulator. You'll need a heatsink on that, I'm sure.

I don't want to explain too much because I might be leading you on the wrong path if this is not how your boards are set up.

Hi TheLaw,
Thanks for the information. i'm going to build this with a 2A transformer. and of course there will be 12v regulator also. once again thanks to share the knowledge about rectifier output(v in*1.41). because i don't know anything abt rectifier's output. So thanks.

 

[davelectronic]

RE Tranformer

You dont need a 5 amp transformer to use the LM338K 5 amp regulator, 5 amps is the maximum power that regulator will provide, anything under is fine, including the transformer you use, rectified smoothed then regulated, some amps will run on rectified dc smoothed only, depends on the amps requirments. Dave.

 

[vinod chandran]

thanks dave. i am going to google for a circuit with lm338k

 

[davelectronic]

The LM338K and 338T Data sheet

Here is the data sheet for the LM338K and LM338T first is a TO3 case, the second the LM338T is a TO220 case PDF file link below. Dave.

http://www.alldatasheet.com/datasheet-pdf/pdf/70444/LINER/LM-338K.html

 

[vinod chandran]

Hi,
Thanks Dave. Last day i've got a 12-0-12 transformer (5A). Now i want a bridge rectifier for that transformer. I mean what type of diodes and capacitors should i use.
-vinod

 

[TheLaw]

Hi,
Thanks Dave. Last day i've got a 12-0-12 transformer (5A). Now i want a bridge rectifier for that transformer. I mean what type of diodes and capacitors should i use.
-vinod

I'm going to simplify it a little bit, so anyone who knows better...I might not be politcally correct.

I don't know the availability in India, but something like a GBU7 (7A) or GBU8 (8A) bridge rectifier will do. You don't really wan to have anything lower for reliability.

The 0V is a shared ground type of thing. It doesn't get hooked up to the rectifier.

Then take each of the 12V wires and hook it up to each of the AC pins on the rectifier. On the DC output of the rectifier, the positive pin will be +17V and the negative pin -17V. The -17V is not the return/ground of the +17V however. The ground is...well..the ground that you already hooked up the 0V of the center tap to.

Capacitors wise, you'll need 2 capacitors, one for each rail. I'm no expert on the matter, but I'd say anywhere from 4700-6300uF would be good. You should use 25V rated capacitors.

 

[vinod chandran]

Thanks TheLow,
But the bridge rectifier is not available in my area. so plz tell me the value of 4 diodes. is a 2 diode rectifier suit for this transformer. could you pls send me an image of your idea?.

 

[TheLaw]

Thanks TheLow,
But the bridge rectifier is not available in my area. so plz tell me the value of 4 diodes. is a 2 diode rectifier suit for this transformer. could you pls send me an image of your idea?.

Okay 6A10 diodes, but that's pretty much a minimum. If you only use 2 diodes, you only get one output. It's full wave, but if you want 2 outputs (~+/-17V), you need 4 diodes.

I'm about to leave for vacation so maybe someone else can help.

 

[vinod chandran]

Okay 6A10 diodes, but that's pretty much a minimum. If you only use 2 diodes, you only get one output. It's full wave, but if you want 2 outputs (~+/-17V), you need 4 diodes.

I'm about to leave for vacation so maybe someone else can help.

Hi TheLaw, enjoy your vacation. i jast need an ordinary single output power supply. I connect my 12-0-12 5A transformer in my sub-woofer project with two 6A Diode and one 2200uF 25V capacitor. but my stereo amp produced a grrrr noise in every second. is that a problem with 5A current or 2200uF capacitor ?. anyhow this power supply giving me an output of 17V. So do i need a 12V regulator?.

 

[TheLaw]

Hi TheLaw, enjoy your vacation. i jast need an ordinary single output power supply. I connect my 12-0-12 5A transformer in my sub-woofer project with two 6A Diode and one 2200uF 25V capacitor. but my stereo amp produced a grrrr noise in every second. is that a problem with 5A current or 2200uF capacitor ?. anyhow this power supply giving me an output of 17V. So do i need a 12V regulator?.

What are you using for your amplifier? A lot of amp chips do require +/- voltages.

 

[vinod chandran]

Hi,
My baas filter board is dameged. today i just turn a preset on that board. after that the board is not working.
-vinod

 

[(*steve*)]

Any reason you don't want to try to fix this one?

My guess is that you changed the biasing in some way that was destructive. It may just need (as a guess) some new transistors and to correctly set up the biasing again (that may not be trivial).

Did smoke come out of anywhere?

 

[vinod chandran]

Hi steve,
No, that was my mistake . i just turned a preset on that subwoofer filter board. after that board is not working. i thought that the preset is to adjust the bass. anyhow my board was damaged. so i need a subwoofer filter circuit. but the next problem is does the new filter circuit suit with my 12v 5A power supply?.By the way i am sorry to say that i post this subject as a new thread in this forum. if that is a mistake, plz ignore or delete that thread. i attached an image of my new idea.

 

[davelectronic]

Rectification

Hi your transformer has a center tap, middle wire on the secondary AC output. there is a link below for a full wave center tapped rectifier. Dave. PS diodes need to be larger on there current and voltage ratings than the transformers current and voltage output.

http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/rectct.html

Sorry for the late reply, hope you got it soughted.

 

[vinod chandran]

Hi Dave,
Thanks for the link. according to that page a 10k resistor will work(My capacitor is 2200uF). And then the rippiling problem is over. But what about the subwoofer filter board?.is there any way to repair it . if no i need a filter circuit..